enum.c

/**********************************************************************

  enum.c -

  $Author$
  created at: Fri Oct  1 15:15:19 JST 1993

  Copyright (C) 1993-2007 Yukihiro Matsumoto

**********************************************************************/

#include "id.h"
#include "internal.h"
#include "internal/compar.h"
#include "internal/enum.h"
#include "internal/hash.h"
#include "internal/imemo.h"
#include "internal/numeric.h"
#include "internal/object.h"
#include "internal/proc.h"
#include "internal/rational.h"
#include "ruby/util.h"
#include "ruby_assert.h"
#include "symbol.h"

VALUE rb_mEnumerable;

static ID id_next;

#define id_div idDiv
#define id_each idEach
#define id_eqq  idEqq
#define id_cmp  idCmp
#define id_lshift idLTLT
#define id_call idCall
#define id_size idSize

VALUE
rb_enum_values_pack(int argc, const VALUE *argv)
{
    if (argc == 0) return Qnil;
    if (argc == 1) return argv[0];
    return rb_ary_new4(argc, argv);
}

#define ENUM_WANT_SVALUE() do { \
    i = rb_enum_values_pack(argc, argv); \
} while (0)

static VALUE
enum_yield(int argc, VALUE ary)
{
    if (argc > 1)
	return rb_yield_force_blockarg(ary);
    if (argc == 1)
	return rb_yield(ary);
    return rb_yield_values2(0, 0);
}

static VALUE
enum_yield_array(VALUE ary)
{
    long len = RARRAY_LEN(ary);

    if (len > 1)
	return rb_yield_force_blockarg(ary);
    if (len == 1)
	return rb_yield(RARRAY_AREF(ary, 0));
    return rb_yield_values2(0, 0);
}

static VALUE
grep_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct MEMO *memo = MEMO_CAST(args);
    ENUM_WANT_SVALUE();

    if (RTEST(rb_funcallv(memo->v1, id_eqq, 1, &i)) == RTEST(memo->u3.value)) {
	rb_ary_push(memo->v2, i);
    }
    return Qnil;
}

static VALUE
grep_iter_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct MEMO *memo = MEMO_CAST(args);
    ENUM_WANT_SVALUE();

    if (RTEST(rb_funcallv(memo->v1, id_eqq, 1, &i)) == RTEST(memo->u3.value)) {
	rb_ary_push(memo->v2, enum_yield(argc, i));
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.grep(pattern)                  -> array
 *     enum.grep(pattern) { |obj| block }  -> array
 *
 *  Returns an array of every element in <i>enum</i> for which
 *  <code>Pattern === element</code>. If the optional <em>block</em> is
 *  supplied, each matching element is passed to it, and the block's
 *  result is stored in the output array.
 *
 *     (1..100).grep 38..44   #=> [38, 39, 40, 41, 42, 43, 44]
 *     c = IO.constants
 *     c.grep(/SEEK/)         #=> [:SEEK_SET, :SEEK_CUR, :SEEK_END]
 *     res = c.grep(/SEEK/) { |v| IO.const_get(v) }
 *     res                    #=> [0, 1, 2]
 *
 */

static VALUE
enum_grep(VALUE obj, VALUE pat)
{
    VALUE ary = rb_ary_new();
    struct MEMO *memo = MEMO_NEW(pat, ary, Qtrue);

    rb_block_call(obj, id_each, 0, 0, rb_block_given_p() ? grep_iter_i : grep_i, (VALUE)memo);

    return ary;
}

/*
 *  call-seq:
 *     enum.grep_v(pattern)                  -> array
 *     enum.grep_v(pattern) { |obj| block }  -> array
 *
 *  Inverted version of Enumerable#grep.
 *  Returns an array of every element in <i>enum</i> for which
 *  not <code>Pattern === element</code>.
 *
 *     (1..10).grep_v 2..5   #=> [1, 6, 7, 8, 9, 10]
 *     res =(1..10).grep_v(2..5) { |v| v * 2 }
 *     res                    #=> [2, 12, 14, 16, 18, 20]
 *
 */

static VALUE
enum_grep_v(VALUE obj, VALUE pat)
{
    VALUE ary = rb_ary_new();
    struct MEMO *memo = MEMO_NEW(pat, ary, Qfalse);

    rb_block_call(obj, id_each, 0, 0, rb_block_given_p() ? grep_iter_i : grep_i, (VALUE)memo);

    return ary;
}

#define COUNT_BIGNUM IMEMO_FL_USER0
#define MEMO_V3_SET(m, v) RB_OBJ_WRITE((m), &(m)->u3.value, (v))

static void
imemo_count_up(struct MEMO *memo)
{
    if (memo->flags & COUNT_BIGNUM) {
	MEMO_V3_SET(memo, rb_int_succ(memo->u3.value));
    }
    else if (++memo->u3.cnt == 0) {
	/* overflow */
	unsigned long buf[2] = {0, 1};
	MEMO_V3_SET(memo, rb_big_unpack(buf, 2));
	memo->flags |= COUNT_BIGNUM;
    }
}

static VALUE
imemo_count_value(struct MEMO *memo)
{
    if (memo->flags & COUNT_BIGNUM) {
	return memo->u3.value;
    }
    else {
	return ULONG2NUM(memo->u3.cnt);
    }
}

static VALUE
count_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memop))
{
    struct MEMO *memo = MEMO_CAST(memop);

    ENUM_WANT_SVALUE();

    if (rb_equal(i, memo->v1)) {
	imemo_count_up(memo);
    }
    return Qnil;
}

static VALUE
count_iter_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memop))
{
    struct MEMO *memo = MEMO_CAST(memop);

    if (RTEST(rb_yield_values2(argc, argv))) {
	imemo_count_up(memo);
    }
    return Qnil;
}

static VALUE
count_all_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memop))
{
    struct MEMO *memo = MEMO_CAST(memop);

    imemo_count_up(memo);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.count                 -> int
 *     enum.count(item)           -> int
 *     enum.count { |obj| block } -> int
 *
 *  Returns the number of items in +enum+ through enumeration.
 *  If an argument is given, the number of items in +enum+ that
 *  are equal to +item+ are counted.  If a block is given, it
 *  counts the number of elements yielding a true value.
 *
 *     ary = [1, 2, 4, 2]
 *     ary.count               #=> 4
 *     ary.count(2)            #=> 2
 *     ary.count{ |x| x%2==0 } #=> 3
 *
 */

static VALUE
enum_count(int argc, VALUE *argv, VALUE obj)
{
    VALUE item = Qnil;
    struct MEMO *memo;
    rb_block_call_func *func;

    if (argc == 0) {
	if (rb_block_given_p()) {
	    func = count_iter_i;
	}
	else {
	    func = count_all_i;
	}
    }
    else {
	rb_scan_args(argc, argv, "1", &item);
	if (rb_block_given_p()) {
	    rb_warn("given block not used");
	}
        func = count_i;
    }

    memo = MEMO_NEW(item, 0, 0);
    rb_block_call(obj, id_each, 0, 0, func, (VALUE)memo);
    return imemo_count_value(memo);
}

static VALUE
find_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memop))
{
    ENUM_WANT_SVALUE();

    if (RTEST(enum_yield(argc, i))) {
	struct MEMO *memo = MEMO_CAST(memop);
	MEMO_V1_SET(memo, i);
	memo->u3.cnt = 1;
	rb_iter_break();
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.detect(ifnone = nil) { |obj| block } -> obj or nil
 *     enum.find(ifnone = nil)   { |obj| block } -> obj or nil
 *     enum.detect(ifnone = nil)                 -> an_enumerator
 *     enum.find(ifnone = nil)                   -> an_enumerator
 *
 *  Passes each entry in <i>enum</i> to <em>block</em>. Returns the
 *  first for which <em>block</em> is not false.  If no
 *  object matches, calls <i>ifnone</i> and returns its result when it
 *  is specified, or returns <code>nil</code> otherwise.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     (1..100).detect  #=> #<Enumerator: 1..100:detect>
 *     (1..100).find    #=> #<Enumerator: 1..100:find>
 *
 *     (1..10).detect         { |i| i % 5 == 0 && i % 7 == 0 }   #=> nil
 *     (1..10).find           { |i| i % 5 == 0 && i % 7 == 0 }   #=> nil
 *     (1..10).detect(-> {0}) { |i| i % 5 == 0 && i % 7 == 0 }   #=> 0
 *     (1..10).find(-> {0})   { |i| i % 5 == 0 && i % 7 == 0 }   #=> 0
 *     (1..100).detect        { |i| i % 5 == 0 && i % 7 == 0 }   #=> 35
 *     (1..100).find          { |i| i % 5 == 0 && i % 7 == 0 }   #=> 35
 *
 */

static VALUE
enum_find(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;
    VALUE if_none;

    if_none = rb_check_arity(argc, 0, 1) ? argv[0] : Qnil;
    RETURN_ENUMERATOR(obj, argc, argv);
    memo = MEMO_NEW(Qundef, 0, 0);
    rb_block_call(obj, id_each, 0, 0, find_i, (VALUE)memo);
    if (memo->u3.cnt) {
	return memo->v1;
    }
    if (!NIL_P(if_none)) {
	return rb_funcallv(if_none, id_call, 0, 0);
    }
    return Qnil;
}

static VALUE
find_index_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memop))
{
    struct MEMO *memo = MEMO_CAST(memop);

    ENUM_WANT_SVALUE();

    if (rb_equal(i, memo->v2)) {
	MEMO_V1_SET(memo, imemo_count_value(memo));
	rb_iter_break();
    }
    imemo_count_up(memo);
    return Qnil;
}

static VALUE
find_index_iter_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memop))
{
    struct MEMO *memo = MEMO_CAST(memop);

    if (RTEST(rb_yield_values2(argc, argv))) {
	MEMO_V1_SET(memo, imemo_count_value(memo));
	rb_iter_break();
    }
    imemo_count_up(memo);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.find_index(value)          -> int or nil
 *     enum.find_index { |obj| block } -> int or nil
 *     enum.find_index                 -> an_enumerator
 *
 *  Compares each entry in <i>enum</i> with <em>value</em> or passes
 *  to <em>block</em>.  Returns the index for the first for which the
 *  evaluated value is non-false.  If no object matches, returns
 *  <code>nil</code>
 *
 *  If neither block nor argument is given, an enumerator is returned instead.
 *
 *     (1..10).find_index  { |i| i % 5 == 0 && i % 7 == 0 }  #=> nil
 *     (1..100).find_index { |i| i % 5 == 0 && i % 7 == 0 }  #=> 34
 *     (1..100).find_index(50)                               #=> 49
 *
 */

static VALUE
enum_find_index(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;	/* [return value, current index, ] */
    VALUE condition_value = Qnil;
    rb_block_call_func *func;

    if (argc == 0) {
        RETURN_ENUMERATOR(obj, 0, 0);
        func = find_index_iter_i;
    }
    else {
	rb_scan_args(argc, argv, "1", &condition_value);
	if (rb_block_given_p()) {
	    rb_warn("given block not used");
	}
        func = find_index_i;
    }

    memo = MEMO_NEW(Qnil, condition_value, 0);
    rb_block_call(obj, id_each, 0, 0, func, (VALUE)memo);
    return memo->v1;
}

static VALUE
find_all_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    ENUM_WANT_SVALUE();

    if (RTEST(enum_yield(argc, i))) {
	rb_ary_push(ary, i);
    }
    return Qnil;
}

static VALUE
enum_size(VALUE self, VALUE args, VALUE eobj)
{
    return rb_check_funcall_default(self, id_size, 0, 0, Qnil);
}

static long
limit_by_enum_size(VALUE obj, long n)
{
    unsigned long limit;
    VALUE size = rb_check_funcall(obj, id_size, 0, 0);
    if (!FIXNUM_P(size)) return n;
    limit = FIX2ULONG(size);
    return ((unsigned long)n > limit) ? (long)limit : n;
}

static int
enum_size_over_p(VALUE obj, long n)
{
    VALUE size = rb_check_funcall(obj, id_size, 0, 0);
    if (!FIXNUM_P(size)) return 0;
    return ((unsigned long)n > FIX2ULONG(size));
}

/*
 *  call-seq:
 *     enum.find_all { |obj| block } -> array
 *     enum.select   { |obj| block } -> array
 *     enum.filter   { |obj| block } -> array
 *     enum.find_all                 -> an_enumerator
 *     enum.select                   -> an_enumerator
 *     enum.filter                   -> an_enumerator
 *
 *  Returns an array containing all elements of +enum+
 *  for which the given +block+ returns a true value.
 *
 *  The <i>find_all</i> and <i>select</i> methods are aliases.
 *  There is no performance benefit to either.
 *
 *  If no block is given, an Enumerator is returned instead.
 *
 *
 *     (1..10).find_all { |i|  i % 3 == 0 }   #=> [3, 6, 9]
 *
 *     [1,2,3,4,5].select { |num|  num.even?  }   #=> [2, 4]
 *
 *     [:foo, :bar].filter { |x| x == :foo }   #=> [:foo]
 *
 *  See also Enumerable#reject, Enumerable#grep.
 */

static VALUE
enum_find_all(VALUE obj)
{
    VALUE ary;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    ary = rb_ary_new();
    rb_block_call(obj, id_each, 0, 0, find_all_i, ary);

    return ary;
}

static VALUE
filter_map_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    i = rb_yield_values2(argc, argv);

    if (RTEST(i)) {
        rb_ary_push(ary, i);
    }

    return Qnil;
}

/*
 *  call-seq:
 *     enum.filter_map { |obj| block } -> array
 *     enum.filter_map                 -> an_enumerator
 *
 *  Returns a new array containing the truthy results (everything except
 *  +false+ or +nil+) of running the +block+ for every element in +enum+.
 *
 *  If no block is given, an Enumerator is returned instead.
 *
 *
 *     (1..10).filter_map { |i| i * 2 if i.even? } #=> [4, 8, 12, 16, 20]
 *
 */
static VALUE
enum_filter_map(VALUE obj)
{
    VALUE ary;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    ary = rb_ary_new();
    rb_block_call(obj, id_each, 0, 0, filter_map_i, ary);

    return ary;
}


static VALUE
reject_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    ENUM_WANT_SVALUE();

    if (!RTEST(enum_yield(argc, i))) {
	rb_ary_push(ary, i);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.reject { |obj| block } -> array
 *     enum.reject                 -> an_enumerator
 *
 *  Returns an array for all elements of +enum+ for which the given
 *  +block+ returns <code>false</code>.
 *
 *  If no block is given, an Enumerator is returned instead.
 *
 *     (1..10).reject { |i|  i % 3 == 0 }   #=> [1, 2, 4, 5, 7, 8, 10]
 *
 *     [1, 2, 3, 4, 5].reject { |num| num.even? } #=> [1, 3, 5]
 *
 *  See also Enumerable#find_all.
 */

static VALUE
enum_reject(VALUE obj)
{
    VALUE ary;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    ary = rb_ary_new();
    rb_block_call(obj, id_each, 0, 0, reject_i, ary);

    return ary;
}

static VALUE
collect_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    rb_ary_push(ary, rb_yield_values2(argc, argv));

    return Qnil;
}

static VALUE
collect_all(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    rb_ary_push(ary, rb_enum_values_pack(argc, argv));

    return Qnil;
}

/*
 *  call-seq:
 *     enum.collect { |obj| block } -> array
 *     enum.map     { |obj| block } -> array
 *     enum.collect                 -> an_enumerator
 *     enum.map                     -> an_enumerator
 *
 *  Returns a new array with the results of running <em>block</em> once
 *  for every element in <i>enum</i>.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     (1..4).map { |i| i*i }      #=> [1, 4, 9, 16]
 *     (1..4).collect { "cat"  }   #=> ["cat", "cat", "cat", "cat"]
 *
 */

static VALUE
enum_collect(VALUE obj)
{
    VALUE ary;
    int min_argc, max_argc;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    ary = rb_ary_new();
    min_argc = rb_block_min_max_arity(&max_argc);
    rb_lambda_call(obj, id_each, 0, 0, collect_i, min_argc, max_argc, ary);

    return ary;
}

static VALUE
flat_map_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    VALUE tmp;

    i = rb_yield_values2(argc, argv);
    tmp = rb_check_array_type(i);

    if (NIL_P(tmp)) {
	rb_ary_push(ary, i);
    }
    else {
	rb_ary_concat(ary, tmp);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.flat_map       { |obj| block } -> array
 *     enum.collect_concat { |obj| block } -> array
 *     enum.flat_map                       -> an_enumerator
 *     enum.collect_concat                 -> an_enumerator
 *
 *  Returns a new array with the concatenated results of running
 *  <em>block</em> once for every element in <i>enum</i>.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     [1, 2, 3, 4].flat_map { |e| [e, -e] } #=> [1, -1, 2, -2, 3, -3, 4, -4]
 *     [[1, 2], [3, 4]].flat_map { |e| e + [100] } #=> [1, 2, 100, 3, 4, 100]
 *
 */

static VALUE
enum_flat_map(VALUE obj)
{
    VALUE ary;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    ary = rb_ary_new();
    rb_block_call(obj, id_each, 0, 0, flat_map_i, ary);

    return ary;
}

/*
 *  call-seq:
 *     enum.to_a(*args)      -> array
 *     enum.entries(*args)   -> array
 *
 *  Returns an array containing the items in <i>enum</i>.
 *
 *     (1..7).to_a                       #=> [1, 2, 3, 4, 5, 6, 7]
 *     { 'a'=>1, 'b'=>2, 'c'=>3 }.to_a   #=> [["a", 1], ["b", 2], ["c", 3]]
 *
 *     require 'prime'
 *     Prime.entries 10                  #=> [2, 3, 5, 7]
 */
static VALUE
enum_to_a(int argc, VALUE *argv, VALUE obj)
{
    VALUE ary = rb_ary_new();

    rb_block_call(obj, id_each, argc, argv, collect_all, ary);

    return ary;
}

static VALUE
enum_hashify(VALUE obj, int argc, const VALUE *argv, rb_block_call_func *iter)
{
    VALUE hash = rb_hash_new();
    rb_block_call(obj, id_each, argc, argv, iter, hash);
    return hash;
}

static VALUE
enum_to_h_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
    ENUM_WANT_SVALUE();
    return rb_hash_set_pair(hash, i);
}

static VALUE
enum_to_h_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
    return rb_hash_set_pair(hash, rb_yield_values2(argc, argv));
}

/*
 *  call-seq:
 *     enum.to_h(*args)        -> hash
 *     enum.to_h(*args) {...}  -> hash
 *
 *  Returns the result of interpreting <i>enum</i> as a list of
 *  <tt>[key, value]</tt> pairs.
 *
 *     %i[hello world].each_with_index.to_h
 *       # => {:hello => 0, :world => 1}
 *
 *  If a block is given, the results of the block on each element of
 *  the enum will be used as pairs.
 *
 *     (1..5).to_h {|x| [x, x ** 2]}
 *       #=> {1=>1, 2=>4, 3=>9, 4=>16, 5=>25}
 */

static VALUE
enum_to_h(int argc, VALUE *argv, VALUE obj)
{
    rb_block_call_func *iter = rb_block_given_p() ? enum_to_h_ii : enum_to_h_i;
    return enum_hashify(obj, argc, argv, iter);
}

static VALUE
inject_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, p))
{
    struct MEMO *memo = MEMO_CAST(p);

    ENUM_WANT_SVALUE();

    if (memo->v1 == Qundef) {
	MEMO_V1_SET(memo, i);
    }
    else {
	MEMO_V1_SET(memo, rb_yield_values(2, memo->v1, i));
    }
    return Qnil;
}

static VALUE
inject_op_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, p))
{
    struct MEMO *memo = MEMO_CAST(p);
    VALUE name;

    ENUM_WANT_SVALUE();

    if (memo->v1 == Qundef) {
	MEMO_V1_SET(memo, i);
    }
    else if (SYMBOL_P(name = memo->u3.value)) {
	const ID mid = SYM2ID(name);
	MEMO_V1_SET(memo, rb_funcallv_public(memo->v1, mid, 1, &i));
    }
    else {
	VALUE args[2];
	args[0] = name;
	args[1] = i;
	MEMO_V1_SET(memo, rb_f_send(numberof(args), args, memo->v1));
    }
    return Qnil;
}

static VALUE
ary_inject_op(VALUE ary, VALUE init, VALUE op)
{
    ID id;
    VALUE v, e;
    long i, n;

    if (RARRAY_LEN(ary) == 0)
        return init == Qundef ? Qnil : init;

    if (init == Qundef) {
        v = RARRAY_AREF(ary, 0);
        i = 1;
        if (RARRAY_LEN(ary) == 1)
            return v;
    }
    else {
        v = init;
        i = 0;
    }

    id = SYM2ID(op);
    if (id == idPLUS) {
	if (RB_INTEGER_TYPE_P(v) &&
	    rb_method_basic_definition_p(rb_cInteger, idPLUS) &&
	    rb_obj_respond_to(v, idPLUS, FALSE)) {
            n = 0;
            for (; i < RARRAY_LEN(ary); i++) {
                e = RARRAY_AREF(ary, i);
                if (FIXNUM_P(e)) {
                    n += FIX2LONG(e); /* should not overflow long type */
                    if (!FIXABLE(n)) {
                        v = rb_big_plus(ULONG2NUM(n), v);
                        n = 0;
                    }
                }
                else if (RB_TYPE_P(e, T_BIGNUM))
                    v = rb_big_plus(e, v);
                else
                    goto not_integer;
            }
            if (n != 0)
                v = rb_fix_plus(LONG2FIX(n), v);
            return v;

          not_integer:
            if (n != 0)
                v = rb_fix_plus(LONG2FIX(n), v);
        }
    }
    for (; i < RARRAY_LEN(ary); i++) {
        VALUE arg = RARRAY_AREF(ary, i);
        v = rb_funcallv_public(v, id, 1, &arg);
    }
    return v;
}

/*
 *  call-seq:
 *     enum.inject(initial, sym) -> obj
 *     enum.inject(sym)          -> obj
 *     enum.inject(initial) { |memo, obj| block }  -> obj
 *     enum.inject          { |memo, obj| block }  -> obj
 *     enum.reduce(initial, sym) -> obj
 *     enum.reduce(sym)          -> obj
 *     enum.reduce(initial) { |memo, obj| block }  -> obj
 *     enum.reduce          { |memo, obj| block }  -> obj
 *
 *  Combines all elements of <i>enum</i> by applying a binary
 *  operation, specified by a block or a symbol that names a
 *  method or operator.
 *
 *  The <i>inject</i> and <i>reduce</i> methods are aliases. There
 *  is no performance benefit to either.
 *
 *  If you specify a block, then for each element in <i>enum</i>
 *  the block is passed an accumulator value (<i>memo</i>) and the element.
 *  If you specify a symbol instead, then each element in the collection
 *  will be passed to the named method of <i>memo</i>.
 *  In either case, the result becomes the new value for <i>memo</i>.
 *  At the end of the iteration, the final value of <i>memo</i> is the
 *  return value for the method.
 *
 *  If you do not explicitly specify an <i>initial</i> value for <i>memo</i>,
 *  then the first element of collection is used as the initial value
 *  of <i>memo</i>.
 *
 *
 *     # Sum some numbers
 *     (5..10).reduce(:+)                             #=> 45
 *     # Same using a block and inject
 *     (5..10).inject { |sum, n| sum + n }            #=> 45
 *     # Multiply some numbers
 *     (5..10).reduce(1, :*)                          #=> 151200
 *     # Same using a block
 *     (5..10).inject(1) { |product, n| product * n } #=> 151200
 *     # find the longest word
 *     longest = %w{ cat sheep bear }.inject do |memo, word|
 *        memo.length > word.length ? memo : word
 *     end
 *     longest                                        #=> "sheep"
 *
 */
static VALUE
enum_inject(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;
    VALUE init, op;
    rb_block_call_func *iter = inject_i;
    ID id;

    switch (rb_scan_args(argc, argv, "02", &init, &op)) {
      case 0:
	init = Qundef;
	break;
      case 1:
	if (rb_block_given_p()) {
	    break;
	}
	id = rb_check_id(&init);
	op = id ? ID2SYM(id) : init;
	init = Qundef;
	iter = inject_op_i;
	break;
      case 2:
	if (rb_block_given_p()) {
	    rb_warning("given block not used");
	}
	id = rb_check_id(&op);
	if (id) op = ID2SYM(id);
	iter = inject_op_i;
	break;
    }

    if (iter == inject_op_i &&
        SYMBOL_P(op) &&
        RB_TYPE_P(obj, T_ARRAY) &&
        rb_method_basic_definition_p(CLASS_OF(obj), id_each)) {
        return ary_inject_op(obj, init, op);
    }

    memo = MEMO_NEW(init, Qnil, op);
    rb_block_call(obj, id_each, 0, 0, iter, (VALUE)memo);
    if (memo->v1 == Qundef) return Qnil;
    return memo->v1;
}

static VALUE
partition_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, arys))
{
    struct MEMO *memo = MEMO_CAST(arys);
    VALUE ary;
    ENUM_WANT_SVALUE();

    if (RTEST(enum_yield(argc, i))) {
	ary = memo->v1;
    }
    else {
	ary = memo->v2;
    }
    rb_ary_push(ary, i);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.partition { |obj| block } -> [ true_array, false_array ]
 *     enum.partition                 -> an_enumerator
 *
 *  Returns two arrays, the first containing the elements of
 *  <i>enum</i> for which the block evaluates to true, the second
 *  containing the rest.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     (1..6).partition { |v| v.even? }  #=> [[2, 4, 6], [1, 3, 5]]
 *
 */

static VALUE
enum_partition(VALUE obj)
{
    struct MEMO *memo;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    memo = MEMO_NEW(rb_ary_new(), rb_ary_new(), 0);
    rb_block_call(obj, id_each, 0, 0, partition_i, (VALUE)memo);

    return rb_assoc_new(memo->v1, memo->v2);
}

static VALUE
group_by_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
    VALUE group;
    VALUE values;

    ENUM_WANT_SVALUE();

    group = enum_yield(argc, i);
    values = rb_hash_aref(hash, group);
    if (!RB_TYPE_P(values, T_ARRAY)) {
	values = rb_ary_new3(1, i);
	rb_hash_aset(hash, group, values);
    }
    else {
	rb_ary_push(values, i);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.group_by { |obj| block } -> a_hash
 *     enum.group_by                 -> an_enumerator
 *
 *  Groups the collection by result of the block.  Returns a hash where the
 *  keys are the evaluated result from the block and the values are
 *  arrays of elements in the collection that correspond to the key.
 *
 *  If no block is given an enumerator is returned.
 *
 *     (1..6).group_by { |i| i%3 }   #=> {0=>[3, 6], 1=>[1, 4], 2=>[2, 5]}
 *
 */

static VALUE
enum_group_by(VALUE obj)
{
    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    return enum_hashify(obj, 0, 0, group_by_i);
}

static void
tally_up(VALUE hash, VALUE group)
{
    VALUE tally = rb_hash_aref(hash, group);
    if (NIL_P(tally)) {
        tally = INT2FIX(1);
    }
    else if (FIXNUM_P(tally) && tally < INT2FIX(FIXNUM_MAX)) {
        tally += INT2FIX(1) & ~FIXNUM_FLAG;
    }
    else {
        tally = rb_big_plus(tally, INT2FIX(1));
    }
    rb_hash_aset(hash, group, tally);
}

static VALUE
tally_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
    ENUM_WANT_SVALUE();
    tally_up(hash, i);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.tally -> a_hash
 *
 *  Tallies the collection, i.e., counts the occurrences of each element.
 *  Returns a hash with the elements of the collection as keys and the
 *  corresponding counts as values.
 *
 *     ["a", "b", "c", "b"].tally  #=> {"a"=>1, "b"=>2, "c"=>1}
 */

static VALUE
enum_tally(VALUE obj)
{
    return enum_hashify(obj, 0, 0, tally_i);
}

NORETURN(static VALUE first_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, params)));
static VALUE
first_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, params))
{
    struct MEMO *memo = MEMO_CAST(params);
    ENUM_WANT_SVALUE();

    MEMO_V1_SET(memo, i);
    rb_iter_break();

    UNREACHABLE_RETURN(Qnil);
}

static VALUE enum_take(VALUE obj, VALUE n);

/*
 *  call-seq:
 *     enum.first       ->  obj or nil
 *     enum.first(n)    ->  an_array
 *
 *  Returns the first element, or the first +n+ elements, of the enumerable.
 *  If the enumerable is empty, the first form returns <code>nil</code>, and the
 *  second form returns an empty array.
 *
 *    %w[foo bar baz].first     #=> "foo"
 *    %w[foo bar baz].first(2)  #=> ["foo", "bar"]
 *    %w[foo bar baz].first(10) #=> ["foo", "bar", "baz"]
 *    [].first                  #=> nil
 *    [].first(10)              #=> []
 *
 */

static VALUE
enum_first(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;
    rb_check_arity(argc, 0, 1);
    if (argc > 0) {
	return enum_take(obj, argv[0]);
    }
    else {
	memo = MEMO_NEW(Qnil, 0, 0);
	rb_block_call(obj, id_each, 0, 0, first_i, (VALUE)memo);
	return memo->v1;
    }
}


/*
 *  call-seq:
 *     enum.sort                  -> array
 *     enum.sort { |a, b| block } -> array
 *
 *  Returns an array containing the items in <i>enum</i> sorted.
 *
 *  Comparisons for the sort will be done using the items' own
 *  <code><=></code> operator or using an optional code block.
 *
 *  The block must implement a comparison between +a+ and +b+ and return
 *  an integer less than 0 when +b+ follows +a+, +0+ when +a+ and +b+
 *  are equivalent, or an integer greater than 0 when +a+ follows +b+.
 *
 *  The result is not guaranteed to be stable.  When the comparison of two
 *  elements returns +0+, the order of the elements is unpredictable.
 *
 *     %w(rhea kea flea).sort           #=> ["flea", "kea", "rhea"]
 *     (1..10).sort { |a, b| b <=> a }  #=> [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
 *
 *  See also Enumerable#sort_by. It implements a Schwartzian transform
 *  which is useful when key computation or comparison is expensive.
 */

static VALUE
enum_sort(VALUE obj)
{
    return rb_ary_sort_bang(enum_to_a(0, 0, obj));
}

#define SORT_BY_BUFSIZE 16
struct sort_by_data {
    const VALUE ary;
    const VALUE buf;
    long n;
};

static VALUE
sort_by_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, _data))
{
    struct sort_by_data *data = (struct sort_by_data *)&MEMO_CAST(_data)->v1;
    VALUE ary = data->ary;
    VALUE v;

    ENUM_WANT_SVALUE();

    v = enum_yield(argc, i);

    if (RBASIC(ary)->klass) {
	rb_raise(rb_eRuntimeError, "sort_by reentered");
    }
    if (RARRAY_LEN(data->buf) != SORT_BY_BUFSIZE*2) {
	rb_raise(rb_eRuntimeError, "sort_by reentered");
    }

    RARRAY_ASET(data->buf, data->n*2, v);
    RARRAY_ASET(data->buf, data->n*2+1, i);
    data->n++;
    if (data->n == SORT_BY_BUFSIZE) {
	rb_ary_concat(ary, data->buf);
	data->n = 0;
    }
    return Qnil;
}

static int
sort_by_cmp(const void *ap, const void *bp, void *data)
{
    struct cmp_opt_data cmp_opt = { 0, 0 };
    VALUE a;
    VALUE b;
    VALUE ary = (VALUE)data;

    if (RBASIC(ary)->klass) {
	rb_raise(rb_eRuntimeError, "sort_by reentered");
    }

    a = *(VALUE *)ap;
    b = *(VALUE *)bp;

    return OPTIMIZED_CMP(a, b, cmp_opt);
}

/*
 *  call-seq:
 *     enum.sort_by { |obj| block }   -> array
 *     enum.sort_by                   -> an_enumerator
 *
 *  Sorts <i>enum</i> using a set of keys generated by mapping the
 *  values in <i>enum</i> through the given block.
 *
 *  The result is not guaranteed to be stable.  When two keys are equal,
 *  the order of the corresponding elements is unpredictable.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     %w{apple pear fig}.sort_by { |word| word.length }
 *                   #=> ["fig", "pear", "apple"]
 *
 *  The current implementation of #sort_by generates an array of
 *  tuples containing the original collection element and the mapped
 *  value. This makes #sort_by fairly expensive when the keysets are
 *  simple.
 *
 *     require 'benchmark'
 *
 *     a = (1..100000).map { rand(100000) }
 *
 *     Benchmark.bm(10) do |b|
 *       b.report("Sort")    { a.sort }
 *       b.report("Sort by") { a.sort_by { |a| a } }
 *     end
 *
 *  <em>produces:</em>
 *
 *     user     system      total        real
 *     Sort        0.180000   0.000000   0.180000 (  0.175469)
 *     Sort by     1.980000   0.040000   2.020000 (  2.013586)
 *
 *  However, consider the case where comparing the keys is a non-trivial
 *  operation. The following code sorts some files on modification time
 *  using the basic #sort method.
 *
 *     files = Dir["*"]
 *     sorted = files.sort { |a, b| File.new(a).mtime <=> File.new(b).mtime }
 *     sorted   #=> ["mon", "tues", "wed", "thurs"]
 *
 *  This sort is inefficient: it generates two new File
 *  objects during every comparison. A slightly better technique is to
 *  use the Kernel#test method to generate the modification
 *  times directly.
 *
 *     files = Dir["*"]
 *     sorted = files.sort { |a, b|
 *       test(?M, a) <=> test(?M, b)
 *     }
 *     sorted   #=> ["mon", "tues", "wed", "thurs"]
 *
 *  This still generates many unnecessary Time objects. A more
 *  efficient technique is to cache the sort keys (modification times
 *  in this case) before the sort. Perl users often call this approach
 *  a Schwartzian transform, after Randal Schwartz. We construct a
 *  temporary array, where each element is an array containing our
 *  sort key along with the filename. We sort this array, and then
 *  extract the filename from the result.
 *
 *     sorted = Dir["*"].collect { |f|
 *        [test(?M, f), f]
 *     }.sort.collect { |f| f[1] }
 *     sorted   #=> ["mon", "tues", "wed", "thurs"]
 *
 *  This is exactly what #sort_by does internally.
 *
 *     sorted = Dir["*"].sort_by { |f| test(?M, f) }
 *     sorted   #=> ["mon", "tues", "wed", "thurs"]
 *
 *  To produce the reverse of a specific order, the following can be used:
 *
 *    ary.sort_by { ... }.reverse!
 */

static VALUE
enum_sort_by(VALUE obj)
{
    VALUE ary, buf;
    struct MEMO *memo;
    long i;
    struct sort_by_data *data;

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    if (RB_TYPE_P(obj, T_ARRAY) && RARRAY_LEN(obj) <= LONG_MAX/2) {
	ary = rb_ary_new2(RARRAY_LEN(obj)*2);
    }
    else {
	ary = rb_ary_new();
    }
    RBASIC_CLEAR_CLASS(ary);
    buf = rb_ary_tmp_new(SORT_BY_BUFSIZE*2);
    rb_ary_store(buf, SORT_BY_BUFSIZE*2-1, Qnil);
    memo = MEMO_NEW(0, 0, 0);
    data = (struct sort_by_data *)&memo->v1;
    RB_OBJ_WRITE(memo, &data->ary, ary);
    RB_OBJ_WRITE(memo, &data->buf, buf);
    data->n = 0;
    rb_block_call(obj, id_each, 0, 0, sort_by_i, (VALUE)memo);
    ary = data->ary;
    buf = data->buf;
    if (data->n) {
	rb_ary_resize(buf, data->n*2);
	rb_ary_concat(ary, buf);
    }
    if (RARRAY_LEN(ary) > 2) {
        RARRAY_PTR_USE(ary, ptr,
                       ruby_qsort(ptr, RARRAY_LEN(ary)/2, 2*sizeof(VALUE),
                                  sort_by_cmp, (void *)ary));
    }
    if (RBASIC(ary)->klass) {
	rb_raise(rb_eRuntimeError, "sort_by reentered");
    }
    for (i=1; i<RARRAY_LEN(ary); i+=2) {
	RARRAY_ASET(ary, i/2, RARRAY_AREF(ary, i));
    }
    rb_ary_resize(ary, RARRAY_LEN(ary)/2);
    RBASIC_SET_CLASS_RAW(ary, rb_cArray);

    return ary;
}

#define ENUMFUNC(name) argc ? name##_eqq : rb_block_given_p() ? name##_iter_i : name##_i

#define MEMO_ENUM_NEW(v1) (rb_check_arity(argc, 0, 1), MEMO_NEW((v1), (argc ? *argv : 0), 0))

#define DEFINE_ENUMFUNCS(name) \
static VALUE enum_##name##_func(VALUE result, struct MEMO *memo); \
\
static VALUE \
name##_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memo)) \
{ \
    return enum_##name##_func(rb_enum_values_pack(argc, argv), MEMO_CAST(memo)); \
} \
\
static VALUE \
name##_iter_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memo)) \
{ \
    return enum_##name##_func(rb_yield_values2(argc, argv), MEMO_CAST(memo));	\
} \
\
static VALUE \
name##_eqq(RB_BLOCK_CALL_FUNC_ARGLIST(i, memo)) \
{ \
    ENUM_WANT_SVALUE(); \
    return enum_##name##_func(rb_funcallv(MEMO_CAST(memo)->v2, id_eqq, 1, &i), MEMO_CAST(memo)); \
} \
\
static VALUE \
enum_##name##_func(VALUE result, struct MEMO *memo)

#define WARN_UNUSED_BLOCK(argc) do { \
    if ((argc) > 0 && rb_block_given_p()) { \
        rb_warn("given block not used"); \
    } \
} while (0)

DEFINE_ENUMFUNCS(all)
{
    if (!RTEST(result)) {
	MEMO_V1_SET(memo, Qfalse);
	rb_iter_break();
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.all? [{ |obj| block } ]   -> true or false
 *     enum.all?(pattern)             -> true or false
 *
 *  Passes each element of the collection to the given block. The method
 *  returns <code>true</code> if the block never returns
 *  <code>false</code> or <code>nil</code>. If the block is not given,
 *  Ruby adds an implicit block of <code>{ |obj| obj }</code> which will
 *  cause #all? to return +true+ when none of the collection members are
 *  +false+ or +nil+.
 *
 *  If instead a pattern is supplied, the method returns whether
 *  <code>pattern === element</code> for every collection member.
 *
 *     %w[ant bear cat].all? { |word| word.length >= 3 } #=> true
 *     %w[ant bear cat].all? { |word| word.length >= 4 } #=> false
 *     %w[ant bear cat].all?(/t/)                        #=> false
 *     [1, 2i, 3.14].all?(Numeric)                       #=> true
 *     [nil, true, 99].all?                              #=> false
 *     [].all?                                           #=> true
 *
 */

static VALUE
enum_all(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo = MEMO_ENUM_NEW(Qtrue);
    WARN_UNUSED_BLOCK(argc);
    rb_block_call(obj, id_each, 0, 0, ENUMFUNC(all), (VALUE)memo);
    return memo->v1;
}

DEFINE_ENUMFUNCS(any)
{
    if (RTEST(result)) {
	MEMO_V1_SET(memo, Qtrue);
	rb_iter_break();
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.any? [{ |obj| block }]   -> true or false
 *     enum.any?(pattern)            -> true or false
 *
 *  Passes each element of the collection to the given block. The method
 *  returns <code>true</code> if the block ever returns a value other
 *  than <code>false</code> or <code>nil</code>. If the block is not
 *  given, Ruby adds an implicit block of <code>{ |obj| obj }</code> that
 *  will cause #any? to return +true+ if at least one of the collection
 *  members is not +false+ or +nil+.
 *
 *  If instead a pattern is supplied, the method returns whether
 *  <code>pattern === element</code> for any collection member.
 *
 *     %w[ant bear cat].any? { |word| word.length >= 3 } #=> true
 *     %w[ant bear cat].any? { |word| word.length >= 4 } #=> true
 *     %w[ant bear cat].any?(/d/)                        #=> false
 *     [nil, true, 99].any?(Integer)                     #=> true
 *     [nil, true, 99].any?                              #=> true
 *     [].any?                                           #=> false
 *
 */

static VALUE
enum_any(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo = MEMO_ENUM_NEW(Qfalse);
    WARN_UNUSED_BLOCK(argc);
    rb_block_call(obj, id_each, 0, 0, ENUMFUNC(any), (VALUE)memo);
    return memo->v1;
}

DEFINE_ENUMFUNCS(one)
{
    if (RTEST(result)) {
	if (memo->v1 == Qundef) {
	    MEMO_V1_SET(memo, Qtrue);
	}
	else if (memo->v1 == Qtrue) {
	    MEMO_V1_SET(memo, Qfalse);
	    rb_iter_break();
	}
    }
    return Qnil;
}

struct nmin_data {
    long n;
    long bufmax;
    long curlen;
    VALUE buf;
    VALUE limit;
    int (*cmpfunc)(const void *, const void *, void *);
    int rev: 1; /* max if 1 */
    int by: 1; /* min_by if 1 */
};

static VALUE
cmpint_reenter_check(struct nmin_data *data, VALUE val)
{
    if (RBASIC(data->buf)->klass) {
        rb_raise(rb_eRuntimeError, "%s%s reentered",
                 data->rev ? "max" : "min",
                 data->by ? "_by" : "");
    }
    return val;
}

static int
nmin_cmp(const void *ap, const void *bp, void *_data)
{
    struct cmp_opt_data cmp_opt = { 0, 0 };
    struct nmin_data *data = (struct nmin_data *)_data;
    VALUE a = *(const VALUE *)ap, b = *(const VALUE *)bp;
#define rb_cmpint(cmp, a, b) rb_cmpint(cmpint_reenter_check(data, (cmp)), a, b)
    return OPTIMIZED_CMP(a, b, cmp_opt);
#undef rb_cmpint
}

static int
nmin_block_cmp(const void *ap, const void *bp, void *_data)
{
    struct nmin_data *data = (struct nmin_data *)_data;
    VALUE a = *(const VALUE *)ap, b = *(const VALUE *)bp;
    VALUE cmp = rb_yield_values(2, a, b);
    cmpint_reenter_check(data, cmp);
    return rb_cmpint(cmp, a, b);
}

static void
nmin_filter(struct nmin_data *data)
{
    long n;
    VALUE *beg;
    int eltsize;
    long numelts;

    long left, right;
    long store_index;

    long i, j;

    if (data->curlen <= data->n)
	return;

    n = data->n;
    beg = RARRAY_PTR(data->buf);
    eltsize = data->by ? 2 : 1;
    numelts = data->curlen;

    left = 0;
    right = numelts-1;

#define GETPTR(i) (beg+(i)*eltsize)

#define SWAP(i, j) do { \
    VALUE tmp[2]; \
    memcpy(tmp, GETPTR(i), sizeof(VALUE)*eltsize); \
    memcpy(GETPTR(i), GETPTR(j), sizeof(VALUE)*eltsize); \
    memcpy(GETPTR(j), tmp, sizeof(VALUE)*eltsize); \
} while (0)

    while (1) {
	long pivot_index = left + (right-left)/2;
	long num_pivots = 1;

	SWAP(pivot_index, right);
	pivot_index = right;

	store_index = left;
	i = left;
	while (i <= right-num_pivots) {
	    int c = data->cmpfunc(GETPTR(i), GETPTR(pivot_index), data);
	    if (data->rev)
		c = -c;
	    if (c == 0) {
	        SWAP(i, right-num_pivots);
		num_pivots++;
		continue;
	    }
	    if (c < 0) {
		SWAP(i, store_index);
		store_index++;
	    }
	    i++;
	}
	j = store_index;
	for (i = right; right-num_pivots < i; i--) {
	    if (i <= j)
	        break;
	    SWAP(j, i);
	    j++;
	}

	if (store_index <= n && n <= store_index+num_pivots)
	    break;

	if (n < store_index) {
	    right = store_index-1;
	}
	else {
	    left = store_index+num_pivots;
	}
    }
#undef GETPTR
#undef SWAP

    data->limit = RARRAY_AREF(data->buf, store_index*eltsize); /* the last pivot */
    data->curlen = data->n;
    rb_ary_resize(data->buf, data->n * eltsize);
}

static VALUE
nmin_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, _data))
{
    struct nmin_data *data = (struct nmin_data *)_data;
    VALUE cmpv;

    ENUM_WANT_SVALUE();

    if (data->by)
	cmpv = enum_yield(argc, i);
    else
	cmpv = i;

    if (data->limit != Qundef) {
        int c = data->cmpfunc(&cmpv, &data->limit, data);
        if (data->rev)
            c = -c;
        if (c >= 0)
            return Qnil;
    }

    if (data->by)
	rb_ary_push(data->buf, cmpv);
    rb_ary_push(data->buf, i);

    data->curlen++;

    if (data->curlen == data->bufmax) {
	nmin_filter(data);
    }

    return Qnil;
}

VALUE
rb_nmin_run(VALUE obj, VALUE num, int by, int rev, int ary)
{
    VALUE result;
    struct nmin_data data;

    data.n = NUM2LONG(num);
    if (data.n < 0)
        rb_raise(rb_eArgError, "negative size (%ld)", data.n);
    if (data.n == 0)
        return rb_ary_new2(0);
    if (LONG_MAX/4/(by ? 2 : 1) < data.n)
        rb_raise(rb_eArgError, "too big size");
    data.bufmax = data.n * 4;
    data.curlen = 0;
    data.buf = rb_ary_tmp_new(data.bufmax * (by ? 2 : 1));
    data.limit = Qundef;
    data.cmpfunc = by ? nmin_cmp :
                   rb_block_given_p() ? nmin_block_cmp :
		   nmin_cmp;
    data.rev = rev;
    data.by = by;
    if (ary) {
	long i;
	for (i = 0; i < RARRAY_LEN(obj); i++) {
	    VALUE args[1];
	    args[0] = RARRAY_AREF(obj, i);
            nmin_i(obj, (VALUE)&data, 1, args, Qundef);
	}
    }
    else {
	rb_block_call(obj, id_each, 0, 0, nmin_i, (VALUE)&data);
    }
    nmin_filter(&data);
    result = data.buf;
    if (by) {
	long i;
        RARRAY_PTR_USE(result, ptr, {
            ruby_qsort(ptr,
                       RARRAY_LEN(result)/2,
                       sizeof(VALUE)*2,
                       data.cmpfunc, (void *)&data);
            for (i=1; i<RARRAY_LEN(result); i+=2) {
                ptr[i/2] = ptr[i];
            }
        });
	rb_ary_resize(result, RARRAY_LEN(result)/2);
    }
    else {
        RARRAY_PTR_USE(result, ptr, {
            ruby_qsort(ptr, RARRAY_LEN(result), sizeof(VALUE),
                       data.cmpfunc, (void *)&data);
        });
    }
    if (rev) {
        rb_ary_reverse(result);
    }
    RBASIC_SET_CLASS(result, rb_cArray);
    return result;

}

/*
 *  call-seq:
 *     enum.one? [{ |obj| block }]   -> true or false
 *     enum.one?(pattern)            -> true or false
 *
 *  Passes each element of the collection to the given block. The method
 *  returns <code>true</code> if the block returns <code>true</code>
 *  exactly once. If the block is not given, <code>one?</code> will return
 *  <code>true</code> only if exactly one of the collection members is
 *  true.
 *
 *  If instead a pattern is supplied, the method returns whether
 *  <code>pattern === element</code> for exactly one collection member.
 *
 *     %w{ant bear cat}.one? { |word| word.length == 4 }  #=> true
 *     %w{ant bear cat}.one? { |word| word.length > 4 }   #=> false
 *     %w{ant bear cat}.one? { |word| word.length < 4 }   #=> false
 *     %w{ant bear cat}.one?(/t/)                         #=> false
 *     [ nil, true, 99 ].one?                             #=> false
 *     [ nil, true, false ].one?                          #=> true
 *     [ nil, true, 99 ].one?(Integer)                    #=> true
 *     [].one?                                            #=> false
 *
 */
static VALUE
enum_one(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo = MEMO_ENUM_NEW(Qundef);
    VALUE result;

    WARN_UNUSED_BLOCK(argc);
    rb_block_call(obj, id_each, 0, 0, ENUMFUNC(one), (VALUE)memo);
    result = memo->v1;
    if (result == Qundef) return Qfalse;
    return result;
}

DEFINE_ENUMFUNCS(none)
{
    if (RTEST(result)) {
	MEMO_V1_SET(memo, Qfalse);
	rb_iter_break();
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.none? [{ |obj| block }]   -> true or false
 *     enum.none?(pattern)            -> true or false
 *
 *  Passes each element of the collection to the given block. The method
 *  returns <code>true</code> if the block never returns <code>true</code>
 *  for all elements. If the block is not given, <code>none?</code> will return
 *  <code>true</code> only if none of the collection members is true.
 *
 *  If instead a pattern is supplied, the method returns whether
 *  <code>pattern === element</code> for none of the collection members.
 *
 *     %w{ant bear cat}.none? { |word| word.length == 5 } #=> true
 *     %w{ant bear cat}.none? { |word| word.length >= 4 } #=> false
 *     %w{ant bear cat}.none?(/d/)                        #=> true
 *     [1, 3.14, 42].none?(Float)                         #=> false
 *     [].none?                                           #=> true
 *     [nil].none?                                        #=> true
 *     [nil, false].none?                                 #=> true
 *     [nil, false, true].none?                           #=> false
 */
static VALUE
enum_none(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo = MEMO_ENUM_NEW(Qtrue);

    WARN_UNUSED_BLOCK(argc);
    rb_block_call(obj, id_each, 0, 0, ENUMFUNC(none), (VALUE)memo);
    return memo->v1;
}

struct min_t {
    VALUE min;
    struct cmp_opt_data cmp_opt;
};

static VALUE
min_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct min_t *memo = MEMO_FOR(struct min_t, args);

    ENUM_WANT_SVALUE();

    if (memo->min == Qundef) {
	memo->min = i;
    }
    else {
	if (OPTIMIZED_CMP(i, memo->min, memo->cmp_opt) < 0) {
	    memo->min = i;
	}
    }
    return Qnil;
}

static VALUE
min_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    VALUE cmp;
    struct min_t *memo = MEMO_FOR(struct min_t, args);

    ENUM_WANT_SVALUE();

    if (memo->min == Qundef) {
	memo->min = i;
    }
    else {
	cmp = rb_yield_values(2, i, memo->min);
	if (rb_cmpint(cmp, i, memo->min) < 0) {
	    memo->min = i;
	}
    }
    return Qnil;
}


/*
 *  call-seq:
 *     enum.min                     -> obj
 *     enum.min { |a, b| block }    -> obj
 *     enum.min(n)                  -> array
 *     enum.min(n) { |a, b| block } -> array
 *
 *  Returns the object in _enum_ with the minimum value. The
 *  first form assumes all objects implement <code><=></code>;
 *  the second uses the block to return <em>a <=> b</em>.
 *
 *     a = %w(albatross dog horse)
 *     a.min                                   #=> "albatross"
 *     a.min { |a, b| a.length <=> b.length }  #=> "dog"
 *
 *  If the +n+ argument is given, minimum +n+ elements are returned
 *  as a sorted array.
 *
 *     a = %w[albatross dog horse]
 *     a.min(2)                                  #=> ["albatross", "dog"]
 *     a.min(2) {|a, b| a.length <=> b.length }  #=> ["dog", "horse"]
 *     [5, 1, 3, 4, 2].min(3)                    #=> [1, 2, 3]
 */

static VALUE
enum_min(int argc, VALUE *argv, VALUE obj)
{
    VALUE memo;
    struct min_t *m = NEW_CMP_OPT_MEMO(struct min_t, memo);
    VALUE result;
    VALUE num;

    if (rb_check_arity(argc, 0, 1) && !NIL_P(num = argv[0]))
       return rb_nmin_run(obj, num, 0, 0, 0);

    m->min = Qundef;
    m->cmp_opt.opt_methods = 0;
    m->cmp_opt.opt_inited = 0;
    if (rb_block_given_p()) {
	rb_block_call(obj, id_each, 0, 0, min_ii, memo);
    }
    else {
	rb_block_call(obj, id_each, 0, 0, min_i, memo);
    }
    result = m->min;
    if (result == Qundef) return Qnil;
    return result;
}

struct max_t {
    VALUE max;
    struct cmp_opt_data cmp_opt;
};

static VALUE
max_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct max_t *memo = MEMO_FOR(struct max_t, args);

    ENUM_WANT_SVALUE();

    if (memo->max == Qundef) {
	memo->max = i;
    }
    else {
	if (OPTIMIZED_CMP(i, memo->max, memo->cmp_opt) > 0) {
	    memo->max = i;
	}
    }
    return Qnil;
}

static VALUE
max_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct max_t *memo = MEMO_FOR(struct max_t, args);
    VALUE cmp;

    ENUM_WANT_SVALUE();

    if (memo->max == Qundef) {
	memo->max = i;
    }
    else {
	cmp = rb_yield_values(2, i, memo->max);
	if (rb_cmpint(cmp, i, memo->max) > 0) {
	    memo->max = i;
	}
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.max                     -> obj
 *     enum.max { |a, b| block }    -> obj
 *     enum.max(n)                  -> array
 *     enum.max(n) { |a, b| block } -> array
 *
 *  Returns the object in _enum_ with the maximum value. The
 *  first form assumes all objects implement <code><=></code>;
 *  the second uses the block to return <em>a <=> b</em>.
 *
 *     a = %w(albatross dog horse)
 *     a.max                                   #=> "horse"
 *     a.max { |a, b| a.length <=> b.length }  #=> "albatross"
 *
 *  If the +n+ argument is given, maximum +n+ elements are returned
 *  as an array, sorted in descending order.
 *
 *     a = %w[albatross dog horse]
 *     a.max(2)                                  #=> ["horse", "dog"]
 *     a.max(2) {|a, b| a.length <=> b.length }  #=> ["albatross", "horse"]
 *     [5, 1, 3, 4, 2].max(3)                    #=> [5, 4, 3]
 */

static VALUE
enum_max(int argc, VALUE *argv, VALUE obj)
{
    VALUE memo;
    struct max_t *m = NEW_CMP_OPT_MEMO(struct max_t, memo);
    VALUE result;
    VALUE num;

    if (rb_check_arity(argc, 0, 1) && !NIL_P(num = argv[0]))
       return rb_nmin_run(obj, num, 0, 1, 0);

    m->max = Qundef;
    m->cmp_opt.opt_methods = 0;
    m->cmp_opt.opt_inited = 0;
    if (rb_block_given_p()) {
	rb_block_call(obj, id_each, 0, 0, max_ii, (VALUE)memo);
    }
    else {
	rb_block_call(obj, id_each, 0, 0, max_i, (VALUE)memo);
    }
    result = m->max;
    if (result == Qundef) return Qnil;
    return result;
}

struct minmax_t {
    VALUE min;
    VALUE max;
    VALUE last;
    struct cmp_opt_data cmp_opt;
};

static void
minmax_i_update(VALUE i, VALUE j, struct minmax_t *memo)
{
    int n;

    if (memo->min == Qundef) {
	memo->min = i;
	memo->max = j;
    }
    else {
	n = OPTIMIZED_CMP(i, memo->min, memo->cmp_opt);
	if (n < 0) {
	    memo->min = i;
	}
	n = OPTIMIZED_CMP(j, memo->max, memo->cmp_opt);
	if (n > 0) {
	    memo->max = j;
	}
    }
}

static VALUE
minmax_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, _memo))
{
    struct minmax_t *memo = MEMO_FOR(struct minmax_t, _memo);
    int n;
    VALUE j;

    ENUM_WANT_SVALUE();

    if (memo->last == Qundef) {
        memo->last = i;
        return Qnil;
    }
    j = memo->last;
    memo->last = Qundef;

    n = OPTIMIZED_CMP(j, i, memo->cmp_opt);
    if (n == 0)
        i = j;
    else if (n < 0) {
        VALUE tmp;
        tmp = i;
        i = j;
        j = tmp;
    }

    minmax_i_update(i, j, memo);

    return Qnil;
}

static void
minmax_ii_update(VALUE i, VALUE j, struct minmax_t *memo)
{
    int n;

    if (memo->min == Qundef) {
	memo->min = i;
	memo->max = j;
    }
    else {
	n = rb_cmpint(rb_yield_values(2, i, memo->min), i, memo->min);
	if (n < 0) {
	    memo->min = i;
	}
	n = rb_cmpint(rb_yield_values(2, j, memo->max), j, memo->max);
	if (n > 0) {
	    memo->max = j;
	}
    }
}

static VALUE
minmax_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, _memo))
{
    struct minmax_t *memo = MEMO_FOR(struct minmax_t, _memo);
    int n;
    VALUE j;

    ENUM_WANT_SVALUE();

    if (memo->last == Qundef) {
        memo->last = i;
        return Qnil;
    }
    j = memo->last;
    memo->last = Qundef;

    n = rb_cmpint(rb_yield_values(2, j, i), j, i);
    if (n == 0)
        i = j;
    else if (n < 0) {
        VALUE tmp;
        tmp = i;
        i = j;
        j = tmp;
    }

    minmax_ii_update(i, j, memo);

    return Qnil;
}

/*
 *  call-seq:
 *     enum.minmax                  -> [min, max]
 *     enum.minmax { |a, b| block } -> [min, max]
 *
 *  Returns a two element array which contains the minimum and the
 *  maximum value in the enumerable.  The first form assumes all
 *  objects implement <code><=></code>; the second uses the
 *  block to return <em>a <=> b</em>.
 *
 *     a = %w(albatross dog horse)
 *     a.minmax                                  #=> ["albatross", "horse"]
 *     a.minmax { |a, b| a.length <=> b.length } #=> ["dog", "albatross"]
 */

static VALUE
enum_minmax(VALUE obj)
{
    VALUE memo;
    struct minmax_t *m = NEW_CMP_OPT_MEMO(struct minmax_t, memo);

    m->min = Qundef;
    m->last = Qundef;
    m->cmp_opt.opt_methods = 0;
    m->cmp_opt.opt_inited = 0;
    if (rb_block_given_p()) {
	rb_block_call(obj, id_each, 0, 0, minmax_ii, memo);
	if (m->last != Qundef)
	    minmax_ii_update(m->last, m->last, m);
    }
    else {
	rb_block_call(obj, id_each, 0, 0, minmax_i, memo);
	if (m->last != Qundef)
	    minmax_i_update(m->last, m->last, m);
    }
    if (m->min != Qundef) {
	return rb_assoc_new(m->min, m->max);
    }
    return rb_assoc_new(Qnil, Qnil);
}

static VALUE
min_by_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct cmp_opt_data cmp_opt = { 0, 0 };
    struct MEMO *memo = MEMO_CAST(args);
    VALUE v;

    ENUM_WANT_SVALUE();

    v = enum_yield(argc, i);
    if (memo->v1 == Qundef) {
	MEMO_V1_SET(memo, v);
	MEMO_V2_SET(memo, i);
    }
    else if (OPTIMIZED_CMP(v, memo->v1, cmp_opt) < 0) {
	MEMO_V1_SET(memo, v);
	MEMO_V2_SET(memo, i);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.min_by {|obj| block }      -> obj
 *     enum.min_by                     -> an_enumerator
 *     enum.min_by(n) {|obj| block }   -> array
 *     enum.min_by(n)                  -> an_enumerator
 *
 *  Returns the object in <i>enum</i> that gives the minimum
 *  value from the given block.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     a = %w(albatross dog horse)
 *     a.min_by { |x| x.length }   #=> "dog"
 *
 *  If the +n+ argument is given, minimum +n+ elements are returned
 *  as an array. These +n+ elements are sorted by the value from the
 *  given block.
 *
 *     a = %w[albatross dog horse]
 *     p a.min_by(2) {|x| x.length } #=> ["dog", "horse"]
 */

static VALUE
enum_min_by(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;
    VALUE num;

    rb_check_arity(argc, 0, 1);

    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enum_size);

    if (argc && !NIL_P(num = argv[0]))
        return rb_nmin_run(obj, num, 1, 0, 0);

    memo = MEMO_NEW(Qundef, Qnil, 0);
    rb_block_call(obj, id_each, 0, 0, min_by_i, (VALUE)memo);
    return memo->v2;
}

static VALUE
max_by_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct cmp_opt_data cmp_opt = { 0, 0 };
    struct MEMO *memo = MEMO_CAST(args);
    VALUE v;

    ENUM_WANT_SVALUE();

    v = enum_yield(argc, i);
    if (memo->v1 == Qundef) {
	MEMO_V1_SET(memo, v);
	MEMO_V2_SET(memo, i);
    }
    else if (OPTIMIZED_CMP(v, memo->v1, cmp_opt) > 0) {
	MEMO_V1_SET(memo, v);
	MEMO_V2_SET(memo, i);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.max_by {|obj| block }      -> obj
 *     enum.max_by                     -> an_enumerator
 *     enum.max_by(n) {|obj| block }   -> obj
 *     enum.max_by(n)                  -> an_enumerator
 *
 *  Returns the object in <i>enum</i> that gives the maximum
 *  value from the given block.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     a = %w(albatross dog horse)
 *     a.max_by { |x| x.length }   #=> "albatross"
 *
 *  If the +n+ argument is given, maximum +n+ elements are returned
 *  as an array. These +n+ elements are sorted by the value from the
 *  given block, in descending order.
 *
 *     a = %w[albatross dog horse]
 *     a.max_by(2) {|x| x.length } #=> ["albatross", "horse"]
 *
 *  enum.max_by(n) can be used to implement weighted random sampling.
 *  Following example implements and use Enumerable#wsample.
 *
 *     module Enumerable
 *       # weighted random sampling.
 *       #
 *       # Pavlos S. Efraimidis, Paul G. Spirakis
 *       # Weighted random sampling with a reservoir
 *       # Information Processing Letters
 *       # Volume 97, Issue 5 (16 March 2006)
 *       def wsample(n)
 *         self.max_by(n) {|v| rand ** (1.0/yield(v)) }
 *       end
 *     end
 *     e = (-20..20).to_a*10000
 *     a = e.wsample(20000) {|x|
 *       Math.exp(-(x/5.0)**2) # normal distribution
 *     }
 *     # a is 20000 samples from e.
 *     p a.length #=> 20000
 *     h = a.group_by {|x| x }
 *     -10.upto(10) {|x| puts "*" * (h[x].length/30.0).to_i if h[x] }
 *     #=> *
 *     #   ***
 *     #   ******
 *     #   ***********
 *     #   ******************
 *     #   *****************************
 *     #   *****************************************
 *     #   ****************************************************
 *     #   ***************************************************************
 *     #   ********************************************************************
 *     #   ***********************************************************************
 *     #   ***********************************************************************
 *     #   **************************************************************
 *     #   ****************************************************
 *     #   ***************************************
 *     #   ***************************
 *     #   ******************
 *     #   ***********
 *     #   *******
 *     #   ***
 *     #   *
 *
 */

static VALUE
enum_max_by(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;
    VALUE num;

    rb_check_arity(argc, 0, 1);

    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enum_size);

    if (argc && !NIL_P(num = argv[0]))
        return rb_nmin_run(obj, num, 1, 1, 0);

    memo = MEMO_NEW(Qundef, Qnil, 0);
    rb_block_call(obj, id_each, 0, 0, max_by_i, (VALUE)memo);
    return memo->v2;
}

struct minmax_by_t {
    VALUE min_bv;
    VALUE max_bv;
    VALUE min;
    VALUE max;
    VALUE last_bv;
    VALUE last;
};

static void
minmax_by_i_update(VALUE v1, VALUE v2, VALUE i1, VALUE i2, struct minmax_by_t *memo)
{
    struct cmp_opt_data cmp_opt = { 0, 0 };

    if (memo->min_bv == Qundef) {
	memo->min_bv = v1;
	memo->max_bv = v2;
	memo->min = i1;
	memo->max = i2;
    }
    else {
	if (OPTIMIZED_CMP(v1, memo->min_bv, cmp_opt) < 0) {
	    memo->min_bv = v1;
	    memo->min = i1;
	}
	if (OPTIMIZED_CMP(v2, memo->max_bv, cmp_opt) > 0) {
	    memo->max_bv = v2;
	    memo->max = i2;
	}
    }
}

static VALUE
minmax_by_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, _memo))
{
    struct cmp_opt_data cmp_opt = { 0, 0 };
    struct minmax_by_t *memo = MEMO_FOR(struct minmax_by_t, _memo);
    VALUE vi, vj, j;
    int n;

    ENUM_WANT_SVALUE();

    vi = enum_yield(argc, i);

    if (memo->last_bv == Qundef) {
        memo->last_bv = vi;
        memo->last = i;
        return Qnil;
    }
    vj = memo->last_bv;
    j = memo->last;
    memo->last_bv = Qundef;

    n = OPTIMIZED_CMP(vj, vi, cmp_opt);
    if (n == 0) {
        i = j;
        vi = vj;
    }
    else if (n < 0) {
        VALUE tmp;
        tmp = i;
        i = j;
        j = tmp;
        tmp = vi;
        vi = vj;
        vj = tmp;
    }

    minmax_by_i_update(vi, vj, i, j, memo);

    return Qnil;
}

/*
 *  call-seq:
 *     enum.minmax_by { |obj| block } -> [min, max]
 *     enum.minmax_by                 -> an_enumerator
 *
 *  Returns a two element array containing the objects in
 *  <i>enum</i> that correspond to the minimum and maximum values respectively
 *  from the given block.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     a = %w(albatross dog horse)
 *     a.minmax_by { |x| x.length }   #=> ["dog", "albatross"]
 */

static VALUE
enum_minmax_by(VALUE obj)
{
    VALUE memo;
    struct minmax_by_t *m = NEW_MEMO_FOR(struct minmax_by_t, memo);

    RETURN_SIZED_ENUMERATOR(obj, 0, 0, enum_size);

    m->min_bv = Qundef;
    m->max_bv = Qundef;
    m->min = Qnil;
    m->max = Qnil;
    m->last_bv = Qundef;
    m->last = Qundef;
    rb_block_call(obj, id_each, 0, 0, minmax_by_i, memo);
    if (m->last_bv != Qundef)
        minmax_by_i_update(m->last_bv, m->last_bv, m->last, m->last, m);
    m = MEMO_FOR(struct minmax_by_t, memo);
    return rb_assoc_new(m->min, m->max);
}

static VALUE
member_i(RB_BLOCK_CALL_FUNC_ARGLIST(iter, args))
{
    struct MEMO *memo = MEMO_CAST(args);

    if (rb_equal(rb_enum_values_pack(argc, argv), memo->v1)) {
	MEMO_V2_SET(memo, Qtrue);
	rb_iter_break();
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.include?(obj)     -> true or false
 *     enum.member?(obj)      -> true or false
 *
 *  Returns <code>true</code> if any member of <i>enum</i> equals
 *  <i>obj</i>. Equality is tested using <code>==</code>.
 *
 *     (1..10).include? 5  #=> true
 *     (1..10).include? 15 #=> false
 *     (1..10).member? 5   #=> true
 *     (1..10).member? 15  #=> false
 *
 */

static VALUE
enum_member(VALUE obj, VALUE val)
{
    struct MEMO *memo = MEMO_NEW(val, Qfalse, 0);

    rb_block_call(obj, id_each, 0, 0, member_i, (VALUE)memo);
    return memo->v2;
}

static VALUE
each_with_index_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memo))
{
    struct MEMO *m = MEMO_CAST(memo);
    VALUE n = imemo_count_value(m);

    imemo_count_up(m);
    return rb_yield_values(2, rb_enum_values_pack(argc, argv), n);
}

/*
 *  call-seq:
 *     enum.each_with_index(*args) { |obj, i| block } ->  enum
 *     enum.each_with_index(*args)                    ->  an_enumerator
 *
 *  Calls <em>block</em> with two arguments, the item and its index,
 *  for each item in <i>enum</i>.  Given arguments are passed through
 *  to #each().
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     hash = Hash.new
 *     %w(cat dog wombat).each_with_index { |item, index|
 *       hash[item] = index
 *     }
 *     hash   #=> {"cat"=>0, "dog"=>1, "wombat"=>2}
 *
 */

static VALUE
enum_each_with_index(int argc, VALUE *argv, VALUE obj)
{
    struct MEMO *memo;

    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enum_size);

    memo = MEMO_NEW(0, 0, 0);
    rb_block_call(obj, id_each, argc, argv, each_with_index_i, (VALUE)memo);
    return obj;
}


/*
 *  call-seq:
 *     enum.reverse_each(*args) { |item| block } ->  enum
 *     enum.reverse_each(*args)                  ->  an_enumerator
 *
 *  Builds a temporary array and traverses that array in reverse order.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     (1..3).reverse_each { |v| p v }
 *
 *  produces:
 *
 *     3
 *     2
 *     1
 */

static VALUE
enum_reverse_each(int argc, VALUE *argv, VALUE obj)
{
    VALUE ary;
    long len;

    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enum_size);

    ary = enum_to_a(argc, argv, obj);

    len = RARRAY_LEN(ary);
    while (len--) {
        long nlen;
        rb_yield(RARRAY_AREF(ary, len));
        nlen = RARRAY_LEN(ary);
        if (nlen < len) {
            len = nlen;
        }
    }

    return obj;
}


static VALUE
each_val_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, p))
{
    ENUM_WANT_SVALUE();
    enum_yield(argc, i);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.each_entry { |obj| block }  -> enum
 *     enum.each_entry                  -> an_enumerator
 *
 *  Calls <i>block</i> once for each element in +self+, passing that
 *  element as a parameter, converting multiple values from yield to an
 *  array.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     class Foo
 *       include Enumerable
 *       def each
 *         yield 1
 *         yield 1, 2
 *         yield
 *       end
 *     end
 *     Foo.new.each_entry{ |o| p o }
 *
 *  produces:
 *
 *     1
 *     [1, 2]
 *     nil
 *
 */

static VALUE
enum_each_entry(int argc, VALUE *argv, VALUE obj)
{
    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enum_size);
    rb_block_call(obj, id_each, argc, argv, each_val_i, 0);
    return obj;
}

static VALUE
add_int(VALUE x, long n)
{
    const VALUE y = LONG2NUM(n);
    if (RB_INTEGER_TYPE_P(x)) return rb_int_plus(x, y);
    return rb_funcallv(x, '+', 1, &y);
}

static VALUE
div_int(VALUE x, long n)
{
    const VALUE y = LONG2NUM(n);
    if (RB_INTEGER_TYPE_P(x)) return rb_int_idiv(x, y);
    return rb_funcallv(x, id_div, 1, &y);
}

#define dont_recycle_block_arg(arity) ((arity) == 1 || (arity) < 0)

static VALUE
each_slice_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, m))
{
    struct MEMO *memo = MEMO_CAST(m);
    VALUE ary = memo->v1;
    VALUE v = Qnil;
    long size = memo->u3.cnt;
    ENUM_WANT_SVALUE();

    rb_ary_push(ary, i);

    if (RARRAY_LEN(ary) == size) {
	v = rb_yield(ary);

	if (memo->v2) {
	    MEMO_V1_SET(memo, rb_ary_new2(size));
	}
	else {
	    rb_ary_clear(ary);
	}
    }

    return v;
}

static VALUE
enum_each_slice_size(VALUE obj, VALUE args, VALUE eobj)
{
    VALUE n, size;
    long slice_size = NUM2LONG(RARRAY_AREF(args, 0));
    ID infinite_p;
    CONST_ID(infinite_p, "infinite?");
    if (slice_size <= 0) rb_raise(rb_eArgError, "invalid slice size");

    size = enum_size(obj, 0, 0);
    if (size == Qnil) return Qnil;
    if (RB_FLOAT_TYPE_P(size) && RTEST(rb_funcall(size, infinite_p, 0))) {
        return size;
    }

    n = add_int(size, slice_size-1);
    return div_int(n, slice_size);
}

/*
 *  call-seq:
 *    enum.each_slice(n) { ... }  ->  nil
 *    enum.each_slice(n)          ->  an_enumerator
 *
 *  Iterates the given block for each slice of <n> elements.  If no
 *  block is given, returns an enumerator.
 *
 *      (1..10).each_slice(3) { |a| p a }
 *      # outputs below
 *      [1, 2, 3]
 *      [4, 5, 6]
 *      [7, 8, 9]
 *      [10]
 *
 */
static VALUE
enum_each_slice(VALUE obj, VALUE n)
{
    long size = NUM2LONG(n);
    VALUE ary;
    struct MEMO *memo;
    int arity;

    if (size <= 0) rb_raise(rb_eArgError, "invalid slice size");
    RETURN_SIZED_ENUMERATOR(obj, 1, &n, enum_each_slice_size);
    size = limit_by_enum_size(obj, size);
    ary = rb_ary_new2(size);
    arity = rb_block_arity();
    memo = MEMO_NEW(ary, dont_recycle_block_arg(arity), size);
    rb_block_call(obj, id_each, 0, 0, each_slice_i, (VALUE)memo);
    ary = memo->v1;
    if (RARRAY_LEN(ary) > 0) rb_yield(ary);

    return Qnil;
}

static VALUE
each_cons_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct MEMO *memo = MEMO_CAST(args);
    VALUE ary = memo->v1;
    VALUE v = Qnil;
    long size = memo->u3.cnt;
    ENUM_WANT_SVALUE();

    if (RARRAY_LEN(ary) == size) {
	rb_ary_shift(ary);
    }
    rb_ary_push(ary, i);
    if (RARRAY_LEN(ary) == size) {
	if (memo->v2) {
	    ary = rb_ary_dup(ary);
	}
	v = rb_yield(ary);
    }
    return v;
}

static VALUE
enum_each_cons_size(VALUE obj, VALUE args, VALUE eobj)
{
    struct cmp_opt_data cmp_opt = { 0, 0 };
    const VALUE zero = LONG2FIX(0);
    VALUE n, size;
    long cons_size = NUM2LONG(RARRAY_AREF(args, 0));
    if (cons_size <= 0) rb_raise(rb_eArgError, "invalid size");

    size = enum_size(obj, 0, 0);
    if (size == Qnil) return Qnil;

    n = add_int(size, 1 - cons_size);
    return (OPTIMIZED_CMP(n, zero, cmp_opt) == -1) ? zero : n;
}

/*
 *  call-seq:
 *    enum.each_cons(n) { ... } ->  nil
 *    enum.each_cons(n)         ->  an_enumerator
 *
 *  Iterates the given block for each array of consecutive <n>
 *  elements.  If no block is given, returns an enumerator.
 *
 *  e.g.:
 *      (1..10).each_cons(3) { |a| p a }
 *      # outputs below
 *      [1, 2, 3]
 *      [2, 3, 4]
 *      [3, 4, 5]
 *      [4, 5, 6]
 *      [5, 6, 7]
 *      [6, 7, 8]
 *      [7, 8, 9]
 *      [8, 9, 10]
 *
 */
static VALUE
enum_each_cons(VALUE obj, VALUE n)
{
    long size = NUM2LONG(n);
    struct MEMO *memo;
    int arity;

    if (size <= 0) rb_raise(rb_eArgError, "invalid size");
    RETURN_SIZED_ENUMERATOR(obj, 1, &n, enum_each_cons_size);
    arity = rb_block_arity();
    if (enum_size_over_p(obj, size)) return Qnil;
    memo = MEMO_NEW(rb_ary_new2(size), dont_recycle_block_arg(arity), size);
    rb_block_call(obj, id_each, 0, 0, each_cons_i, (VALUE)memo);

    return Qnil;
}

static VALUE
each_with_object_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, memo))
{
    ENUM_WANT_SVALUE();
    return rb_yield_values(2, i, memo);
}

/*
 *  call-seq:
 *    enum.each_with_object(obj) { |(*args), memo_obj| ... }  ->  obj
 *    enum.each_with_object(obj)                              ->  an_enumerator
 *
 *  Iterates the given block for each element with an arbitrary
 *  object given, and returns the initially given object.
 *
 *  If no block is given, returns an enumerator.
 *
 *      evens = (1..10).each_with_object([]) { |i, a| a << i*2 }
 *      #=> [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
 *
 */
static VALUE
enum_each_with_object(VALUE obj, VALUE memo)
{
    RETURN_SIZED_ENUMERATOR(obj, 1, &memo, enum_size);

    rb_block_call(obj, id_each, 0, 0, each_with_object_i, memo);

    return memo;
}

static VALUE
zip_ary(RB_BLOCK_CALL_FUNC_ARGLIST(val, memoval))
{
    struct MEMO *memo = (struct MEMO *)memoval;
    VALUE result = memo->v1;
    VALUE args = memo->v2;
    long n = memo->u3.cnt++;
    VALUE tmp;
    int i;

    tmp = rb_ary_new2(RARRAY_LEN(args) + 1);
    rb_ary_store(tmp, 0, rb_enum_values_pack(argc, argv));
    for (i=0; i<RARRAY_LEN(args); i++) {
	VALUE e = RARRAY_AREF(args, i);

	if (RARRAY_LEN(e) <= n) {
	    rb_ary_push(tmp, Qnil);
	}
	else {
	    rb_ary_push(tmp, RARRAY_AREF(e, n));
	}
    }
    if (NIL_P(result)) {
	enum_yield_array(tmp);
    }
    else {
	rb_ary_push(result, tmp);
    }

    RB_GC_GUARD(args);

    return Qnil;
}

static VALUE
call_next(VALUE w)
{
    VALUE *v = (VALUE *)w;
    return v[0] = rb_funcallv(v[1], id_next, 0, 0);
}

static VALUE
call_stop(VALUE w, VALUE _)
{
    VALUE *v = (VALUE *)w;
    return v[0] = Qundef;
}

static VALUE
zip_i(RB_BLOCK_CALL_FUNC_ARGLIST(val, memoval))
{
    struct MEMO *memo = (struct MEMO *)memoval;
    VALUE result = memo->v1;
    VALUE args = memo->v2;
    VALUE tmp;
    int i;

    tmp = rb_ary_new2(RARRAY_LEN(args) + 1);
    rb_ary_store(tmp, 0, rb_enum_values_pack(argc, argv));
    for (i=0; i<RARRAY_LEN(args); i++) {
	if (NIL_P(RARRAY_AREF(args, i))) {
	    rb_ary_push(tmp, Qnil);
	}
	else {
	    VALUE v[2];

	    v[1] = RARRAY_AREF(args, i);
	    rb_rescue2(call_next, (VALUE)v, call_stop, (VALUE)v, rb_eStopIteration, (VALUE)0);
	    if (v[0] == Qundef) {
		RARRAY_ASET(args, i, Qnil);
		v[0] = Qnil;
	    }
	    rb_ary_push(tmp, v[0]);
	}
    }
    if (NIL_P(result)) {
	enum_yield_array(tmp);
    }
    else {
	rb_ary_push(result, tmp);
    }

    RB_GC_GUARD(args);

    return Qnil;
}

/*
 *  call-seq:
 *     enum.zip(arg, ...)                  -> an_array_of_array
 *     enum.zip(arg, ...) { |arr| block }  -> nil
 *
 *  Takes one element from <i>enum</i> and merges corresponding
 *  elements from each <i>args</i>.  This generates a sequence of
 *  <em>n</em>-element arrays, where <em>n</em> is one more than the
 *  count of arguments.  The length of the resulting sequence will be
 *  <code>enum#size</code>.  If the size of any argument is less than
 *  <code>enum#size</code>, <code>nil</code> values are supplied. If
 *  a block is given, it is invoked for each output array, otherwise
 *  an array of arrays is returned.
 *
 *     a = [ 4, 5, 6 ]
 *     b = [ 7, 8, 9 ]
 *
 *     a.zip(b)                 #=> [[4, 7], [5, 8], [6, 9]]
 *     [1, 2, 3].zip(a, b)      #=> [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
 *     [1, 2].zip(a, b)         #=> [[1, 4, 7], [2, 5, 8]]
 *     a.zip([1, 2], [8])       #=> [[4, 1, 8], [5, 2, nil], [6, nil, nil]]
 *
 *     c = []
 *     a.zip(b) { |x, y| c << x + y }  #=> nil
 *     c                               #=> [11, 13, 15]
 *
 */

static VALUE
enum_zip(int argc, VALUE *argv, VALUE obj)
{
    int i;
    ID conv;
    struct MEMO *memo;
    VALUE result = Qnil;
    VALUE args = rb_ary_new4(argc, argv);
    int allary = TRUE;

    argv = RARRAY_PTR(args);
    for (i=0; i<argc; i++) {
	VALUE ary = rb_check_array_type(argv[i]);
	if (NIL_P(ary)) {
	    allary = FALSE;
	    break;
	}
	argv[i] = ary;
    }
    if (!allary) {
	static const VALUE sym_each = STATIC_ID2SYM(id_each);
	CONST_ID(conv, "to_enum");
	for (i=0; i<argc; i++) {
	    if (!rb_respond_to(argv[i], id_each)) {
		rb_raise(rb_eTypeError, "wrong argument type %"PRIsVALUE" (must respond to :each)",
			 rb_obj_class(argv[i]));
            }
	    argv[i] = rb_funcallv(argv[i], conv, 1, &sym_each);
	}
    }
    if (!rb_block_given_p()) {
	result = rb_ary_new();
    }

    /* TODO: use NODE_DOT2 as memo(v, v, -) */
    memo = MEMO_NEW(result, args, 0);
    rb_block_call(obj, id_each, 0, 0, allary ? zip_ary : zip_i, (VALUE)memo);

    return result;
}

static VALUE
take_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct MEMO *memo = MEMO_CAST(args);
    rb_ary_push(memo->v1, rb_enum_values_pack(argc, argv));
    if (--memo->u3.cnt == 0) rb_iter_break();
    return Qnil;
}

/*
 *  call-seq:
 *     enum.take(n)               -> array
 *
 *  Returns first n elements from <i>enum</i>.
 *
 *     a = [1, 2, 3, 4, 5, 0]
 *     a.take(3)             #=> [1, 2, 3]
 *     a.take(30)            #=> [1, 2, 3, 4, 5, 0]
 *
 */

static VALUE
enum_take(VALUE obj, VALUE n)
{
    struct MEMO *memo;
    VALUE result;
    long len = NUM2LONG(n);

    if (len < 0) {
	rb_raise(rb_eArgError, "attempt to take negative size");
    }

    if (len == 0) return rb_ary_new2(0);
    result = rb_ary_new2(len);
    memo = MEMO_NEW(result, 0, len);
    rb_block_call(obj, id_each, 0, 0, take_i, (VALUE)memo);
    return result;
}


static VALUE
take_while_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    if (!RTEST(rb_yield_values2(argc, argv))) rb_iter_break();
    rb_ary_push(ary, rb_enum_values_pack(argc, argv));
    return Qnil;
}

/*
 *  call-seq:
 *     enum.take_while { |obj| block } -> array
 *     enum.take_while                 -> an_enumerator
 *
 *  Passes elements to the block until the block returns +nil+ or +false+,
 *  then stops iterating and returns an array of all prior elements.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     a = [1, 2, 3, 4, 5, 0]
 *     a.take_while { |i| i < 3 }   #=> [1, 2]
 *
 */

static VALUE
enum_take_while(VALUE obj)
{
    VALUE ary;

    RETURN_ENUMERATOR(obj, 0, 0);
    ary = rb_ary_new();
    rb_block_call(obj, id_each, 0, 0, take_while_i, ary);
    return ary;
}

static VALUE
drop_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct MEMO *memo = MEMO_CAST(args);
    if (memo->u3.cnt == 0) {
	rb_ary_push(memo->v1, rb_enum_values_pack(argc, argv));
    }
    else {
	memo->u3.cnt--;
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.drop(n)               -> array
 *
 *  Drops first n elements from <i>enum</i>, and returns rest elements
 *  in an array.
 *
 *     a = [1, 2, 3, 4, 5, 0]
 *     a.drop(3)             #=> [4, 5, 0]
 *
 */

static VALUE
enum_drop(VALUE obj, VALUE n)
{
    VALUE result;
    struct MEMO *memo;
    long len = NUM2LONG(n);

    if (len < 0) {
	rb_raise(rb_eArgError, "attempt to drop negative size");
    }

    result = rb_ary_new();
    memo = MEMO_NEW(result, 0, len);
    rb_block_call(obj, id_each, 0, 0, drop_i, (VALUE)memo);
    return result;
}


static VALUE
drop_while_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    struct MEMO *memo = MEMO_CAST(args);
    ENUM_WANT_SVALUE();

    if (!memo->u3.state && !RTEST(enum_yield(argc, i))) {
	memo->u3.state = TRUE;
    }
    if (memo->u3.state) {
	rb_ary_push(memo->v1, i);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.drop_while { |obj| block }  -> array
 *     enum.drop_while                  -> an_enumerator
 *
 *  Drops elements up to, but not including, the first element for
 *  which the block returns +nil+ or +false+ and returns an array
 *  containing the remaining elements.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     a = [1, 2, 3, 4, 5, 0]
 *     a.drop_while { |i| i < 3 }   #=> [3, 4, 5, 0]
 *
 */

static VALUE
enum_drop_while(VALUE obj)
{
    VALUE result;
    struct MEMO *memo;

    RETURN_ENUMERATOR(obj, 0, 0);
    result = rb_ary_new();
    memo = MEMO_NEW(result, 0, FALSE);
    rb_block_call(obj, id_each, 0, 0, drop_while_i, (VALUE)memo);
    return result;
}

static VALUE
cycle_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, ary))
{
    ENUM_WANT_SVALUE();

    rb_ary_push(ary, argc > 1 ? i : rb_ary_new_from_values(argc, argv));
    enum_yield(argc, i);
    return Qnil;
}

static VALUE
enum_cycle_size(VALUE self, VALUE args, VALUE eobj)
{
    long mul = 0;
    VALUE n = Qnil;
    VALUE size;

    if (args && (RARRAY_LEN(args) > 0)) {
	n = RARRAY_AREF(args, 0);
	if (!NIL_P(n)) mul = NUM2LONG(n);
    }

    size = enum_size(self, args, 0);
    if (NIL_P(size) || FIXNUM_ZERO_P(size)) return size;

    if (NIL_P(n)) return DBL2NUM(HUGE_VAL);
    if (mul <= 0) return INT2FIX(0);
    n = LONG2FIX(mul);
    return rb_funcallv(size, '*', 1, &n);
}

/*
 *  call-seq:
 *     enum.cycle(n=nil) { |obj| block }  ->  nil
 *     enum.cycle(n=nil)                  ->  an_enumerator
 *
 *  Calls <i>block</i> for each element of <i>enum</i> repeatedly _n_
 *  times or forever if none or +nil+ is given.  If a non-positive
 *  number is given or the collection is empty, does nothing.  Returns
 *  +nil+ if the loop has finished without getting interrupted.
 *
 *  Enumerable#cycle saves elements in an internal array so changes
 *  to <i>enum</i> after the first pass have no effect.
 *
 *  If no block is given, an enumerator is returned instead.
 *
 *     a = ["a", "b", "c"]
 *     a.cycle { |x| puts x }  # print, a, b, c, a, b, c,.. forever.
 *     a.cycle(2) { |x| puts x }  # print, a, b, c, a, b, c.
 *
 */

static VALUE
enum_cycle(int argc, VALUE *argv, VALUE obj)
{
    VALUE ary;
    VALUE nv = Qnil;
    long n, i, len;

    rb_check_arity(argc, 0, 1);

    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enum_cycle_size);
    if (!argc || NIL_P(nv = argv[0])) {
        n = -1;
    }
    else {
        n = NUM2LONG(nv);
        if (n <= 0) return Qnil;
    }
    ary = rb_ary_new();
    RBASIC_CLEAR_CLASS(ary);
    rb_block_call(obj, id_each, 0, 0, cycle_i, ary);
    len = RARRAY_LEN(ary);
    if (len == 0) return Qnil;
    while (n < 0 || 0 < --n) {
        for (i=0; i<len; i++) {
	    enum_yield_array(RARRAY_AREF(ary, i));
        }
    }
    return Qnil;
}

struct chunk_arg {
    VALUE categorize;
    VALUE prev_value;
    VALUE prev_elts;
    VALUE yielder;
};

static VALUE
chunk_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, _argp))
{
    struct chunk_arg *argp = MEMO_FOR(struct chunk_arg, _argp);
    VALUE v, s;
    VALUE alone = ID2SYM(rb_intern("_alone"));
    VALUE separator = ID2SYM(rb_intern("_separator"));

    ENUM_WANT_SVALUE();

    v = rb_funcallv(argp->categorize, id_call, 1, &i);

    if (v == alone) {
        if (!NIL_P(argp->prev_value)) {
	    s = rb_assoc_new(argp->prev_value, argp->prev_elts);
            rb_funcallv(argp->yielder, id_lshift, 1, &s);
            argp->prev_value = argp->prev_elts = Qnil;
        }
	v = rb_assoc_new(v, rb_ary_new3(1, i));
        rb_funcallv(argp->yielder, id_lshift, 1, &v);
    }
    else if (NIL_P(v) || v == separator) {
        if (!NIL_P(argp->prev_value)) {
	    v = rb_assoc_new(argp->prev_value, argp->prev_elts);
            rb_funcallv(argp->yielder, id_lshift, 1, &v);
            argp->prev_value = argp->prev_elts = Qnil;
        }
    }
    else if (SYMBOL_P(v) && (s = rb_sym2str(v), RSTRING_PTR(s)[0] == '_')) {
	rb_raise(rb_eRuntimeError, "symbols beginning with an underscore are reserved");
    }
    else {
        if (NIL_P(argp->prev_value)) {
            argp->prev_value = v;
            argp->prev_elts = rb_ary_new3(1, i);
        }
        else {
            if (rb_equal(argp->prev_value, v)) {
                rb_ary_push(argp->prev_elts, i);
            }
            else {
		s = rb_assoc_new(argp->prev_value, argp->prev_elts);
                rb_funcallv(argp->yielder, id_lshift, 1, &s);
                argp->prev_value = v;
                argp->prev_elts = rb_ary_new3(1, i);
            }
        }
    }
    return Qnil;
}

static VALUE
chunk_i(RB_BLOCK_CALL_FUNC_ARGLIST(yielder, enumerator))
{
    VALUE enumerable;
    VALUE arg;
    struct chunk_arg *memo = NEW_MEMO_FOR(struct chunk_arg, arg);

    enumerable = rb_ivar_get(enumerator, rb_intern("chunk_enumerable"));
    memo->categorize = rb_ivar_get(enumerator, rb_intern("chunk_categorize"));
    memo->prev_value = Qnil;
    memo->prev_elts = Qnil;
    memo->yielder = yielder;

    rb_block_call(enumerable, id_each, 0, 0, chunk_ii, arg);
    memo = MEMO_FOR(struct chunk_arg, arg);
    if (!NIL_P(memo->prev_elts)) {
	arg = rb_assoc_new(memo->prev_value, memo->prev_elts);
	rb_funcallv(memo->yielder, id_lshift, 1, &arg);
    }
    return Qnil;
}

/*
 *  call-seq:
 *     enum.chunk { |elt| ... }                       -> an_enumerator
 *
 *  Enumerates over the items, chunking them together based on the return
 *  value of the block.
 *
 *  Consecutive elements which return the same block value are chunked together.
 *
 *  For example, consecutive even numbers and odd numbers can be
 *  chunked as follows.
 *
 *    [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5].chunk { |n|
 *      n.even?
 *    }.each { |even, ary|
 *      p [even, ary]
 *    }
 *    #=> [false, [3, 1]]
 *    #   [true, [4]]
 *    #   [false, [1, 5, 9]]
 *    #   [true, [2, 6]]
 *    #   [false, [5, 3, 5]]
 *
 *  This method is especially useful for sorted series of elements.
 *  The following example counts words for each initial letter.
 *
 *    open("/usr/share/dict/words", "r:iso-8859-1") { |f|
 *      f.chunk { |line| line.upcase.ord }.each { |ch, lines| p [ch.chr, lines.length] }
 *    }
 *    #=> ["\n", 1]
 *    #   ["A", 1327]
 *    #   ["B", 1372]
 *    #   ["C", 1507]
 *    #   ["D", 791]
 *    #   ...
 *
 *  The following key values have special meaning:
 *  - +nil+ and +:_separator+ specifies that the elements should be dropped.
 *  - +:_alone+ specifies that the element should be chunked by itself.
 *
 *  Any other symbols that begin with an underscore will raise an error:
 *
 *    items.chunk { |item| :_underscore }
 *    #=> RuntimeError: symbols beginning with an underscore are reserved
 *
 *  +nil+ and +:_separator+ can be used to ignore some elements.
 *
 *  For example, the sequence of hyphens in svn log can be eliminated as follows:
 *
 *    sep = "-"*72 + "\n"
 *    IO.popen("svn log README") { |f|
 *      f.chunk { |line|
 *        line != sep || nil
 *      }.each { |_, lines|
 *        pp lines
 *      }
 *    }
 *    #=> ["r20018 | knu | 2008-10-29 13:20:42 +0900 (Wed, 29 Oct 2008) | 2 lines\n",
 *    #    "\n",
 *    #    "* README, README.ja: Update the portability section.\n",
 *    #    "\n"]
 *    #   ["r16725 | knu | 2008-05-31 23:34:23 +0900 (Sat, 31 May 2008) | 2 lines\n",
 *    #    "\n",
 *    #    "* README, README.ja: Add a note about default C flags.\n",
 *    #    "\n"]
 *    #   ...
 *
 *  Paragraphs separated by empty lines can be parsed as follows:
 *
 *    File.foreach("README").chunk { |line|
 *      /\A\s*\z/ !~ line || nil
 *    }.each { |_, lines|
 *      pp lines
 *    }
 *
 *  +:_alone+ can be used to force items into their own chunk.
 *  For example, you can put lines that contain a URL by themselves,
 *  and chunk the rest of the lines together, like this:
 *
 *    pattern = /http/
 *    open(filename) { |f|
 *      f.chunk { |line| line =~ pattern ? :_alone : true }.each { |key, lines|
 *        pp lines
 *      }
 *    }
 *
 *  If no block is given, an enumerator to `chunk` is returned instead.
 */
static VALUE
enum_chunk(VALUE enumerable)
{
    VALUE enumerator;

    RETURN_SIZED_ENUMERATOR(enumerable, 0, 0, enum_size);

    enumerator = rb_obj_alloc(rb_cEnumerator);
    rb_ivar_set(enumerator, rb_intern("chunk_enumerable"), enumerable);
    rb_ivar_set(enumerator, rb_intern("chunk_categorize"), rb_block_proc());
    rb_block_call(enumerator, idInitialize, 0, 0, chunk_i, enumerator);
    return enumerator;
}


struct slicebefore_arg {
    VALUE sep_pred;
    VALUE sep_pat;
    VALUE prev_elts;
    VALUE yielder;
};

static VALUE
slicebefore_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, _argp))
{
    struct slicebefore_arg *argp = MEMO_FOR(struct slicebefore_arg, _argp);
    VALUE header_p;

    ENUM_WANT_SVALUE();

    if (!NIL_P(argp->sep_pat))
        header_p = rb_funcallv(argp->sep_pat, id_eqq, 1, &i);
    else
        header_p = rb_funcallv(argp->sep_pred, id_call, 1, &i);
    if (RTEST(header_p)) {
        if (!NIL_P(argp->prev_elts))
            rb_funcallv(argp->yielder, id_lshift, 1, &argp->prev_elts);
        argp->prev_elts = rb_ary_new3(1, i);
    }
    else {
        if (NIL_P(argp->prev_elts))
            argp->prev_elts = rb_ary_new3(1, i);
        else
            rb_ary_push(argp->prev_elts, i);
    }

    return Qnil;
}

static VALUE
slicebefore_i(RB_BLOCK_CALL_FUNC_ARGLIST(yielder, enumerator))
{
    VALUE enumerable;
    VALUE arg;
    struct slicebefore_arg *memo = NEW_MEMO_FOR(struct slicebefore_arg, arg);

    enumerable = rb_ivar_get(enumerator, rb_intern("slicebefore_enumerable"));
    memo->sep_pred = rb_attr_get(enumerator, rb_intern("slicebefore_sep_pred"));
    memo->sep_pat = NIL_P(memo->sep_pred) ? rb_ivar_get(enumerator, rb_intern("slicebefore_sep_pat")) : Qnil;
    memo->prev_elts = Qnil;
    memo->yielder = yielder;

    rb_block_call(enumerable, id_each, 0, 0, slicebefore_ii, arg);
    memo = MEMO_FOR(struct slicebefore_arg, arg);
    if (!NIL_P(memo->prev_elts))
        rb_funcallv(memo->yielder, id_lshift, 1, &memo->prev_elts);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.slice_before(pattern)                             -> an_enumerator
 *     enum.slice_before { |elt| bool }                       -> an_enumerator
 *
 *  Creates an enumerator for each chunked elements.
 *  The beginnings of chunks are defined by _pattern_ and the block.

 *  If <code>_pattern_ === _elt_</code> returns <code>true</code> or the block
 *  returns <code>true</code> for the element, the element is beginning of a
 *  chunk.

 *  The <code>===</code> and _block_ is called from the first element to the last
 *  element of _enum_.  The result for the first element is ignored.

 *  The result enumerator yields the chunked elements as an array.
 *  So +each+ method can be called as follows:
 *
 *    enum.slice_before(pattern).each { |ary| ... }
 *    enum.slice_before { |elt| bool }.each { |ary| ... }
 *
 *  Other methods of the Enumerator class and Enumerable module,
 *  such as +to_a+, +map+, etc., are also usable.
 *
 *  For example, iteration over ChangeLog entries can be implemented as
 *  follows:
 *
 *    # iterate over ChangeLog entries.
 *    open("ChangeLog") { |f|
 *      f.slice_before(/\A\S/).each { |e| pp e }
 *    }
 *
 *    # same as above.  block is used instead of pattern argument.
 *    open("ChangeLog") { |f|
 *      f.slice_before { |line| /\A\S/ === line }.each { |e| pp e }
 *    }
 *
 *
 *  "svn proplist -R" produces multiline output for each file.
 *  They can be chunked as follows:
 *
 *    IO.popen([{"LC_ALL"=>"C"}, "svn", "proplist", "-R"]) { |f|
 *      f.lines.slice_before(/\AProp/).each { |lines| p lines }
 *    }
 *    #=> ["Properties on '.':\n", "  svn:ignore\n", "  svk:merge\n"]
 *    #   ["Properties on 'goruby.c':\n", "  svn:eol-style\n"]
 *    #   ["Properties on 'complex.c':\n", "  svn:mime-type\n", "  svn:eol-style\n"]
 *    #   ["Properties on 'regparse.c':\n", "  svn:eol-style\n"]
 *    #   ...
 *
 *  If the block needs to maintain state over multiple elements,
 *  local variables can be used.
 *  For example, three or more consecutive increasing numbers can be squashed
 *  as follows (see +chunk_while+ for a better way):
 *
 *    a = [0, 2, 3, 4, 6, 7, 9]
 *    prev = a[0]
 *    p a.slice_before { |e|
 *      prev, prev2 = e, prev
 *      prev2 + 1 != e
 *    }.map { |es|
 *      es.length <= 2 ? es.join(",") : "#{es.first}-#{es.last}"
 *    }.join(",")
 *    #=> "0,2-4,6,7,9"
 *
 *  However local variables should be used carefully
 *  if the result enumerator is enumerated twice or more.
 *  The local variables should be initialized for each enumeration.
 *  Enumerator.new can be used to do it.
 *
 *    # Word wrapping.  This assumes all characters have same width.
 *    def wordwrap(words, maxwidth)
 *      Enumerator.new {|y|
 *        # cols is initialized in Enumerator.new.
 *        cols = 0
 *        words.slice_before { |w|
 *          cols += 1 if cols != 0
 *          cols += w.length
 *          if maxwidth < cols
 *            cols = w.length
 *            true
 *          else
 *            false
 *          end
 *        }.each {|ws| y.yield ws }
 *      }
 *    end
 *    text = (1..20).to_a.join(" ")
 *    enum = wordwrap(text.split(/\s+/), 10)
 *    puts "-"*10
 *    enum.each { |ws| puts ws.join(" ") } # first enumeration.
 *    puts "-"*10
 *    enum.each { |ws| puts ws.join(" ") } # second enumeration generates same result as the first.
 *    puts "-"*10
 *    #=> ----------
 *    #   1 2 3 4 5
 *    #   6 7 8 9 10
 *    #   11 12 13
 *    #   14 15 16
 *    #   17 18 19
 *    #   20
 *    #   ----------
 *    #   1 2 3 4 5
 *    #   6 7 8 9 10
 *    #   11 12 13
 *    #   14 15 16
 *    #   17 18 19
 *    #   20
 *    #   ----------
 *
 *  mbox contains series of mails which start with Unix From line.
 *  So each mail can be extracted by slice before Unix From line.
 *
 *    # parse mbox
 *    open("mbox") { |f|
 *      f.slice_before { |line|
 *        line.start_with? "From "
 *      }.each { |mail|
 *        unix_from = mail.shift
 *        i = mail.index("\n")
 *        header = mail[0...i]
 *        body = mail[(i+1)..-1]
 *        body.pop if body.last == "\n"
 *        fields = header.slice_before { |line| !" \t".include?(line[0]) }.to_a
 *        p unix_from
 *        pp fields
 *        pp body
 *      }
 *    }
 *
 *    # split mails in mbox (slice before Unix From line after an empty line)
 *    open("mbox") { |f|
 *      emp = true
 *      f.slice_before { |line|
 *        prevemp = emp
 *        emp = line == "\n"
 *        prevemp && line.start_with?("From ")
 *      }.each { |mail|
 *        mail.pop if mail.last == "\n"
 *        pp mail
 *      }
 *    }
 *
 */
static VALUE
enum_slice_before(int argc, VALUE *argv, VALUE enumerable)
{
    VALUE enumerator;

    if (rb_block_given_p()) {
        if (argc != 0)
            rb_error_arity(argc, 0, 0);
        enumerator = rb_obj_alloc(rb_cEnumerator);
        rb_ivar_set(enumerator, rb_intern("slicebefore_sep_pred"), rb_block_proc());
    }
    else {
        VALUE sep_pat;
        rb_scan_args(argc, argv, "1", &sep_pat);
        enumerator = rb_obj_alloc(rb_cEnumerator);
        rb_ivar_set(enumerator, rb_intern("slicebefore_sep_pat"), sep_pat);
    }
    rb_ivar_set(enumerator, rb_intern("slicebefore_enumerable"), enumerable);
    rb_block_call(enumerator, idInitialize, 0, 0, slicebefore_i, enumerator);
    return enumerator;
}


struct sliceafter_arg {
    VALUE pat;
    VALUE pred;
    VALUE prev_elts;
    VALUE yielder;
};

static VALUE
sliceafter_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, _memo))
{
#define UPDATE_MEMO ((void)(memo = MEMO_FOR(struct sliceafter_arg, _memo)))
    struct sliceafter_arg *memo;
    int split_p;
    UPDATE_MEMO;

    ENUM_WANT_SVALUE();

    if (NIL_P(memo->prev_elts)) {
        memo->prev_elts = rb_ary_new3(1, i);
    }
    else {
        rb_ary_push(memo->prev_elts, i);
    }

    if (NIL_P(memo->pred)) {
        split_p = RTEST(rb_funcallv(memo->pat, id_eqq, 1, &i));
        UPDATE_MEMO;
    }
    else {
        split_p = RTEST(rb_funcallv(memo->pred, id_call, 1, &i));
        UPDATE_MEMO;
    }

    if (split_p) {
        rb_funcallv(memo->yielder, id_lshift, 1, &memo->prev_elts);
        UPDATE_MEMO;
        memo->prev_elts = Qnil;
    }

    return Qnil;
#undef UPDATE_MEMO
}

static VALUE
sliceafter_i(RB_BLOCK_CALL_FUNC_ARGLIST(yielder, enumerator))
{
    VALUE enumerable;
    VALUE arg;
    struct sliceafter_arg *memo = NEW_MEMO_FOR(struct sliceafter_arg, arg);

    enumerable = rb_ivar_get(enumerator, rb_intern("sliceafter_enum"));
    memo->pat = rb_ivar_get(enumerator, rb_intern("sliceafter_pat"));
    memo->pred = rb_attr_get(enumerator, rb_intern("sliceafter_pred"));
    memo->prev_elts = Qnil;
    memo->yielder = yielder;

    rb_block_call(enumerable, id_each, 0, 0, sliceafter_ii, arg);
    memo = MEMO_FOR(struct sliceafter_arg, arg);
    if (!NIL_P(memo->prev_elts))
        rb_funcallv(memo->yielder, id_lshift, 1, &memo->prev_elts);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.slice_after(pattern)       -> an_enumerator
 *     enum.slice_after { |elt| bool } -> an_enumerator
 *
 *  Creates an enumerator for each chunked elements.
 *  The ends of chunks are defined by _pattern_ and the block.
 *
 *  If <code>_pattern_ === _elt_</code> returns <code>true</code> or the block
 *  returns <code>true</code> for the element, the element is end of a
 *  chunk.
 *
 *  The <code>===</code> and _block_ is called from the first element to the last
 *  element of _enum_.
 *
 *  The result enumerator yields the chunked elements as an array.
 *  So +each+ method can be called as follows:
 *
 *    enum.slice_after(pattern).each { |ary| ... }
 *    enum.slice_after { |elt| bool }.each { |ary| ... }
 *
 *  Other methods of the Enumerator class and Enumerable module,
 *  such as +map+, etc., are also usable.
 *
 *  For example, continuation lines (lines end with backslash) can be
 *  concatenated as follows:
 *
 *    lines = ["foo\n", "bar\\\n", "baz\n", "\n", "qux\n"]
 *    e = lines.slice_after(/(?<!\\)\n\z/)
 *    p e.to_a
 *    #=> [["foo\n"], ["bar\\\n", "baz\n"], ["\n"], ["qux\n"]]
 *    p e.map {|ll| ll[0...-1].map {|l| l.sub(/\\\n\z/, "") }.join + ll.last }
 *    #=>["foo\n", "barbaz\n", "\n", "qux\n"]
 *
 */

static VALUE
enum_slice_after(int argc, VALUE *argv, VALUE enumerable)
{
    VALUE enumerator;
    VALUE pat = Qnil, pred = Qnil;

    if (rb_block_given_p()) {
        if (0 < argc)
            rb_raise(rb_eArgError, "both pattern and block are given");
        pred = rb_block_proc();
    }
    else {
        rb_scan_args(argc, argv, "1", &pat);
    }

    enumerator = rb_obj_alloc(rb_cEnumerator);
    rb_ivar_set(enumerator, rb_intern("sliceafter_enum"), enumerable);
    rb_ivar_set(enumerator, rb_intern("sliceafter_pat"), pat);
    rb_ivar_set(enumerator, rb_intern("sliceafter_pred"), pred);

    rb_block_call(enumerator, idInitialize, 0, 0, sliceafter_i, enumerator);
    return enumerator;
}

struct slicewhen_arg {
    VALUE pred;
    VALUE prev_elt;
    VALUE prev_elts;
    VALUE yielder;
    int inverted; /* 0 for slice_when and 1 for chunk_while. */
};

static VALUE
slicewhen_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, _memo))
{
#define UPDATE_MEMO ((void)(memo = MEMO_FOR(struct slicewhen_arg, _memo)))
    struct slicewhen_arg *memo;
    int split_p;
    UPDATE_MEMO;

    ENUM_WANT_SVALUE();

    if (memo->prev_elt == Qundef) {
        /* The first element */
        memo->prev_elt = i;
        memo->prev_elts = rb_ary_new3(1, i);
    }
    else {
	VALUE args[2];
	args[0] = memo->prev_elt;
	args[1] = i;
        split_p = RTEST(rb_funcallv(memo->pred, id_call, 2, args));
        UPDATE_MEMO;

        if (memo->inverted)
            split_p = !split_p;

        if (split_p) {
            rb_funcallv(memo->yielder, id_lshift, 1, &memo->prev_elts);
            UPDATE_MEMO;
            memo->prev_elts = rb_ary_new3(1, i);
        }
        else {
            rb_ary_push(memo->prev_elts, i);
        }

        memo->prev_elt = i;
    }

    return Qnil;
#undef UPDATE_MEMO
}

static VALUE
slicewhen_i(RB_BLOCK_CALL_FUNC_ARGLIST(yielder, enumerator))
{
    VALUE enumerable;
    VALUE arg;
    struct slicewhen_arg *memo =
	NEW_PARTIAL_MEMO_FOR(struct slicewhen_arg, arg, inverted);

    enumerable = rb_ivar_get(enumerator, rb_intern("slicewhen_enum"));
    memo->pred = rb_attr_get(enumerator, rb_intern("slicewhen_pred"));
    memo->prev_elt = Qundef;
    memo->prev_elts = Qnil;
    memo->yielder = yielder;
    memo->inverted = RTEST(rb_attr_get(enumerator, rb_intern("slicewhen_inverted")));

    rb_block_call(enumerable, id_each, 0, 0, slicewhen_ii, arg);
    memo = MEMO_FOR(struct slicewhen_arg, arg);
    if (!NIL_P(memo->prev_elts))
        rb_funcallv(memo->yielder, id_lshift, 1, &memo->prev_elts);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.slice_when {|elt_before, elt_after| bool } -> an_enumerator
 *
 *  Creates an enumerator for each chunked elements.
 *  The beginnings of chunks are defined by the block.
 *
 *  This method splits each chunk using adjacent elements,
 *  _elt_before_ and _elt_after_,
 *  in the receiver enumerator.
 *  This method split chunks between _elt_before_ and _elt_after_ where
 *  the block returns <code>true</code>.
 *
 *  The block is called the length of the receiver enumerator minus one.
 *
 *  The result enumerator yields the chunked elements as an array.
 *  So +each+ method can be called as follows:
 *
 *    enum.slice_when { |elt_before, elt_after| bool }.each { |ary| ... }
 *
 *  Other methods of the Enumerator class and Enumerable module,
 *  such as +to_a+, +map+, etc., are also usable.
 *
 *  For example, one-by-one increasing subsequence can be chunked as follows:
 *
 *    a = [1,2,4,9,10,11,12,15,16,19,20,21]
 *    b = a.slice_when {|i, j| i+1 != j }
 *    p b.to_a #=> [[1, 2], [4], [9, 10, 11, 12], [15, 16], [19, 20, 21]]
 *    c = b.map {|a| a.length < 3 ? a : "#{a.first}-#{a.last}" }
 *    p c #=> [[1, 2], [4], "9-12", [15, 16], "19-21"]
 *    d = c.join(",")
 *    p d #=> "1,2,4,9-12,15,16,19-21"
 *
 *  Near elements (threshold: 6) in sorted array can be chunked as follows:
 *
 *    a = [3, 11, 14, 25, 28, 29, 29, 41, 55, 57]
 *    p a.slice_when {|i, j| 6 < j - i }.to_a
 *    #=> [[3], [11, 14], [25, 28, 29, 29], [41], [55, 57]]
 *
 *  Increasing (non-decreasing) subsequence can be chunked as follows:
 *
 *    a = [0, 9, 2, 2, 3, 2, 7, 5, 9, 5]
 *    p a.slice_when {|i, j| i > j }.to_a
 *    #=> [[0, 9], [2, 2, 3], [2, 7], [5, 9], [5]]
 *
 *  Adjacent evens and odds can be chunked as follows:
 *  (Enumerable#chunk is another way to do it.)
 *
 *    a = [7, 5, 9, 2, 0, 7, 9, 4, 2, 0]
 *    p a.slice_when {|i, j| i.even? != j.even? }.to_a
 *    #=> [[7, 5, 9], [2, 0], [7, 9], [4, 2, 0]]
 *
 *  Paragraphs (non-empty lines with trailing empty lines) can be chunked as follows:
 *  (See Enumerable#chunk to ignore empty lines.)
 *
 *    lines = ["foo\n", "bar\n", "\n", "baz\n", "qux\n"]
 *    p lines.slice_when {|l1, l2| /\A\s*\z/ =~ l1 && /\S/ =~ l2 }.to_a
 *    #=> [["foo\n", "bar\n", "\n"], ["baz\n", "qux\n"]]
 *
 *  Enumerable#chunk_while does the same, except splitting when the block
 *  returns <code>false</code> instead of <code>true</code>.
 */
static VALUE
enum_slice_when(VALUE enumerable)
{
    VALUE enumerator;
    VALUE pred;

    pred = rb_block_proc();

    enumerator = rb_obj_alloc(rb_cEnumerator);
    rb_ivar_set(enumerator, rb_intern("slicewhen_enum"), enumerable);
    rb_ivar_set(enumerator, rb_intern("slicewhen_pred"), pred);
    rb_ivar_set(enumerator, rb_intern("slicewhen_inverted"), Qfalse);

    rb_block_call(enumerator, idInitialize, 0, 0, slicewhen_i, enumerator);
    return enumerator;
}

/*
 *  call-seq:
 *     enum.chunk_while {|elt_before, elt_after| bool } -> an_enumerator
 *
 *  Creates an enumerator for each chunked elements.
 *  The beginnings of chunks are defined by the block.
 *
 *  This method splits each chunk using adjacent elements,
 *  _elt_before_ and _elt_after_,
 *  in the receiver enumerator.
 *  This method split chunks between _elt_before_ and _elt_after_ where
 *  the block returns <code>false</code>.
 *
 *  The block is called the length of the receiver enumerator minus one.
 *
 *  The result enumerator yields the chunked elements as an array.
 *  So +each+ method can be called as follows:
 *
 *    enum.chunk_while { |elt_before, elt_after| bool }.each { |ary| ... }
 *
 *  Other methods of the Enumerator class and Enumerable module,
 *  such as +to_a+, +map+, etc., are also usable.
 *
 *  For example, one-by-one increasing subsequence can be chunked as follows:
 *
 *    a = [1,2,4,9,10,11,12,15,16,19,20,21]
 *    b = a.chunk_while {|i, j| i+1 == j }
 *    p b.to_a #=> [[1, 2], [4], [9, 10, 11, 12], [15, 16], [19, 20, 21]]
 *    c = b.map {|a| a.length < 3 ? a : "#{a.first}-#{a.last}" }
 *    p c #=> [[1, 2], [4], "9-12", [15, 16], "19-21"]
 *    d = c.join(",")
 *    p d #=> "1,2,4,9-12,15,16,19-21"
 *
 *  Increasing (non-decreasing) subsequence can be chunked as follows:
 *
 *    a = [0, 9, 2, 2, 3, 2, 7, 5, 9, 5]
 *    p a.chunk_while {|i, j| i <= j }.to_a
 *    #=> [[0, 9], [2, 2, 3], [2, 7], [5, 9], [5]]
 *
 *  Adjacent evens and odds can be chunked as follows:
 *  (Enumerable#chunk is another way to do it.)
 *
 *    a = [7, 5, 9, 2, 0, 7, 9, 4, 2, 0]
 *    p a.chunk_while {|i, j| i.even? == j.even? }.to_a
 *    #=> [[7, 5, 9], [2, 0], [7, 9], [4, 2, 0]]
 *
 *  Enumerable#slice_when does the same, except splitting when the block
 *  returns <code>true</code> instead of <code>false</code>.
 */
static VALUE
enum_chunk_while(VALUE enumerable)
{
    VALUE enumerator;
    VALUE pred;

    pred = rb_block_proc();

    enumerator = rb_obj_alloc(rb_cEnumerator);
    rb_ivar_set(enumerator, rb_intern("slicewhen_enum"), enumerable);
    rb_ivar_set(enumerator, rb_intern("slicewhen_pred"), pred);
    rb_ivar_set(enumerator, rb_intern("slicewhen_inverted"), Qtrue);

    rb_block_call(enumerator, idInitialize, 0, 0, slicewhen_i, enumerator);
    return enumerator;
}

struct enum_sum_memo {
    VALUE v, r;
    long n;
    double f, c;
    int block_given;
    int float_value;
};

static void
sum_iter_normalize_memo(struct enum_sum_memo *memo)
{
    assert(FIXABLE(memo->n));
    memo->v = rb_fix_plus(LONG2FIX(memo->n), memo->v);
    memo->n = 0;

    switch (TYPE(memo->r)) {
      case T_RATIONAL: memo->v = rb_rational_plus(memo->r, memo->v); break;
      case T_UNDEF:    break;
      default:         UNREACHABLE; /* or ...? */
    }
    memo->r = Qundef;
}

static void
sum_iter_fixnum(VALUE i, struct enum_sum_memo *memo)
{
    memo->n += FIX2LONG(i); /* should not overflow long type */
    if (! FIXABLE(memo->n)) {
        memo->v = rb_big_plus(LONG2NUM(memo->n), memo->v);
        memo->n = 0;
    }
}

static void
sum_iter_bignum(VALUE i, struct enum_sum_memo *memo)
{
    memo->v = rb_big_plus(i, memo->v);
}

static void
sum_iter_rational(VALUE i, struct enum_sum_memo *memo)
{
    if (memo->r == Qundef) {
        memo->r = i;
    }
    else {
        memo->r = rb_rational_plus(memo->r, i);
    }
}

static void
sum_iter_some_value(VALUE i, struct enum_sum_memo *memo)
{
    memo->v = rb_funcallv(memo->v, idPLUS, 1, &i);
}

static void
sum_iter_Kahan_Babuska(VALUE i, struct enum_sum_memo *memo)
{
    /*
     * Kahan-Babuska balancing compensated summation algorithm
     * See https://link.springer.com/article/10.1007/s00607-005-0139-x
     */
    double x;

    switch (TYPE(i)) {
      case T_FLOAT:    x = RFLOAT_VALUE(i); break;
      case T_FIXNUM:   x = FIX2LONG(i);     break;
      case T_BIGNUM:   x = rb_big2dbl(i);   break;
      case T_RATIONAL: x = rb_num2dbl(i);   break;
      default:
        memo->v = DBL2NUM(memo->f);
        memo->float_value = 0;
        sum_iter_some_value(i, memo);
        return;
    }

    double f = memo->f;

    if (isnan(f)) {
        return;
    }
    else if (! isfinite(x)) {
        if (isinf(x) && isinf(f) && signbit(x) != signbit(f)) {
            i = DBL2NUM(f);
            x = nan("");
        }
        memo->v = i;
        memo->f = x;
        return;
    }
    else if (isinf(f)) {
        return;
    }

    double c = memo->c;
    double t = f + x;

    if (fabs(f) >= fabs(x)) {
        c += ((f - t) + x);
    }
    else {
        c += ((x - t) + f);
    }
    f = t;

    memo->f = f;
    memo->c = c;
}

static void
sum_iter(VALUE i, struct enum_sum_memo *memo)
{
    assert(memo != NULL);
    if (memo->block_given) {
        i = rb_yield(i);
    }

    if (memo->float_value) {
        sum_iter_Kahan_Babuska(i, memo);
    }
    else switch (TYPE(memo->v)) {
      default:      sum_iter_some_value(i, memo);    return;
      case T_FLOAT: sum_iter_Kahan_Babuska(i, memo); return;
      case T_FIXNUM:
      case T_BIGNUM:
      case T_RATIONAL:
        switch (TYPE(i)) {
          case T_FIXNUM:   sum_iter_fixnum(i, memo);   return;
          case T_BIGNUM:   sum_iter_bignum(i, memo);   return;
          case T_RATIONAL: sum_iter_rational(i, memo); return;
          case T_FLOAT:
            sum_iter_normalize_memo(memo);
            memo->f = NUM2DBL(memo->v);
            memo->c = 0.0;
            memo->float_value = 1;
            sum_iter_Kahan_Babuska(i, memo);
            return;
          default:
            sum_iter_normalize_memo(memo);
            sum_iter_some_value(i, memo);
            return;
        }
    }
}

static VALUE
enum_sum_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, args))
{
    ENUM_WANT_SVALUE();
    sum_iter(i, (struct enum_sum_memo *) args);
    return Qnil;
}

static int
hash_sum_i(VALUE key, VALUE value, VALUE arg)
{
    sum_iter(rb_assoc_new(key, value), (struct enum_sum_memo *) arg);
    return ST_CONTINUE;
}

static void
hash_sum(VALUE hash, struct enum_sum_memo *memo)
{
    assert(RB_TYPE_P(hash, T_HASH));
    assert(memo != NULL);

    rb_hash_foreach(hash, hash_sum_i, (VALUE)memo);
}

static VALUE
int_range_sum(VALUE beg, VALUE end, int excl, VALUE init)
{
    if (excl) {
        if (FIXNUM_P(end))
            end = LONG2FIX(FIX2LONG(end) - 1);
        else
            end = rb_big_minus(end, LONG2FIX(1));
    }

    if (rb_int_ge(end, beg)) {
        VALUE a;
        a = rb_int_plus(rb_int_minus(end, beg), LONG2FIX(1));
        a = rb_int_mul(a, rb_int_plus(end, beg));
        a = rb_int_idiv(a, LONG2FIX(2));
        return rb_int_plus(init, a);
    }

    return init;
}

/*
 * call-seq:
 *   enum.sum(init=0)                   -> number
 *   enum.sum(init=0) {|e| expr }       -> number
 *
 * Returns the sum of elements in an Enumerable.
 *
 * If a block is given, the block is applied to each element
 * before addition.
 *
 * If <i>enum</i> is empty, it returns <i>init</i>.
 *
 * For example:
 *
 *   { 1 => 10, 2 => 20 }.sum {|k, v| k * v }  #=> 50
 *   (1..10).sum                               #=> 55
 *   (1..10).sum {|v| v * 2 }                  #=> 110
 *   ('a'..'z').sum                            #=> TypeError
 *
 * This method can be used for non-numeric objects by
 * explicit <i>init</i> argument.
 *
 *   { 1 => 10, 2 => 20 }.sum([])                   #=> [1, 10, 2, 20]
 *   "a\nb\nc".each_line.lazy.map(&:chomp).sum("")  #=> "abc"
 *
 * If the method is applied to an Integer range without a block,
 * the sum is not done by iteration, but instead using Gauss's summation
 * formula.
 *
 * Enumerable#sum method may not respect method redefinition of "+"
 * methods such as Integer#+, or "each" methods such as Range#each.
 */
static VALUE
enum_sum(int argc, VALUE* argv, VALUE obj)
{
    struct enum_sum_memo memo;
    VALUE beg, end;
    int excl;

    memo.v = (rb_check_arity(argc, 0, 1) == 0) ? LONG2FIX(0) : argv[0];
    memo.block_given = rb_block_given_p();
    memo.n = 0;
    memo.r = Qundef;

    if ((memo.float_value = RB_FLOAT_TYPE_P(memo.v))) {
        memo.f = RFLOAT_VALUE(memo.v);
        memo.c = 0.0;
    }
    else {
        memo.f = 0.0;
        memo.c = 0.0;
    }

    if (RTEST(rb_range_values(obj, &beg, &end, &excl))) {
        if (!memo.block_given && !memo.float_value &&
                (FIXNUM_P(beg) || RB_TYPE_P(beg, T_BIGNUM)) &&
                (FIXNUM_P(end) || RB_TYPE_P(end, T_BIGNUM))) {
            return int_range_sum(beg, end, excl, memo.v);
        }
    }

    if (RB_TYPE_P(obj, T_HASH) &&
            rb_method_basic_definition_p(CLASS_OF(obj), id_each))
        hash_sum(obj, &memo);
    else
        rb_block_call(obj, id_each, 0, 0, enum_sum_i, (VALUE)&memo);

    if (memo.float_value) {
        return DBL2NUM(memo.f + memo.c);
    }
    else {
        if (memo.n != 0)
            memo.v = rb_fix_plus(LONG2FIX(memo.n), memo.v);
        if (memo.r != Qundef) {
            memo.v = rb_rational_plus(memo.r, memo.v);
        }
        return memo.v;
    }
}

static VALUE
uniq_func(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
    ENUM_WANT_SVALUE();
    rb_hash_add_new_element(hash, i, i);
    return Qnil;
}

static VALUE
uniq_iter(RB_BLOCK_CALL_FUNC_ARGLIST(i, hash))
{
    ENUM_WANT_SVALUE();
    rb_hash_add_new_element(hash, rb_yield_values2(argc, argv), i);
    return Qnil;
}

/*
 *  call-seq:
 *     enum.uniq                -> new_ary
 *     enum.uniq { |item| ... } -> new_ary
 *
 *  Returns a new array by removing duplicate values in +self+.
 *
 *  See also Array#uniq.
 */

static VALUE
enum_uniq(VALUE obj)
{
    VALUE hash, ret;
    rb_block_call_func *const func =
	rb_block_given_p() ? uniq_iter : uniq_func;

    hash = rb_obj_hide(rb_hash_new());
    rb_block_call(obj, id_each, 0, 0, func, hash);
    ret = rb_hash_values(hash);
    rb_hash_clear(hash);
    return ret;
}

/*
 *  The Enumerable mixin provides collection classes with several
 *  traversal and searching methods, and with the ability to sort. The
 *  class must provide a method #each, which yields
 *  successive members of the collection. If Enumerable#max, #min, or
 *  #sort is used, the objects in the collection must also implement a
 *  meaningful <code><=></code> operator, as these methods rely on an
 *  ordering between members of the collection.
 */

void
Init_Enumerable(void)
{
    rb_mEnumerable = rb_define_module("Enumerable");

    rb_define_method(rb_mEnumerable, "to_a", enum_to_a, -1);
    rb_define_method(rb_mEnumerable, "entries", enum_to_a, -1);
    rb_define_method(rb_mEnumerable, "to_h", enum_to_h, -1);

    rb_define_method(rb_mEnumerable, "sort", enum_sort, 0);
    rb_define_method(rb_mEnumerable, "sort_by", enum_sort_by, 0);
    rb_define_method(rb_mEnumerable, "grep", enum_grep, 1);
    rb_define_method(rb_mEnumerable, "grep_v", enum_grep_v, 1);
    rb_define_method(rb_mEnumerable, "count", enum_count, -1);
    rb_define_method(rb_mEnumerable, "find", enum_find, -1);
    rb_define_method(rb_mEnumerable, "detect", enum_find, -1);
    rb_define_method(rb_mEnumerable, "find_index", enum_find_index, -1);
    rb_define_method(rb_mEnumerable, "find_all", enum_find_all, 0);
    rb_define_method(rb_mEnumerable, "select", enum_find_all, 0);
    rb_define_method(rb_mEnumerable, "filter", enum_find_all, 0);
    rb_define_method(rb_mEnumerable, "filter_map", enum_filter_map, 0);
    rb_define_method(rb_mEnumerable, "reject", enum_reject, 0);
    rb_define_method(rb_mEnumerable, "collect", enum_collect, 0);
    rb_define_method(rb_mEnumerable, "map", enum_collect, 0);
    rb_define_method(rb_mEnumerable, "flat_map", enum_flat_map, 0);
    rb_define_method(rb_mEnumerable, "collect_concat", enum_flat_map, 0);
    rb_define_method(rb_mEnumerable, "inject", enum_inject, -1);
    rb_define_method(rb_mEnumerable, "reduce", enum_inject, -1);
    rb_define_method(rb_mEnumerable, "partition", enum_partition, 0);
    rb_define_method(rb_mEnumerable, "group_by", enum_group_by, 0);
    rb_define_method(rb_mEnumerable, "tally", enum_tally, 0);
    rb_define_method(rb_mEnumerable, "first", enum_first, -1);
    rb_define_method(rb_mEnumerable, "all?", enum_all, -1);
    rb_define_method(rb_mEnumerable, "any?", enum_any, -1);
    rb_define_method(rb_mEnumerable, "one?", enum_one, -1);
    rb_define_method(rb_mEnumerable, "none?", enum_none, -1);
    rb_define_method(rb_mEnumerable, "min", enum_min, -1);
    rb_define_method(rb_mEnumerable, "max", enum_max, -1);
    rb_define_method(rb_mEnumerable, "minmax", enum_minmax, 0);
    rb_define_method(rb_mEnumerable, "min_by", enum_min_by, -1);
    rb_define_method(rb_mEnumerable, "max_by", enum_max_by, -1);
    rb_define_method(rb_mEnumerable, "minmax_by", enum_minmax_by, 0);
    rb_define_method(rb_mEnumerable, "member?", enum_member, 1);
    rb_define_method(rb_mEnumerable, "include?", enum_member, 1);
    rb_define_method(rb_mEnumerable, "each_with_index", enum_each_with_index, -1);
    rb_define_method(rb_mEnumerable, "reverse_each", enum_reverse_each, -1);
    rb_define_method(rb_mEnumerable, "each_entry", enum_each_entry, -1);
    rb_define_method(rb_mEnumerable, "each_slice", enum_each_slice, 1);
    rb_define_method(rb_mEnumerable, "each_cons", enum_each_cons, 1);
    rb_define_method(rb_mEnumerable, "each_with_object", enum_each_with_object, 1);
    rb_define_method(rb_mEnumerable, "zip", enum_zip, -1);
    rb_define_method(rb_mEnumerable, "take", enum_take, 1);
    rb_define_method(rb_mEnumerable, "take_while", enum_take_while, 0);
    rb_define_method(rb_mEnumerable, "drop", enum_drop, 1);
    rb_define_method(rb_mEnumerable, "drop_while", enum_drop_while, 0);
    rb_define_method(rb_mEnumerable, "cycle", enum_cycle, -1);
    rb_define_method(rb_mEnumerable, "chunk", enum_chunk, 0);
    rb_define_method(rb_mEnumerable, "slice_before", enum_slice_before, -1);
    rb_define_method(rb_mEnumerable, "slice_after", enum_slice_after, -1);
    rb_define_method(rb_mEnumerable, "slice_when", enum_slice_when, 0);
    rb_define_method(rb_mEnumerable, "chunk_while", enum_chunk_while, 0);
    rb_define_method(rb_mEnumerable, "sum", enum_sum, -1);
    rb_define_method(rb_mEnumerable, "uniq", enum_uniq, 0);

    id_next = rb_intern_const("next");
}