index.js

var TINF_OK = 0;
var TINF_DATA_ERROR = -3;

function Tree() {
  this.table = new Uint16Array(16);   /* table of code length counts */
  this.trans = new Uint16Array(288);  /* code -> symbol translation table */
}

function Data(source, dest) {
  this.source = source;
  this.sourceIndex = 0;
  this.tag = 0;
  this.bitcount = 0;
  
  this.dest = dest;
  this.destLen = 0;
  
  this.ltree = new Tree();  /* dynamic length/symbol tree */
  this.dtree = new Tree();  /* dynamic distance tree */
}

/* --------------------------------------------------- *
 * -- uninitialized global data (static structures) -- *
 * --------------------------------------------------- */

var sltree = new Tree();
var sdtree = new Tree();

/* extra bits and base tables for length codes */
var length_bits = new Uint8Array(30);
var length_base = new Uint16Array(30);

/* extra bits and base tables for distance codes */
var dist_bits = new Uint8Array(30);
var dist_base = new Uint16Array(30);

/* special ordering of code length codes */
var clcidx = new Uint8Array([
  16, 17, 18, 0, 8, 7, 9, 6,
  10, 5, 11, 4, 12, 3, 13, 2,
  14, 1, 15
]);

/* used by tinf_decode_trees, avoids allocations every call */
var code_tree = new Tree();
var lengths = new Uint8Array(288 + 32);

/* ----------------------- *
 * -- utility functions -- *
 * ----------------------- */

/* build extra bits and base tables */
function tinf_build_bits_base(bits, base, delta, first) {
  var i, sum;

  /* build bits table */
  for (i = 0; i < delta; ++i) bits[i] = 0;
  for (i = 0; i < 30 - delta; ++i) bits[i + delta] = i / delta | 0;

  /* build base table */
  for (sum = first, i = 0; i < 30; ++i) {
    base[i] = sum;
    sum += 1 << bits[i];
  }
}

/* build the fixed huffman trees */
function tinf_build_fixed_trees(lt, dt) {
  var i;

  /* build fixed length tree */
  for (i = 0; i < 7; ++i) lt.table[i] = 0;

  lt.table[7] = 24;
  lt.table[8] = 152;
  lt.table[9] = 112;

  for (i = 0; i < 24; ++i) lt.trans[i] = 256 + i;
  for (i = 0; i < 144; ++i) lt.trans[24 + i] = i;
  for (i = 0; i < 8; ++i) lt.trans[24 + 144 + i] = 280 + i;
  for (i = 0; i < 112; ++i) lt.trans[24 + 144 + 8 + i] = 144 + i;

  /* build fixed distance tree */
  for (i = 0; i < 5; ++i) dt.table[i] = 0;

  dt.table[5] = 32;

  for (i = 0; i < 32; ++i) dt.trans[i] = i;
}

/* given an array of code lengths, build a tree */
var offs = new Uint16Array(16);

function tinf_build_tree(t, lengths, off, num) {
  var i, sum;

  /* clear code length count table */
  for (i = 0; i < 16; ++i) t.table[i] = 0;

  /* scan symbol lengths, and sum code length counts */
  for (i = 0; i < num; ++i) t.table[lengths[off + i]]++;

  t.table[0] = 0;

  /* compute offset table for distribution sort */
  for (sum = 0, i = 0; i < 16; ++i) {
    offs[i] = sum;
    sum += t.table[i];
  }

  /* create code->symbol translation table (symbols sorted by code) */
  for (i = 0; i < num; ++i) {
    if (lengths[off + i]) t.trans[offs[lengths[off + i]]++] = i;
  }
}

/* ---------------------- *
 * -- decode functions -- *
 * ---------------------- */

/* get one bit from source stream */
function tinf_getbit(d) {
  /* check if tag is empty */
  if (!d.bitcount--) {
    /* load next tag */
    d.tag = d.source[d.sourceIndex++];
    d.bitcount = 7;
  }

  /* shift bit out of tag */
  var bit = d.tag & 1;
  d.tag >>>= 1;

  return bit;
}

/* read a num bit value from a stream and add base */
function tinf_read_bits(d, num, base) {
  if (!num)
    return base;

  while (d.bitcount < 24) {
    d.tag |= d.source[d.sourceIndex++] << d.bitcount;
    d.bitcount += 8;
  }

  var val = d.tag & (0xffff >>> (16 - num));
  d.tag >>>= num;
  d.bitcount -= num;
  return val + base;
}

/* given a data stream and a tree, decode a symbol */
function tinf_decode_symbol(d, t) {
  while (d.bitcount < 24) {
    d.tag |= d.source[d.sourceIndex++] << d.bitcount;
    d.bitcount += 8;
  }
  
  var sum = 0, cur = 0, len = 0;
  var tag = d.tag;

  /* get more bits while code value is above sum */
  do {
    cur = 2 * cur + (tag & 1);
    tag >>>= 1;
    ++len;

    sum += t.table[len];
    cur -= t.table[len];
  } while (cur >= 0);
  
  d.tag = tag;
  d.bitcount -= len;

  return t.trans[sum + cur];
}

/* given a data stream, decode dynamic trees from it */
function tinf_decode_trees(d, lt, dt) {
  var hlit, hdist, hclen;
  var i, num, length;

  /* get 5 bits HLIT (257-286) */
  hlit = tinf_read_bits(d, 5, 257);

  /* get 5 bits HDIST (1-32) */
  hdist = tinf_read_bits(d, 5, 1);

  /* get 4 bits HCLEN (4-19) */
  hclen = tinf_read_bits(d, 4, 4);

  for (i = 0; i < 19; ++i) lengths[i] = 0;

  /* read code lengths for code length alphabet */
  for (i = 0; i < hclen; ++i) {
    /* get 3 bits code length (0-7) */
    var clen = tinf_read_bits(d, 3, 0);
    lengths[clcidx[i]] = clen;
  }

  /* build code length tree */
  tinf_build_tree(code_tree, lengths, 0, 19);

  /* decode code lengths for the dynamic trees */
  for (num = 0; num < hlit + hdist;) {
    var sym = tinf_decode_symbol(d, code_tree);

    switch (sym) {
      case 16:
        /* copy previous code length 3-6 times (read 2 bits) */
        var prev = lengths[num - 1];
        for (length = tinf_read_bits(d, 2, 3); length; --length) {
          lengths[num++] = prev;
        }
        break;
      case 17:
        /* repeat code length 0 for 3-10 times (read 3 bits) */
        for (length = tinf_read_bits(d, 3, 3); length; --length) {
          lengths[num++] = 0;
        }
        break;
      case 18:
        /* repeat code length 0 for 11-138 times (read 7 bits) */
        for (length = tinf_read_bits(d, 7, 11); length; --length) {
          lengths[num++] = 0;
        }
        break;
      default:
        /* values 0-15 represent the actual code lengths */
        lengths[num++] = sym;
        break;
    }
  }

  /* build dynamic trees */
  tinf_build_tree(lt, lengths, 0, hlit);
  tinf_build_tree(dt, lengths, hlit, hdist);
}

/* ----------------------------- *
 * -- block inflate functions -- *
 * ----------------------------- */

/* given a stream and two trees, inflate a block of data */
function tinf_inflate_block_data(d, lt, dt) {
  while (1) {
    var sym = tinf_decode_symbol(d, lt);

    /* check for end of block */
    if (sym === 256) {
      return TINF_OK;
    }

    if (sym < 256) {
      d.dest[d.destLen++] = sym;
    } else {
      var length, dist, offs;
      var i;

      sym -= 257;

      /* possibly get more bits from length code */
      length = tinf_read_bits(d, length_bits[sym], length_base[sym]);

      dist = tinf_decode_symbol(d, dt);

      /* possibly get more bits from distance code */
      offs = d.destLen - tinf_read_bits(d, dist_bits[dist], dist_base[dist]);

      /* copy match */
      for (i = offs; i < offs + length; ++i) {
        d.dest[d.destLen++] = d.dest[i];
      }
    }
  }
}

/* inflate an uncompressed block of data */
function tinf_inflate_uncompressed_block(d) {
  var length, invlength;
  var i;
  
  /* unread from bitbuffer */
  while (d.bitcount > 8) {
    d.sourceIndex--;
    d.bitcount -= 8;
  }

  /* get length */
  length = d.source[d.sourceIndex + 1];
  length = 256 * length + d.source[d.sourceIndex];

  /* get one's complement of length */
  invlength = d.source[d.sourceIndex + 3];
  invlength = 256 * invlength + d.source[d.sourceIndex + 2];

  /* check length */
  if (length !== (~invlength & 0x0000ffff))
    return TINF_DATA_ERROR;

  d.sourceIndex += 4;

  /* copy block */
  for (i = length; i; --i)
    d.dest[d.destLen++] = d.source[d.sourceIndex++];

  /* make sure we start next block on a byte boundary */
  d.bitcount = 0;

  return TINF_OK;
}

/* inflate stream from source to dest */
function tinf_uncompress(source, dest) {
  var d = new Data(source, dest);
  var bfinal, btype, res;

  do {
    /* read final block flag */
    bfinal = tinf_getbit(d);

    /* read block type (2 bits) */
    btype = tinf_read_bits(d, 2, 0);

    /* decompress block */
    switch (btype) {
      case 0:
        /* decompress uncompressed block */
        res = tinf_inflate_uncompressed_block(d);
        break;
      case 1:
        /* decompress block with fixed huffman trees */
        res = tinf_inflate_block_data(d, sltree, sdtree);
        break;
      case 2:
        /* decompress block with dynamic huffman trees */
        tinf_decode_trees(d, d.ltree, d.dtree);
        res = tinf_inflate_block_data(d, d.ltree, d.dtree);
        break;
      default:
        res = TINF_DATA_ERROR;
    }

    if (res !== TINF_OK)
      throw new Error('Data error');

  } while (!bfinal);

  if (d.destLen < d.dest.length) {
    if (typeof d.dest.slice === 'function')
      return d.dest.slice(0, d.destLen);
    else
      return d.dest.subarray(0, d.destLen);
  }
  
  return d.dest;
}

/* -------------------- *
 * -- initialization -- *
 * -------------------- */

/* build fixed huffman trees */
tinf_build_fixed_trees(sltree, sdtree);

/* build extra bits and base tables */
tinf_build_bits_base(length_bits, length_base, 4, 3);
tinf_build_bits_base(dist_bits, dist_base, 2, 1);

/* fix a special case */
length_bits[28] = 0;
length_base[28] = 258;

module.exports = tinf_uncompress;