pager.rb

# frozen_string_literal: true

# A pager is an `IO`-like object that accepts text and either prints it
# immediately, prints it one page at a time, or streams it to an external
# program to print one page at a time.
class Pry
  class Pager
    class StopPaging < StandardError
    end

    # @return [String] the 'less' executable and its options
    LESS_PAGER = 'less --RAW-CONTROL-CHARS --raw-control-chars -F -X'

    attr_reader :pry_instance

    def initialize(pry_instance)
      @pry_instance = pry_instance
    end

    # Send the given text through the best available pager (if
    # `Pry.config.pager` is enabled). If you want to send text through in
    # chunks as you generate it, use `open` to get a writable object
    # instead.
    #
    # @param [String] text
    #   Text to run through a pager.
    #
    def page(text)
      open do |pager|
        pager << text
      end
    end

    # Yields a pager object (`NullPager`, `SimplePager`, or `SystemPager`).
    # All pagers accept output with `#puts`, `#print`, `#write`, and `#<<`.
    def open
      pager = best_available
      yield pager
    rescue StopPaging # rubocop:disable Lint/HandleExceptions
    ensure
      pager.close if pager
    end

    private

    def enabled?
      !!@enabled
    end

    attr_reader :output

    # Return an instance of the "best" available pager class --
    # `SystemPager` if possible, `SimplePager` if `SystemPager` isn't
    # available, and `NullPager` if the user has disabled paging. All
    # pagers accept output with `#puts`, `#print`, `#write`, and `#<<`. You
    # must call `#close` when you're done writing output to a pager, and
    # you must rescue `Pry::Pager::StopPaging`. These requirements can be
    # avoided by using `.open` instead.
    def best_available
      if !pry_instance.config.pager
        NullPager.new(pry_instance.output)
      elsif !SystemPager.available? || Helpers::Platform.jruby?
        SimplePager.new(pry_instance.output)
      else
        SystemPager.new(pry_instance.output)
      end
    end

    # `NullPager` is a "pager" that actually just prints all output as it
    # comes in. Used when `Pry.config.pager` is false.
    class NullPager
      def initialize(out)
        @out = out
      end

      def puts(str)
        print "#{str.chomp}\n"
      end

      def print(str)
        write str
      end
      alias << print

      def write(str)
        @out.write str
      end

      def close; end

      private

      def height
        @height ||= @out.height
      end

      def width
        @width ||= @out.width
      end
    end

    # `SimplePager` is a straightforward pure-Ruby pager. We use it on
    # JRuby and when we can't find a usable external pager.
    class SimplePager < NullPager
      def initialize(*)
        super
        @tracker = PageTracker.new(height - 3, width)
      end

      def write(str)
        str.lines.each do |line|
          @out.print line
          @tracker.record line

          next unless @tracker.page?

          @out.print "\n"
          @out.print "\e[0m"
          @out.print "<page break> --- Press enter to continue " \
                     "( q<enter> to break ) --- <page break>\n"
          raise StopPaging if Readline.readline("").chomp == "q"

          @tracker.reset
        end
      end
    end

    # `SystemPager` buffers output until we're pretty sure it's at least a
    # page long, then invokes an external pager and starts streaming output
    # to it. If `#close` is called before then, it just prints out the
    # buffered content.
    class SystemPager < NullPager
      def self.default_pager
        pager = Pry::Env['PAGER'] || ''

        # Default to less.
        pager = LESS_PAGER if pager.strip.empty? || pager =~ /^less\b/

        pager
      end

      @system_pager = nil

      def self.available?
        if @system_pager.nil?
          @system_pager =
            begin
              pager_executable = default_pager.split(' ').first
              if Helpers::Platform.windows? || Helpers::Platform.windows_ansi?
                `where /Q #{pager_executable}`
              else
                `which #{pager_executable}`
              end
              $CHILD_STATUS.success?
            rescue StandardError
              false
            end
        else
          @system_pager
        end
      end

      def initialize(*)
        super
        @tracker = PageTracker.new(height, width)
        @buffer  = ""
        @pager   = nil
      end

      def write(str)
        if invoked_pager?
          write_to_pager str
        else
          @tracker.record str
          @buffer += str

          write_to_pager @buffer if @tracker.page?
        end
      rescue Errno::EPIPE
        raise StopPaging
      end

      def close
        if invoked_pager?
          pager.close
        else
          @out.puts @buffer
        end
      end

      private

      def write_to_pager(text)
        pager.write @out.decolorize_maybe(text)
      end

      def invoked_pager?
        @pager
      end

      def pager
        @pager ||= IO.popen(self.class.default_pager, 'w')
      end
    end

    # `PageTracker` tracks output to determine whether it's likely to take
    # up a whole page. This doesn't need to be super precise, but we can
    # use it for `SimplePager` and to avoid invoking the system pager
    # unnecessarily.
    #
    # One simplifying assumption is that we don't need `#page?` to return
    # `true` on the basis of an incomplete line. Long lines should be
    # counted as multiple lines, but we don't have to transition from
    # `false` to `true` until we see a newline.
    class PageTracker
      def initialize(rows, cols)
        @rows = rows
        @cols = cols
        reset
      end

      def record(str)
        str.lines.each do |line|
          if line.end_with? "\n"
            @row += ((@col + line_length(line) - 1) / @cols) + 1
            @col  = 0
          else
            @col += line_length(line)
          end
        end
      end

      def page?
        @row >= @rows
      end

      def reset
        @row = 0
        @col = 0
      end

      private

      # Approximation of the printable length of a given line, without the
      # newline and without ANSI color codes.
      def line_length(line)
        line.chomp.gsub(/\e\[[\d;]*m/, '').length
      end
    end
  end
end