# coding: utf-8

module PDF

  class Reader

    # A general class for decoding LZW compressed data. LZW can be
    # used in PDF files to compresses streams, usually for image data sourced
    # from a TIFF file.
    #
    # See the following links for more information:
    #
    #   ref http://www.fileformat.info/format/tiff/corion-lzw.htm
    #   ref http://marknelson.us/1989/10/01/lzw-data-compression/
    #
    # The PDF spec also has some data on the algorithm.
    #
    class LZW # :nodoc:

      # Wraps an LZW encoded string
      class BitStream # :nodoc:

        def initialize(data, bits_in_chunk)
          @data = data
          @data.force_encoding("BINARY")
          @bits_in_chunk = bits_in_chunk
          @current_pos = 0
          @bits_left_in_byte = 8
        end

        def set_bits_in_chunk(bits_in_chunk)
          @bits_in_chunk = bits_in_chunk
        end

        def read
          bits_left_in_chunk = @bits_in_chunk
          chunk = nil
          while bits_left_in_chunk > 0 and @current_pos < @data.size
            chunk = 0 if chunk.nil?
            codepoint = @data[@current_pos, 1].unpack("C*")[0]
            current_byte = codepoint & (2**@bits_left_in_byte - 1) #clear consumed bits
            dif = bits_left_in_chunk - @bits_left_in_byte
            if dif > 0 then  current_byte <<= dif
            elsif dif < 0 then  current_byte >>= dif.abs
            end
            chunk |= current_byte #add bits to result
            bits_left_in_chunk = if dif >= 0 then dif else 0 end
            @bits_left_in_byte = if dif < 0 then dif.abs else 0 end
            if @bits_left_in_byte.zero? #next byte
              @current_pos += 1
              @bits_left_in_byte = 8
            end
          end
          chunk
        end
      end

      CODE_EOD = 257 #end of data
      CODE_CLEAR_TABLE = 256 #clear table

      # stores de pairs code => string
      class StringTable < Hash # :nodoc:
        attr_reader :string_table_pos

        def initialize
          super
          @string_table_pos = 258 #initial code
        end

        #if code less than 258 return fixed string
        def [](key)
          if key > 257 then super else key.chr end
        end

        def add(string)
          store(@string_table_pos, string)
          @string_table_pos += 1
        end
      end

      # Decompresses a LZW compressed string.
      #
      def self.decode(data)
        stream = BitStream.new data.to_s, 9 # size of codes between 9 and 12 bits
        result = ''
        until (code = stream.read) == CODE_EOD
          if code == CODE_CLEAR_TABLE
            stream.set_bits_in_chunk(9)
            string_table = StringTable.new
            code = stream.read
            break if code == CODE_EOD
            result << string_table[code]
            old_code = code
          else
            string = string_table[code]
            if string
              result << string
              string_table.add create_new_string(string_table, old_code, code)
              old_code = code
            else
              new_string = create_new_string(string_table, old_code, old_code)
              result << new_string
              string_table.add new_string
              old_code = code
            end
            #increase de size of the codes when limit reached
            if string_table.string_table_pos == 511
              stream.set_bits_in_chunk(10)
            elsif string_table.string_table_pos == 1023
              stream.set_bits_in_chunk(11)
            elsif string_table.string_table_pos == 2047
              stream.set_bits_in_chunk(12)
            end
          end
        end
        result
      end

      private

      def self.create_new_string(string_table,some_code, other_code)
        string_table[some_code] + string_table[other_code][0].chr
      end

    end
  end
end