module HexaPDF::Filter::ASCII85Decode

def self.decoder(source, _ = nil)
  Fiber.new do
    rest = nil
    finished = false
    while !finished && source.alive? && (data = source.resume)
      data.tr!(HexaPDF::Tokenizer::WHITESPACE, '')
      if data.index(/[^!-uz~]/)
        raise FilterError, "Invalid characters in ASCII85 stream"
      end
      if rest
        data = rest << data
        rest = nil
      end
      result = []
      scanner = StringScanner.new(data)
      until scanner.eos?
        if (m = scanner.scan(/[!-u]{5}/))
          num = (m.getbyte(0) * POW85_4 + m.getbyte(1) * POW85_3 +
            m.getbyte(2) * POW85_2 + m.getbyte(3) * POW85_1 +
            m.getbyte(4)) - FIXED_SUBTRAHEND
          if num > MAX_VALUE
            raise FilterError, "Value outside range in ASCII85 stream"
          end
          result << num
        elsif scanner.scan(/z/)
          result << 0
        elsif scanner.scan(/([!-u]{0,4})~>/)
          rest = scanner[1] unless scanner[1].empty?
          finished = true
          break
        else
          rest = scanner.scan(/.+/)
        end
      end
      Fiber.yield(result.pack('N*')) unless result.empty?
    end
    if rest
      if rest.index('z') || rest.index('~')
        raise FilterError, "End of ASCII85 encoded stream is invalid"
      end
      rlen = rest.length
      rest << "u" * (5 - rlen)
      num = (rest.getbyte(0) * POW85_4 + rest.getbyte(1) * POW85_3 +
        rest.getbyte(2) * POW85_2 + rest.getbyte(3) * POW85_1 +
        rest.getbyte(4)) - FIXED_SUBTRAHEND
      if num > MAX_VALUE
        raise FilterError, "Value outside base-85 range in ASCII85 stream"
      end
      [num].pack('N')[0, rlen - 1]
    end
  end
end

def self.encoder(source, _ = nil)
  Fiber.new do
    rest = nil
    while source.alive? && (data = source.resume)
      data = rest << data if rest
      rlen = data.length % 4
      rest = (rlen != 0 ? data.slice!(-rlen, rlen) : nil)
      next if data.length < 4
      data = data.unpack('N*').inject(''.b) do |memo, num|
        memo << if num == 0
                  'z'
                else
                  VALUE_TO_CHAR[num / POW85_4 % 85] + VALUE_TO_CHAR[num / POW85_3 % 85] <<
                    VALUE_TO_CHAR[num / POW85_2 % 85] << VALUE_TO_CHAR[num / POW85_1 % 85] <<
                    VALUE_TO_CHAR[num % 85]
                end
      end
      Fiber.yield(data)
    end
    if rest
      rlen = rest.length
      num = (rest + "\0" * (4 - rlen)).unpack1('N')
      ((VALUE_TO_CHAR[num / POW85_4 % 85] + VALUE_TO_CHAR[num / POW85_3 % 85] <<
        VALUE_TO_CHAR[num / POW85_2 % 85] << VALUE_TO_CHAR[num / POW85_1 % 85] <<
        VALUE_TO_CHAR[num % 85])[0, rlen + 1] << "~>").force_encoding(Encoding::BINARY)
    else
      "~>".b
    end
  end
end