module HexaPDF::Filter::RunLengthDecode

def self.decoder(source, _ = nil)
  Fiber.new do
    i = 0
    result = ''.b
    data = source.resume
    while data && i < data.length
      length = data.getbyte(i)
      if length < 128 && i + length + 1 < data.length # no byte run and enough bytes
        result << data[i + 1, length + 1]
        i += length + 2
      elsif length > 128 && i + 1 < data.length # byte run and enough bytes
        result << data[i + 1] * (257 - length)
        i += 2
      elsif length != 128 # not enough bytes in data
        Fiber.yield(result)
        if source.alive? && (new_data = source.resume)
          data = data[i..-1] << new_data
        else
          raise FilterError, "Missing data for run length encoded stream"
        end
        i = 0
        result = ''.b
      else # EOD reached
        break
      end
      if i == data.length && source.alive? && (data = source.resume)
        Fiber.yield(result)
        i = 0
        result = ''.b
      end
    end
    result unless result.empty?
  end
end

def self.encoder(source, _ = nil)
  Fiber.new do
    while source.alive? && (data = source.resume)
      result = ''.b
      strscan = StringScanner.new(data)
      until strscan.eos?
        if strscan.scan(/(.)\1{1,127}/m) # a run of <= 128 same characters
          result << (257 - strscan.matched_size).chr << strscan[1]
        else # a run of characters until two same characters or length > 128
          match = strscan.scan(/.{1,128}?(?=(.)\1|\z)|.{128}/m)
          result << (match.length - 1).chr << match
        end
      end
      Fiber.yield(result)
    end
    EOD
  end
end