class PDF::Reader::Filter::Depredict

def filter(data)
  predictor = @options[:Predictor].to_i
  case predictor
  when 0, 1 then
    data
  when 2    then
    tiff_depredict(data)
  when 10, 11, 12, 13, 14, 15 then
    png_depredict(data)
  else
    raise  MalformedPDFError, "Unrecognised predictor value (#{predictor})"
  end
end

def initialize(options = {})
  @options = options
end

def png_depredict(data)
  return data if @options[:Predictor].to_i < 10
  data = data.unpack("C*")
  pixel_bytes     = @options[:Colors] || 1
  scanline_length = (pixel_bytes * @options[:Columns]) + 1
  row = 0
  pixels = []
  paeth, pa, pb, pc = 0, 0, 0, 0
  until data.empty? do
    row_data = data.slice! 0, scanline_length
    filter = row_data.shift
    case filter
    when 0 # None
    when 1 # Sub
      row_data.each_with_index do |byte, index|
        left = index < pixel_bytes ? 0 : row_data[index - pixel_bytes]
        row_data[index] = (byte + left) % 256
        #p [byte, left, row_data[index]]
      end
    when 2 # Up
      row_data.each_with_index do |byte, index|
        col = index / pixel_bytes
        upper = row == 0 ? 0 : pixels[row-1][col][index % pixel_bytes]
        row_data[index] = (upper + byte) % 256
      end
    when 3  # Average
      row_data.each_with_index do |byte, index|
        col = index / pixel_bytes
        upper = row == 0 ? 0 : pixels[row-1][col][index % pixel_bytes]
        left = index < pixel_bytes ? 0 : row_data[index - pixel_bytes]
        row_data[index] = (byte + ((left + upper)/2).floor) % 256
      end
    when 4 # Paeth
      left = upper = upper_left = 0
      row_data.each_with_index do |byte, index|
        col = index / pixel_bytes
        left = index < pixel_bytes ? 0 : Integer(row_data[index - pixel_bytes])
        if row.zero?
          upper = upper_left = 0
        else
          upper = Integer(pixels[row-1][col][index % pixel_bytes])
          upper_left = col.zero? ? 0 :
            Integer(pixels[row-1][col-1][index % pixel_bytes])
        end
        p = left + upper - upper_left
        pa = (p - left).abs
        pb = (p - upper).abs
        pc = (p - upper_left).abs
        paeth = if pa <= pb && pa <= pc
                  left
                elsif pb <= pc
                  upper
                else
                  upper_left
                end
        row_data[index] = (byte + paeth) % 256
      end
    else
      raise MalformedPDFError, "Invalid filter algorithm #{filter}"
    end
    s = []
    row_data.each_slice pixel_bytes do |slice|
      s << slice
    end
    pixels << s
    row += 1
  end
  pixels.map { |bytes| bytes.flatten.pack("C*") }.join("")
end

def tiff_depredict(data)
  data        = data.unpack("C*")
  unfiltered  = ''
  bpc         = @options[:BitsPerComponent] || 8
  pixel_bits  = bpc * @options[:Colors]
  pixel_bytes = pixel_bits / 8
  line_len    = (pixel_bytes * @options[:Columns])
  pos         = 0
  if bpc != 8
    raise UnsupportedFeatureError, "TIFF predictor onlys supports 8 Bits Per Component"
  end
  until pos > data.size
    row_data = data[pos, line_len]
    row_data.each_with_index do |byte, index|
      left = index < pixel_bytes ? 0 : row_data[index - pixel_bytes]
      row_data[index] = (byte + left) % 256
    end
    unfiltered += row_data.pack("C*")
    pos += line_len
  end
  unfiltered
end