module ChunkyPNG::Canvas::PNGEncoding

def determine_png_encoding(constraints = {})
  encoding = case constraints
    when :fast_rgb         then {color_mode: ChunkyPNG::COLOR_TRUECOLOR, compression: Zlib::BEST_SPEED}
    when :fast_rgba        then {color_mode: ChunkyPNG::COLOR_TRUECOLOR_ALPHA, compression: Zlib::BEST_SPEED}
    when :best_compression then {compression: Zlib::BEST_COMPRESSION, filtering: ChunkyPNG::FILTER_PAETH}
    when :good_compression then {compression: Zlib::BEST_COMPRESSION, filtering: ChunkyPNG::FILTER_NONE}
    when :no_compression   then {compression: Zlib::NO_COMPRESSION}
    when :black_and_white  then {color_mode: ChunkyPNG::COLOR_GRAYSCALE, bit_depth: 1}
    when Hash              then constraints
    else raise ChunkyPNG::Exception, "Unknown encoding preset: #{constraints.inspect}"
  end
  # Do not create a palette when the encoding is given and does not require a palette.
  if encoding[:color_mode]
    if encoding[:color_mode] == ChunkyPNG::COLOR_INDEXED
      self.encoding_palette = palette
      encoding[:bit_depth] ||= encoding_palette.determine_bit_depth
    else
      encoding[:bit_depth] ||= 8
    end
  else
    self.encoding_palette = palette
    suggested_color_mode, suggested_bit_depth = encoding_palette.best_color_settings
    encoding[:color_mode] ||= suggested_color_mode
    encoding[:bit_depth]  ||= suggested_bit_depth
  end
  # Use Zlib's default for compression unless otherwise provided.
  encoding[:compression] ||= Zlib::DEFAULT_COMPRESSION
  encoding[:interlace] = case encoding[:interlace]
    when nil, false then ChunkyPNG::INTERLACING_NONE
    when true then ChunkyPNG::INTERLACING_ADAM7
    else encoding[:interlace]
  end
  encoding[:filtering] ||= case encoding[:compression]
    when Zlib::BEST_COMPRESSION then ChunkyPNG::FILTER_PAETH
    when Zlib::NO_COMPRESSION..Zlib::BEST_SPEED then ChunkyPNG::FILTER_NONE
    else ChunkyPNG::FILTER_UP
  end
  encoding
end

def encode_png_image_pass_to_stream(stream, color_mode, bit_depth, filtering)
  start_pos  = stream.bytesize
  pixel_size = Color.pixel_bytesize(color_mode)
  line_width = Color.scanline_bytesize(color_mode, bit_depth, width)
  # Determine the filter method
  encode_method = encode_png_pixels_to_scanline_method(color_mode, bit_depth)
  filter_method = case filtering
    when ChunkyPNG::FILTER_NONE    then nil
    when ChunkyPNG::FILTER_SUB     then :encode_png_str_scanline_sub
    when ChunkyPNG::FILTER_UP      then :encode_png_str_scanline_up
    when ChunkyPNG::FILTER_AVERAGE then :encode_png_str_scanline_average
    when ChunkyPNG::FILTER_PAETH   then :encode_png_str_scanline_paeth
    else raise ArgumentError, "Filtering method #{filtering} is not supported"
  end
  0.upto(height - 1) do |y|
    stream << send(encode_method, row(y))
  end
  # Now, apply filtering if any
  if filter_method
    (height - 1).downto(0) do |y|
      pos = start_pos + y * (line_width + 1)
      prev_pos = y == 0 ? nil : pos - (line_width + 1)
      send(filter_method, stream, pos, prev_pos, line_width, pixel_size)
    end
  end
end

def encode_png_image_with_interlacing(color_mode, bit_depth = 8, filtering = ChunkyPNG::FILTER_NONE)
  stream = "".b
  0.upto(6) do |pass|
    subcanvas = self.class.adam7_extract_pass(pass, self)
    subcanvas.encoding_palette = encoding_palette
    subcanvas.encode_png_image_pass_to_stream(stream, color_mode, bit_depth, filtering)
  end
  stream
end

def encode_png_image_without_interlacing(color_mode, bit_depth = 8, filtering = ChunkyPNG::FILTER_NONE)
  stream = "".b
  encode_png_image_pass_to_stream(stream, color_mode, bit_depth, filtering)
  stream
end

def encode_png_pixels_to_scanline_grayscale_1bit(pixels)
  chars = []
  pixels.each_slice(8) do |p1, p2, p3, p4, p5, p6, p7, p8|
    chars << ((p1.nil? ? 0 : (p1 & 0x0000ffff) >> 15 << 7) |
              (p2.nil? ? 0 : (p2 & 0x0000ffff) >> 15 << 6) |
              (p3.nil? ? 0 : (p3 & 0x0000ffff) >> 15 << 5) |
              (p4.nil? ? 0 : (p4 & 0x0000ffff) >> 15 << 4) |
              (p5.nil? ? 0 : (p5 & 0x0000ffff) >> 15 << 3) |
              (p6.nil? ? 0 : (p6 & 0x0000ffff) >> 15 << 2) |
              (p7.nil? ? 0 : (p7 & 0x0000ffff) >> 15 << 1) |
              (p8.nil? ? 0 : (p8 & 0x0000ffff) >> 15))
  end
  chars.pack("xC*")
end

def encode_png_pixels_to_scanline_grayscale_2bit(pixels)
  chars = []
  pixels.each_slice(4) do |p1, p2, p3, p4|
    chars << ((p1.nil? ? 0 : (p1 & 0x0000ffff) >> 14 << 6) |
              (p2.nil? ? 0 : (p2 & 0x0000ffff) >> 14 << 4) |
              (p3.nil? ? 0 : (p3 & 0x0000ffff) >> 14 << 2) |
              (p4.nil? ? 0 : (p4 & 0x0000ffff) >> 14))
  end
  chars.pack("xC*")
end

def encode_png_pixels_to_scanline_grayscale_4bit(pixels)
  chars = []
  pixels.each_slice(2) do |p1, p2|
    chars << ((p1.nil? ? 0 : ((p1 & 0x0000ffff) >> 12) << 4) | (p2.nil? ? 0 : ((p2 & 0x0000ffff) >> 12)))
  end
  chars.pack("xC*")
end

def encode_png_pixels_to_scanline_grayscale_8bit(pixels)
  pixels.map { |p| p >> 8 }.pack("xC#{width}")
end

def encode_png_pixels_to_scanline_grayscale_alpha_8bit(pixels)
  pixels.pack("xn#{width}")
end

def encode_png_pixels_to_scanline_indexed_1bit(pixels)
  chars = []
  pixels.each_slice(8) do |p1, p2, p3, p4, p5, p6, p7, p8|
    chars << (
      (encoding_palette.index(p1) << 7) |
      (encoding_palette.index(p2) << 6) |
      (encoding_palette.index(p3) << 5) |
      (encoding_palette.index(p4) << 4) |
      (encoding_palette.index(p5) << 3) |
      (encoding_palette.index(p6) << 2) |
      (encoding_palette.index(p7) << 1) |
      encoding_palette.index(p8)
    )
  end
  chars.pack("xC*")
end

def encode_png_pixels_to_scanline_indexed_2bit(pixels)
  chars = []
  pixels.each_slice(4) do |p1, p2, p3, p4|
    chars << (
      (encoding_palette.index(p1) << 6) |
      (encoding_palette.index(p2) << 4) |
      (encoding_palette.index(p3) << 2) |
      encoding_palette.index(p4)
    )
  end
  chars.pack("xC*")
end

def encode_png_pixels_to_scanline_indexed_4bit(pixels)
  chars = []
  pixels.each_slice(2) do |p1, p2|
    chars << ((encoding_palette.index(p1) << 4) | encoding_palette.index(p2))
  end
  chars.pack("xC*")
end

def encode_png_pixels_to_scanline_indexed_8bit(pixels)
  pixels.map { |p| encoding_palette.index(p) }.pack("xC#{width}")
end

def encode_png_pixels_to_scanline_method(color_mode, depth)
  encoder_method = case color_mode
    when ChunkyPNG::COLOR_TRUECOLOR       then :"encode_png_pixels_to_scanline_truecolor_#{depth}bit"
    when ChunkyPNG::COLOR_TRUECOLOR_ALPHA then :"encode_png_pixels_to_scanline_truecolor_alpha_#{depth}bit"
    when ChunkyPNG::COLOR_INDEXED         then :"encode_png_pixels_to_scanline_indexed_#{depth}bit"
    when ChunkyPNG::COLOR_GRAYSCALE       then :"encode_png_pixels_to_scanline_grayscale_#{depth}bit"
    when ChunkyPNG::COLOR_GRAYSCALE_ALPHA then :"encode_png_pixels_to_scanline_grayscale_alpha_#{depth}bit"
  end
  raise ChunkyPNG::NotSupported, "No encoder found for color mode #{color_mode} and #{depth}-bit depth!" unless respond_to?(encoder_method, true)
  encoder_method
end

def encode_png_pixels_to_scanline_truecolor_8bit(pixels)
  pixels.pack("x" + ("NX" * width))
end

def encode_png_pixels_to_scanline_truecolor_alpha_8bit(pixels)
  pixels.pack("xN#{width}")
end

def encode_png_pixelstream(color_mode = ChunkyPNG::COLOR_TRUECOLOR, bit_depth = 8, interlace = ChunkyPNG::INTERLACING_NONE, filtering = ChunkyPNG::FILTER_NONE)
  if color_mode == ChunkyPNG::COLOR_INDEXED
    raise ChunkyPNG::ExpectationFailed, "This palette is not suitable for encoding!" if encoding_palette.nil? || !encoding_palette.can_encode?
    raise ChunkyPNG::ExpectationFailed, "This palette has too many colors!" if encoding_palette.size > (1 << bit_depth)
  end
  case interlace
    when ChunkyPNG::INTERLACING_NONE  then encode_png_image_without_interlacing(color_mode, bit_depth, filtering)
    when ChunkyPNG::INTERLACING_ADAM7 then encode_png_image_with_interlacing(color_mode, bit_depth, filtering)
    else raise ChunkyPNG::NotSupported, "Unknown interlacing method: #{interlace}!"
  end
end

def encode_png_str_scanline_average(stream, pos, prev_pos, line_width, pixel_size)
  line_width.downto(1) do |i|
    a = i > pixel_size ? stream.getbyte(pos + i - pixel_size) : 0
    b = prev_pos ? stream.getbyte(prev_pos + i) : 0
    stream.setbyte(pos + i, (stream.getbyte(pos + i) - ((a + b) >> 1)) & 0xff)
  end
  stream.setbyte(pos, ChunkyPNG::FILTER_AVERAGE)
end

def encode_png_str_scanline_none(stream, pos, prev_pos, line_width, pixel_size)
  # noop - this method shouldn't get called at all.
end

def encode_png_str_scanline_paeth(stream, pos, prev_pos, line_width, pixel_size)
  line_width.downto(1) do |i|
    a = i > pixel_size ? stream.getbyte(pos + i - pixel_size) : 0
    b = prev_pos ? stream.getbyte(prev_pos + i) : 0
    c = prev_pos && i > pixel_size ? stream.getbyte(prev_pos + i - pixel_size) : 0
    p = a + b - c
    pa = (p - a).abs
    pb = (p - b).abs
    pc = (p - c).abs
    pr = if pa <= pb && pa <= pc
      a
    else
      pb <= pc ? b : c
    end
    stream.setbyte(pos + i, (stream.getbyte(pos + i) - pr) & 0xff)
  end
  stream.setbyte(pos, ChunkyPNG::FILTER_PAETH)
end

def encode_png_str_scanline_sub(stream, pos, prev_pos, line_width, pixel_size)
  line_width.downto(1) do |i|
    a = i > pixel_size ? stream.getbyte(pos + i - pixel_size) : 0
    stream.setbyte(pos + i, (stream.getbyte(pos + i) - a) & 0xff)
  end
  stream.setbyte(pos, ChunkyPNG::FILTER_SUB)
end

def encode_png_str_scanline_up(stream, pos, prev_pos, line_width, pixel_size)
  line_width.downto(1) do |i|
    b = prev_pos ? stream.getbyte(prev_pos + i) : 0
    stream.setbyte(pos + i, (stream.getbyte(pos + i) - b) & 0xff)
  end
  stream.setbyte(pos, ChunkyPNG::FILTER_UP)
end

def save(filename, constraints = {})
  File.open(filename, "wb") { |io| write(io, constraints) }
end

def to_blob(constraints = {})
  to_datastream(constraints).to_blob
end

def to_datastream(constraints = {})
  encoding = determine_png_encoding(constraints)
  ds = Datastream.new
  ds.header_chunk = Chunk::Header.new(
    width: width,
    height: height,
    color: encoding[:color_mode],
    depth: encoding[:bit_depth],
    interlace: encoding[:interlace]
  )
  if encoding[:color_mode] == ChunkyPNG::COLOR_INDEXED
    ds.palette_chunk      = encoding_palette.to_plte_chunk
    ds.transparency_chunk = encoding_palette.to_trns_chunk unless encoding_palette.opaque?
  end
  data           = encode_png_pixelstream(encoding[:color_mode], encoding[:bit_depth], encoding[:interlace], encoding[:filtering])
  ds.data_chunks = Chunk::ImageData.split_in_chunks(data, encoding[:compression])
  ds.end_chunk   = Chunk::End.new
  ds
end

def write(io, constraints = {})
  to_datastream(constraints).write(io)
end