module ChunkyPNG

  # A palette describes the set of colors that is being used for an image.
  #
  # A PNG image can contain an explicit palette which defines the colors of that image,
  # but can also use an implicit palette, e.g. all truecolor colors or all grayscale colors.
  #
  # This palette supports decoding colors from a palette if an explicit palette is provided 
  # in a PNG datastream, and it supports encoding colors to an explicit matrix.
  #
  # @see ChunkyPNG::Pixel
  class Palette < SortedSet
    
    # Builds a new palette given a set (Enumerable instance) of colors.
    #
    # @param [Enumerbale<ChunkyPNG::Pixel>] enum The set of colors to include in this palette.
    #   This Enumerbale can contains duplicates.
    # @param [Array] decoding_map An array of colors in the exact order at which 
    #   they appeared in the palette chunk, so that this array can be used for decoding.
    def initialize(enum, decoding_map = nil)
      super(enum)
      @decoding_map = decoding_map if decoding_map
    end
    
    # Builds a palette instance from a PLTE chunk and optionally a tRNS chunk 
    # from a PNG datastream.
    #
    # This method will cerate a palette that is suitable for decoding an image.
    #
    # @param [ChunkyPNG::Chunk::Palette] The palette chunk to load from
    # @param [ChunkyPNG::Chunk::Transparency, nil] The optional transparency chunk.
    # @return [ChunkyPNG::Palette] The loaded palette instance. 
    # @see ChunkyPNG::Palette#can_decode?
    def self.from_chunks(palette_chunk, transparency_chunk = nil)
      return nil if palette_chunk.nil?
      
      decoding_map = []
      index = 0
      
      palatte_bytes = palette_chunk.content.unpack('C*')
      if transparency_chunk
        alpha_channel = transparency_chunk.content.unpack('C*')
      else
        alpha_channel = Array.new(palatte_bytes.size / 3, 255)
      end
      
      index = 0
      palatte_bytes.each_slice(3) do |bytes|
        bytes << alpha_channel[index]
        decoding_map << ChunkyPNG::Pixel.rgba(*bytes)
        index += 1
      end
      
      self.new(decoding_map, decoding_map)
    end
    
    # Builds a palette instance from a given pixel matrix.
    # @param [ChunkyPNG::PixelMatrix] pixel_matrix The pixel matrix to create a palette for.
    # @return [ChunkyPNG::Palette] The palette instance.
    def self.from_pixel_matrix(pixel_matrix)
      self.new(pixel_matrix.pixels)
    end
    
    # Builds a palette instance from a given set of pixels.
    # @param [Enumerable<ChunkyPNG::Pixel>] pixels An enumeration of pixels to create a palette for
    # @return [ChunkyPNG::Palette] The palette instance.
    def self.from_pixels(pixels)
      self.new(pixels)
    end
    
    # Checks whether the size of this palette is suitable for indexed storage.
    # @return [true, false] True if the number of colors in this palette is less than 256.
    def indexable?
      size < 256
    end
    
    # Check whether this pelette only contains opaque colors.
    # @return [true, false] True if all colors in this palette are opaque.
    # @see ChunkyPNG::Pixel#opaque?
    def opaque?
      all? { |pixel| pixel.opaque? }
    end
    
    # Check whether this pelette only contains grayscale colors.
    # @return [true, false] True if all colors in this palette are grayscale teints.
    # @see ChunkyPNG::Pixel#grayscale??
    def grayscale?
      all? { |pixel| pixel.grayscale? }
    end

    # Checks whether this palette is suitable for decoding an image from a datastream.
    #
    # This requires that the positions of the colors in the original palette chunk is known,
    # which is stored as an array in the +@decoding_map+ instance variable.
    #
    # @return [true, false] True if a decoding map was built when this palette was loaded.
    def can_decode?
      !@decoding_map.nil?
    end
    
    # Checks whether this palette is suitable for encoding an image from to datastream.
    #
    # This requires that the position of the color in the future palette chunk is known,
    # which is stored as a hash in the +@encoding_map+ instance variable.
    #
    # @return [true, false] True if a encoding map was built when this palette was loaded.
    def can_encode?
      !@encoding_map.nil?
    end
    
    # Returns a color, given the position in the original palette chunk.
    # @param [Fixnum] index The 0-based position of the color in the palette.
    # @return [ChunkyPNG::Pixel] The color that is stored in the palette under the given index
    # @see ChunkyPNG::Palette#can_decode?
    def [](index)
      @decoding_map[index]
    end
    
    # Returns the position of a color in the palette
    # @param [ChunkyPNG::Pixel] color The color for which to look up the index.
    # @return [Fixnum] The 0-based position of the color in the palette.
    # @see ChunkyPNG::Palette#can_encode?
    def index(color)
      @encoding_map[color]
    end
    
    # Creates a tRNS chunk that corresponds with this palette to store the
    # alpha channel of all colors.
    #
    # Note that this chunk can be left out of every color in the palette is 
    # opaque, and the image is encoded using indexed colors.
    #
    # @return [ChunkyPNG::Chunk::Transparency] The tRNS chunk.
    def to_trns_chunk
      ChunkyPNG::Chunk::Transparency.new('tRNS', map(&:a).pack('C*'))
    end
    
    # Creates a PLTE chunk that corresponds with this palette to store the
    # r, g and b channels of all colors.
    #
    # Note that a PLTE chunk should only be included if the image is 
    # encoded using index colors. After this chunk has been built, the
    # palette becomes suitable for encoding an image.
    #
    # @return [ChunkyPNG::Chunk::Palette] The PLTE chunk.
    # @see ChunkyPNG::Palette#can_encode?
    def to_plte_chunk
      @encoding_map = {}
      colors     = []
      
      each_with_index do |color, index|
        @encoding_map[color] = index
        colors += color.to_truecolor_bytes
      end
      
      ChunkyPNG::Chunk::Palette.new('PLTE', colors.pack('C*'))
    end
    
    # Determines the most suitable colormode for this palette.
    # @return [Fixnum] The colormode which would create the smalles possible
    #    file for images that use this exact palette.
    def best_colormode
      if grayscale?
        if opaque?
          ChunkyPNG::COLOR_GRAYSCALE
        else
          ChunkyPNG::COLOR_GRAYSCALE_ALPHA
        end
      elsif indexable?
        ChunkyPNG::COLOR_INDEXED
      elsif opaque?
        ChunkyPNG::COLOR_TRUECOLOR
      else
        ChunkyPNG::COLOR_TRUECOLOR_ALPHA
      end
    end
  end
end