module ChunkyPNG

  # Factory method to return a color value, based on the arguments given.
  #
  # @overload Color(r, g, b, a)
  #   @param (see ChunkyPNG::Color.rgba)
  #   @return [Integer] The rgba color value.
  #
  # @overload Color(r, g, b)
  #   @param (see ChunkyPNG::Color.rgb)
  #   @return [Integer] The rgb color value.
  #
  # @overload Color(hex_value, opacity = nil)
  #   @param (see ChunkyPNG::Color.from_hex)
  #   @return [Integer] The hex color value, with the opacity applied if one
  #     was given.
  #
  # @overload Color(color_name, opacity = nil)
  #   @param (see ChunkyPNG::Color.html_color)
  #   @return [Integer] The hex color value, with the opacity applied if one
  #     was given.
  #
  # @overload Color(color_value, opacity = nil)
  #   @param [Integer, :to_i] The color value.
  #   @return [Integer] The color value, with the opacity applied if one was
  #     given.
  #
  # @return [Integer] The determined color value as RGBA integer.
  # @raise [ArgumentError] if the arguments weren't understood as a color.
  # @see ChunkyPNG::Color
  # @see ChunkyPNG::Color.parse
  def self.Color(*args)
    case args.length
      when 1; ChunkyPNG::Color.parse(args.first)
      when 2; (ChunkyPNG::Color.parse(args.first) & 0xffffff00) | args[1].to_i
      when 3; ChunkyPNG::Color.rgb(*args)
      when 4; ChunkyPNG::Color.rgba(*args)
      else raise ArgumentError, "Don't know how to create a color from #{args.inspect}!"
    end
  end

  # The Color module defines methods for handling colors. Within the ChunkyPNG
  # library, the concepts of pixels and colors are both used, and they are
  # both represented by a Integer.
  #
  # Pixels/colors are represented in RGBA components. Each of the four
  # components is stored with a depth of 8 bits (maximum value = 255 =
  # {ChunkyPNG::Color::MAX}). Together, these components are stored in a 4-byte
  # Integer.
  #
  # A color will always be represented using these 4 components in memory.
  # When the image is encoded, a more suitable representation can be used
  # (e.g. rgb, grayscale, palette-based), for which several conversion methods
  # are provided in this module.
  module Color
    extend self

    # @return [Integer] The maximum value of each color component.
    MAX = 0xff

    # @private
    # @return [Regexp] The regexp to parse 3-digit hex color values.
    HEX3_COLOR_REGEXP  = /\A(?:#|0x)?([0-9a-f]{3})\z/i

    # @private
    # @return [Regexp] The regexp to parse 6- and 8-digit hex color values.
    HEX6_COLOR_REGEXP  = /\A(?:#|0x)?([0-9a-f]{6})([0-9a-f]{2})?\z/i

    # @private
    # @return [Regexp] The regexp to parse named color values.
    HTML_COLOR_REGEXP = /^([a-z][a-z_ ]+[a-z])(?:\ ?\@\ ?(1\.0|0\.\d+))?$/i

    ####################################################################
    # CONSTRUCTING COLOR VALUES
    ####################################################################

    # Parses a color value given a numeric or string argument.
    #
    # It supports color numbers, colors in hex notation and named HTML colors.
    #
    # @param [Integer, String] The color value.
    # @return [Integer] The color value, with the opacity applied if one was
    #   given.
    def parse(source)
      return source if source.kind_of?(Integer)
      case source.to_s
        when /^\d+$/; source.to_s.to_i
        when HEX3_COLOR_REGEXP, HEX6_COLOR_REGEXP; from_hex(source.to_s)
        when HTML_COLOR_REGEXP; html_color(source.to_s)
        else raise ArgumentError, "Don't know how to create a color from #{source.inspect}!"
      end
    end

    # Creates a new color using an r, g, b triple and an alpha value.
    # @param [Integer] r The r-component (0-255)
    # @param [Integer] g The g-component (0-255)
    # @param [Integer] b The b-component (0-255)
    # @param [Integer] a The opacity (0-255)
    # @return [Integer] The newly constructed color value.
    def rgba(r, g, b, a)
      r << 24 | g << 16 | b << 8 | a
    end

    # Creates a new color using an r, g, b triple.
    # @param [Integer] r The r-component (0-255)
    # @param [Integer] g The g-component (0-255)
    # @param [Integer] b The b-component (0-255)
    # @return [Integer] The newly constructed color value.
    def rgb(r, g, b)
      r << 24 | g << 16 | b << 8 | 0xff
    end

    # Creates a new color using a grayscale teint.
    # @param [Integer] teint The grayscale teint (0-255), will be used as r, g,
    #   and b value.
    # @return [Integer] The newly constructed color value.
    def grayscale(teint)
      teint << 24 | teint << 16 | teint << 8 | 0xff
    end

    # Creates a new color using a grayscale teint and alpha value.
    # @param [Integer] teint The grayscale teint (0-255), will be used as r, g,
    #   and b value.
    # @param [Integer] a The opacity (0-255)
    # @return [Integer] The newly constructed color value.
    def grayscale_alpha(teint, a)
      teint << 24 | teint << 16 | teint << 8 | a
    end

    ####################################################################
    # COLOR IMPORTING
    ####################################################################

    # Creates a color by unpacking an rgb triple from a string.
    #
    # @param [String] stream The string to load the color from. It should be
    #     at least 3 + pos bytes long.
    # @param [Integer] pos The position in the string to load the triple from.
    # @return [Integer] The newly constructed color value.
    def from_rgb_stream(stream, pos = 0)
      rgb(*stream.unpack("@#{pos}C3"))
    end

    # Creates a color by unpacking an rgba triple from a string
    #
    # @param [String] stream The string to load the color from. It should be
    #      at least 4 + pos bytes long.
    # @param [Integer] pos The position in the string to load the triple from.
    # @return [Integer] The newly constructed color value.
    def from_rgba_stream(stream, pos = 0)
      rgba(*stream.unpack("@#{pos}C4"))
    end

    # Creates a color by converting it from a string in hex notation.
    #
    # It supports colors with (#rrggbbaa) or without (#rrggbb) alpha channel
    # as well as the 3-digit short format (#rgb) for those without.
    # Color strings may include the prefix "0x" or "#".
    #
    # @param [String] str The color in hex notation. @return [Integer] The
    #   converted color value.
    # @param [Integer] opacity The opacity value for the color. Overrides any
    #    opacity value given in the hex value if given.
    # @return [Integer] The color value.
    # @raise [ArgumentError] if the value given is not a hex color notation.
    def from_hex(hex_value, opacity = nil)
      base_color = case hex_value
                   when HEX3_COLOR_REGEXP
                     $1.gsub(/([0-9a-f])/i, '\1\1').hex << 8
                   when HEX6_COLOR_REGEXP
                     $1.hex << 8
                   else
                     raise ArgumentError, "Not a valid hex color notation: #{hex_value.inspect}!"
                   end
      opacity  ||= $2 ? $2.hex : 0xff
      base_color | opacity
    end

    # Creates a new color from an HSV triple.
    #
    # Create a new color using an HSV (sometimes also called HSB) triple. The
    # words `value` and `brightness` are used interchangeably and synonymously
    # in descriptions of this colorspace. This implementation follows the modern
    # convention of 0 degrees hue indicating red.
    #
    # @param [Fixnum] hue The hue component (0-360)
    # @param [Fixnum] saturation The saturation component (0-1)
    # @param [Fixnum] value The value (brightness) component (0-1)
    # @param [Fixnum] alpha Defaults to opaque (255).
    # @return [Integer] The newly constructed color value.
    # @raise [ArgumentError] if the hsv triple is invalid.
    # @see http://en.wikipedia.org/wiki/HSL_and_HSV
    def from_hsv(hue, saturation, value, alpha = 255)
      raise ArgumentError, "Hue must be between 0 and 360" unless (0..360).include?(hue)
      raise ArgumentError, "Saturation must be between 0 and 1" unless (0..1).include?(saturation)
      raise ArgumentError, "Value/brightness must be between 0 and 1" unless (0..1).include?(value)
      chroma = value * saturation
      rgb    = cylindrical_to_cubic(hue, saturation, value, chroma)
      rgb.map! { |component| ((component + value - chroma) * 255).to_i }
      rgb << alpha
      self.rgba(*rgb)
    end
    alias_method :from_hsb, :from_hsv

    # Creates a new color from an HSL triple.
    #
    # This implementation follows the modern convention of 0 degrees hue
    # indicating red.
    #
    # @param [Fixnum] hue The hue component (0-360)
    # @param [Fixnum] saturation The saturation component (0-1)
    # @param [Fixnum] lightness The lightness component (0-1)
    # @param [Fixnum] alpha Defaults to opaque (255).
    # @return [Integer] The newly constructed color value.
    # @raise [ArgumentError] if the hsl triple is invalid.
    # @see http://en.wikipedia.org/wiki/HSL_and_HSV
    def from_hsl(hue, saturation, lightness, alpha = 255)
      raise ArgumentError, "Hue #{hue} was not between 0 and 360" unless (0..360).include?(hue)
      raise ArgumentError, "Saturation #{saturation} was not between 0 and 1" unless (0..1).include?(saturation)
      raise ArgumentError, "Lightness #{lightness} was not between 0 and 1" unless (0..1).include?(lightness)
      chroma = (1 - (2 * lightness - 1).abs) * saturation
      rgb    = cylindrical_to_cubic(hue, saturation, lightness, chroma)
      rgb.map! { |component| ((component + lightness - 0.5 * chroma) * 255).to_i }
      rgb << alpha
      self.rgba(*rgb)
    end

    # Convert one HSL or HSV triple and associated chroma to a scaled rgb triple
    #
    # This method encapsulates the shared mathematical operations needed to
    # convert coordinates from a cylindrical colorspace such as HSL or HSV into
    # coordinates of the RGB colorspace.
    #
    # Even though chroma values are derived from the other three coordinates,
    # the formula for calculating chroma differs for each colorspace.  Since it
    # is calculated differently for each colorspace, it must be passed in as
    # a parameter.
    #
    # @param [Fixnum] hue The hue-component (0-360)
    # @param [Fixnum] saturation The saturation-component (0-1)
    # @param [Fixnum] y_component The y_component can represent either lightness
    #     or brightness/value (0-1) depending on which scheme (HSV/HSL) is being used.
    # @param [Fixnum] chroma The associated chroma value.
    # @return [Array<Fixnum>] A scaled r,g,b triple. Scheme-dependent
    #    adjustments are still needed to reach the true r,g,b values.
    # @see http://en.wikipedia.org/wiki/HSL_and_HSV
    # @see http://www.tomjewett.com/colors/hsb.html
    # @private
    def cylindrical_to_cubic(hue, saturation, y_component, chroma)
      hue_prime = hue.fdiv(60)
      x = chroma * (1 - (hue_prime % 2 - 1).abs)

      case hue_prime
      when (0...1); [chroma, x, 0]
      when (1...2); [x, chroma, 0]
      when (2...3); [0, chroma, x]
      when (3...4); [0, x, chroma]
      when (4...5); [x, 0, chroma]
      when (5...6); [chroma, 0, x]
      end
    end
    private :cylindrical_to_cubic

    ####################################################################
    # PROPERTIES
    ####################################################################

    # Returns the red-component from the color value.
    #
    # @param [Integer] value The color value.
    # @return [Integer] A value between 0 and MAX.
    def r(value)
      (value & 0xff000000) >> 24
    end

    # Returns the green-component from the color value.
    #
    # @param [Integer] value The color value.
    # @return [Integer] A value between 0 and MAX.
    def g(value)
      (value & 0x00ff0000) >> 16
    end

    # Returns the blue-component from the color value.
    #
    # @param [Integer] value The color value.
    # @return [Integer] A value between 0 and MAX.
    def b(value)
      (value & 0x0000ff00) >> 8
    end

    # Returns the alpha channel value for the color value.
    #
    # @param [Integer] value The color value.
    # @return [Integer] A value between 0 and MAX.
    def a(value)
      value & 0x000000ff
    end

    # Returns true if this color is fully opaque.
    #
    # @param [Integer] value The color to test.
    # @return [true, false] True if the alpha channel equals MAX.
    def opaque?(value)
      a(value) == 0x000000ff
    end

    # Returns the opaque value of this color by removing the alpha channel.
    # @param [Integer] value The color to transform.
    # @return [Integer] The opaque color
    def opaque!(value)
      value | 0x000000ff
    end

    # Returns true if this color is fully transparent.
    #
    # @param [Integer] value The color to test.
    # @return [true, false] True if the r, g and b component are equal.
    def grayscale?(value)
      r(value) == b(value) && b(value) == g(value)
    end

    # Returns true if this color is fully transparent.
    #
    # @param [Integer] value The color to test.
    # @return [true, false] True if the alpha channel equals 0.
    def fully_transparent?(value)
      a(value) == 0x00000000
    end

    ####################################################################
    # ALPHA COMPOSITION
    ####################################################################

    # Multiplies two fractions using integer math, where the fractions are
    # stored using an integer between 0 and 255. This method is used as a
    # helper method for compositing colors using integer math.
    #
    # This is a quicker implementation of ((a * b) / 255.0).round.
    #
    # @param [Integer] a The first fraction.
    # @param [Integer] b The second fraction.
    # @return [Integer] The result of the multiplication.
    def int8_mult(a, b)
      t = a * b + 0x80
      ((t >> 8) + t) >> 8
    end

    # Composes two colors with an alpha channel using integer math.
    #
    # This version is faster than the version based on floating point math, so
    # this compositing function is used by default.
    #
    # @param [Integer] fg The foreground color.
    # @param [Integer] bg The background color.
    # @return [Integer] The composited color.
    # @see ChunkyPNG::Color#compose_precise
    def compose_quick(fg, bg)
      return fg if opaque?(fg) || fully_transparent?(bg)
      return bg if fully_transparent?(fg)

      a_com = int8_mult(0xff - a(fg), a(bg))
      new_r = int8_mult(a(fg), r(fg)) + int8_mult(a_com, r(bg))
      new_g = int8_mult(a(fg), g(fg)) + int8_mult(a_com, g(bg))
      new_b = int8_mult(a(fg), b(fg)) + int8_mult(a_com, b(bg))
      new_a = a(fg) + a_com
      rgba(new_r, new_g, new_b, new_a)
    end

    # Composes two colors with an alpha channel using floating point math.
    #
    # This method uses more precise floating point math, but this precision is
    # lost when the result is converted back to an integer. Because it is
    # slower than the version based on integer math, that version is preferred.
    #
    # @param [Integer] fg The foreground color.
    # @param [Integer] bg The background color.
    # @return [Integer] The composited color.
    # @see ChunkyPNG::Color#compose_quick
    def compose_precise(fg, bg)
      return fg if opaque?(fg) || fully_transparent?(bg)
      return bg if fully_transparent?(fg)

      fg_a  = a(fg).to_f / MAX
      bg_a  = a(bg).to_f / MAX
      a_com = (1.0 - fg_a) * bg_a

      new_r = (fg_a * r(fg) + a_com * r(bg)).round
      new_g = (fg_a * g(fg) + a_com * g(bg)).round
      new_b = (fg_a * b(fg) + a_com * b(bg)).round
      new_a = ((fg_a + a_com) * MAX).round
      rgba(new_r, new_g, new_b, new_a)
    end

    alias :compose :compose_quick

    # Blends the foreground and background color by taking the average of
    # the components.
    #
    # @param [Integer] fg The foreground color.
    # @param [Integer] bg The foreground color.
    # @return [Integer] The blended color.
    def blend(fg, bg)
      (fg + bg) >> 1
    end

    # Interpolates the foreground and background colors by the given alpha
    # value. This also blends the alpha channels themselves.
    #
    # A blending factor of 255 will give entirely the foreground,
    # while a blending factor of 0 will give the background.
    #
    # @param [Integer] fg The foreground color.
    # @param [Integer] bg The background color.
    # @param [Integer] alpha The blending factor (fixed 8bit)
    # @param [Integer] The interpolated color.
    def interpolate_quick(fg, bg, alpha)
      return fg if alpha >= 255
      return bg if alpha <= 0

      alpha_com = 255 - alpha

      new_r = int8_mult(alpha, r(fg)) + int8_mult(alpha_com, r(bg))
      new_g = int8_mult(alpha, g(fg)) + int8_mult(alpha_com, g(bg))
      new_b = int8_mult(alpha, b(fg)) + int8_mult(alpha_com, b(bg))
      new_a = int8_mult(alpha, a(fg)) + int8_mult(alpha_com, a(bg))

      rgba(new_r, new_g, new_b, new_a)
    end

    # Calculates the grayscale teint of an RGB color.
    #
    # @param [Integer] color The color to convert.
    # @return [Integer] The grayscale teint of the input color, 0-255.
    def grayscale_teint(color)
      (r(color) * 0.3 + g(color) * 0.59 + b(color) * 0.11).round
    end

    # Converts a color to a fiting grayscale value. It will conserve the alpha
    # channel.
    #
    # This method will return a full color value, with the R, G, and B value
    # set to the grayscale teint calcuated from the input color's R, G and B
    # values.
    #
    # @param [Integer] color The color to convert.
    # @return [Integer] The input color, converted to the best fitting
    #   grayscale.
    # @see #grayscale_teint
    def to_grayscale(color)
      grayscale_alpha(grayscale_teint(color), a(color))
    end

    # Lowers the intensity of a color, by lowering its alpha by a given factor.
    # @param [Integer] color The color to adjust.
    # @param [Integer] factor Fade factor as an integer between 0 and 255.
    # @return [Integer] The faded color.
    def fade(color, factor)
      new_alpha = int8_mult(a(color), factor)
      (color & 0xffffff00) | new_alpha
    end

    # Decomposes a color, given a color, a mask color and a background color.
    # The returned color will be a variant of the mask color, with the alpha
    # channel set to the best fitting value. This basically is the reverse
    # operation if alpha composition.
    #
    # If the color cannot be decomposed, this method will return the fully
    # transparent variant of the mask color.
    #
    # @param [Integer] color The color that was the result of compositing.
    # @param [Integer] mask The opaque variant of the color that was being
    #   composed
    # @param [Integer] bg The background color on which the color was composed.
    # @param [Integer] tolerance The decomposition tolerance level, a value
    #   between 0 and 255.
    # @return [Integer] The decomposed color, a variant of the masked color
    #   with the alpha channel set to an appropriate value.
    def decompose_color(color, mask, bg, tolerance = 1)
      if alpha_decomposable?(color, mask, bg, tolerance)
        mask & 0xffffff00 | decompose_alpha(color, mask, bg)
      else
        mask & 0xffffff00
      end
    end

    # Checks whether an alpha channel value can successfully be composed
    # given the resulting color, the mask color and a background color,
    # all of which should be opaque.
    #
    # @param [Integer] color The color that was the result of compositing.
    # @param [Integer] mask The opaque variant of the color that was being
    #   composed
    # @param [Integer] bg The background color on which the color was composed.
    # @param [Integer] tolerance The decomposition tolerance level, a value
    #   between 0 and 255.
    # @return [Boolean] True if the alpha component can be decomposed
    #   successfully.
    # @see #decompose_alpha
    def alpha_decomposable?(color, mask, bg, tolerance = 1)
      components = decompose_alpha_components(color, mask, bg)
      sum = components.inject(0) { |a,b| a + b }
      max = components.max * 3
      components.max <= 255 && components.min >= 0 && (sum + tolerance * 3) >= max
    end

    # Decomposes the alpha channel value given the resulting color, the mask
    # color and a background color, all of which should be opaque.
    #
    # Make sure to call {#alpha_decomposable?} first to see if the alpha
    # channel value can successfully decomposed with a given tolerance,
    # otherwise the return value of this method is undefined.
    #
    # @param [Integer] color The color that was the result of compositing.
    # @param [Integer] mask The opaque variant of the color that was being
    #   composed
    # @param [Integer] bg The background color on which the color was composed.
    # @return [Integer] The best fitting alpha channel, a value between 0 and
    #   255.
    # @see #alpha_decomposable?
    def decompose_alpha(color, mask, bg)
      components = decompose_alpha_components(color, mask, bg)
      (components.inject(0) { |a,b| a + b } / 3.0).round
    end

    # Decomposes an alpha channel for either the r, g or b color channel.
    # @param [:r, :g, :b] channel The channel to decompose the alpha channel
    #   from.
    # @param [Integer] color The color that was the result of compositing.
    # @param [Integer] mask The opaque variant of the color that was being
    #   composed
    # @param [Integer] bg The background color on which the color was composed.
    # @return [Integer] The decomposed alpha value for the channel.
    def decompose_alpha_component(channel, color, mask, bg)
      cc, mc, bc = send(channel, color), send(channel, mask), send(channel, bg)

      return 0x00 if bc == cc
      return 0xff if bc == mc
      return 0xff if cc == mc

      (((bc - cc).to_f / (bc - mc).to_f) * MAX).round
    end

    # Decomposes the alpha channels for the r, g and b color channel.
    # @param [Integer] color The color that was the result of compositing.
    # @param [Integer] mask The opaque variant of the color that was being
    #   composed
    # @param [Integer] bg The background color on which the color was composed.
    # @return [Array<Integer>] The decomposed alpha values for the r, g and b
    #   channels.
    def decompose_alpha_components(color, mask, bg)
      [
        decompose_alpha_component(:r, color, mask, bg),
        decompose_alpha_component(:g, color, mask, bg),
        decompose_alpha_component(:b, color, mask, bg)
      ]
    end

    ####################################################################
    # CONVERSIONS
    ####################################################################

    # Returns a string representing this color using hex notation (i.e.
    # #rrggbbaa).
    #
    # @param [Integer] value The color to convert.
    # @return [String] The color in hex notation, starting with a pound sign.
    def to_hex(color, include_alpha = true)
      include_alpha ? ('#%08x' % color) : ('#%06x' % [color >> 8])
    end

    # Returns an array with the separate HSV components of a color.
    #
    # Because ChunkyPNG internally handles colors as Integers for performance
    # reasons, some rounding  occurs when importing or exporting HSV colors
    # whose coordinates are float-based.  Because of this rounding, #to_hsv and
    # #from_hsv may not be perfect inverses.
    #
    # This implementation follows the modern convention of 0 degrees hue
    # indicating red.
    #
    # @param [Integer] color The ChunkyPNG color to convert.
    # @param [Boolean] include_alpha Flag indicates whether a fourth element
    #    representing alpha channel should be included in the returned array.
    # @return [Array[0]] The hue of the color (0-360)
    # @return [Array[1]] The saturation of the color (0-1)
    # @return [Array[2]] The value of the color (0-1)
    # @return [Array[3]] Optional fourth element for alpha, included if
    #    include_alpha=true (0-255)
    # @see http://en.wikipedia.org/wiki/HSL_and_HSV
    def to_hsv(color, include_alpha = false)
      hue, chroma, max, min = hue_and_chroma(color)
      value      = max
      saturation = chroma.zero? ? 0.0 : chroma.fdiv(value)

      include_alpha ? [hue, saturation, value, a(color)] :
                      [hue, saturation, value]
    end
    alias_method :to_hsb, :to_hsv

    # Returns an array with the separate HSL components of a color.
    #
    # Because ChunkyPNG internally handles colors as Integers for performance
    # reasons, some rounding  occurs when importing or exporting HSL colors
    # whose coordinates are float-based.  Because of this rounding, #to_hsl and
    # #from_hsl may not be perfect inverses.
    #
    # This implementation follows the modern convention of 0 degrees hue indicating red.
    #
    # @param [Integer] color The ChunkyPNG color to convert.
    # @param [Boolean] include_alpha Flag indicates whether a fourth element
    #     representing alpha channel should be included in the returned array.
    # @return [Array<Fixnum>[0]] The hue of the color (0-360)
    # @return [Array<Fixnum>[1]] The saturation of the color (0-1)
    # @return [Array<Fixnum>[2]] The lightness of the color (0-1)
    # @return [Array<Fixnum>[3]] Optional fourth element for alpha, included if
    #     include_alpha=true (0-255)
    # @see http://en.wikipedia.org/wiki/HSL_and_HSV
    def to_hsl(color, include_alpha = false)
      hue, chroma, max, min = hue_and_chroma(color)
      lightness  = 0.5 * (max + min)
      saturation = chroma.zero? ? 0.0 : chroma.fdiv(1 - (2 * lightness - 1).abs)

      include_alpha ? [hue, saturation, lightness, a(color)] :
                      [hue, saturation, lightness]
    end

    # This method encapsulates the logic needed to extract hue and chroma from
    # a ChunkPNG color. This logic is shared by the cylindrical HSV/HSB and HSL
    # color space models.
    #
    # @param [Integer] A ChunkyPNG color.
    # @return [Fixnum] hue The hue of the color (0-360)
    # @return [Fixnum] chroma The chroma of the color (0-1)
    # @return [Fixnum] max The magnitude of the largest scaled rgb component (0-1)
    # @return [Fixnum] min The magnitude of the smallest scaled rgb component (0-1)
    # @private
    def hue_and_chroma(color)
      scaled_rgb      = to_truecolor_bytes(color)
      scaled_rgb.map! { |component| component.fdiv(255) }
      min, max = scaled_rgb.minmax
      chroma   = max - min

      r, g, b   = scaled_rgb
      hue_prime = chroma.zero? ? 0 : case max
                                     when r; (g - b).fdiv(chroma)
                                     when g; (b - r).fdiv(chroma) + 2
                                     when b; (r - g).fdiv(chroma) + 4
                                     else 0
                                     end
      hue = 60 * hue_prime

      return hue.round, chroma, max, min
    end
    private :hue_and_chroma

    # Returns an array with the separate RGBA values for this color.
    #
    # @param [Integer] color The color to convert.
    # @return [Array<Integer>] An array with 4 Integer elements.
    def to_truecolor_alpha_bytes(color)
      [r(color), g(color), b(color), a(color)]
    end

    # Returns an array with the separate RGB values for this color. The alpha
    # channel will be discarded.
    #
    # @param [Integer] color The color to convert.
    # @return [Array<Integer>] An array with 3 Integer elements.
    def to_truecolor_bytes(color)
      [r(color), g(color), b(color)]
    end

    # Returns an array with the grayscale teint value for this color.
    #
    # This method expects the r, g, and b value to be equal, and the alpha
    # channel will be discarded.
    #
    # @param [Integer] color The grayscale color to convert.
    # @return [Array<Integer>] An array with 1 Integer element.
    def to_grayscale_bytes(color)
      [b(color)] # assumption r == g == b
    end

    # Returns an array with the grayscale teint and alpha channel values for
    # this color.
    #
    # This method expects the color to be grayscale, i.e. r, g, and b value to
    # be equal and uses only the B channel. If you need to convert a color to
    # grayscale first, see {#to_grayscale}.
    #
    # @param [Integer] color The grayscale color to convert.
    # @return [Array<Integer>] An array with 2 Integer elements.
    # @see #to_grayscale
    def to_grayscale_alpha_bytes(color)
      [b(color), a(color)] # assumption r == g == b
    end

    ####################################################################
    # COMPARISON
    ####################################################################

    # Compute the Euclidean distance between 2 colors in RGBA
    #
    # This method simply takes the Euclidean distance between the RGBA channels
    # of 2 colors, which gives us a measure of how different the two colors
    # are.
    #
    # Although it would be more perceptually accurate to calculate a proper
    # Delta E in Lab colorspace, this method should serve many use-cases while
    # avoiding the overhead of converting RGBA to Lab.
    #
    # @param pixel_after [Integer]
    # @param pixel_before [Integer]
    # @return [Float]
    def euclidean_distance_rgba(pixel_after, pixel_before)
      Math.sqrt(
        (r(pixel_after) - r(pixel_before))**2 +
        (g(pixel_after) - g(pixel_before))**2 +
        (b(pixel_after) - b(pixel_before))**2 +
        (a(pixel_after) - a(pixel_before))**2
      )
    end

    # Could be simplified as MAX * 2, but this format mirrors the math in
    # {#euclidean_distance_rgba}
    # @return [Float] The maximum Euclidean distance of two RGBA colors.
    MAX_EUCLIDEAN_DISTANCE_RGBA = Math.sqrt(MAX**2 * 4)

    ####################################################################
    # COLOR CONSTANTS
    ####################################################################

    # @return [Hash<Symbol, Integer>] All the predefined color names in HTML.
    PREDEFINED_COLORS = {
      :aliceblue            => 0xf0f8ff00,
      :antiquewhite         => 0xfaebd700,
      :aqua                 => 0x00ffff00,
      :aquamarine           => 0x7fffd400,
      :azure                => 0xf0ffff00,
      :beige                => 0xf5f5dc00,
      :bisque               => 0xffe4c400,
      :black                => 0x00000000,
      :blanchedalmond       => 0xffebcd00,
      :blue                 => 0x0000ff00,
      :blueviolet           => 0x8a2be200,
      :brown                => 0xa52a2a00,
      :burlywood            => 0xdeb88700,
      :cadetblue            => 0x5f9ea000,
      :chartreuse           => 0x7fff0000,
      :chocolate            => 0xd2691e00,
      :coral                => 0xff7f5000,
      :cornflowerblue       => 0x6495ed00,
      :cornsilk             => 0xfff8dc00,
      :crimson              => 0xdc143c00,
      :cyan                 => 0x00ffff00,
      :darkblue             => 0x00008b00,
      :darkcyan             => 0x008b8b00,
      :darkgoldenrod        => 0xb8860b00,
      :darkgray             => 0xa9a9a900,
      :darkgrey             => 0xa9a9a900,
      :darkgreen            => 0x00640000,
      :darkkhaki            => 0xbdb76b00,
      :darkmagenta          => 0x8b008b00,
      :darkolivegreen       => 0x556b2f00,
      :darkorange           => 0xff8c0000,
      :darkorchid           => 0x9932cc00,
      :darkred              => 0x8b000000,
      :darksalmon           => 0xe9967a00,
      :darkseagreen         => 0x8fbc8f00,
      :darkslateblue        => 0x483d8b00,
      :darkslategray        => 0x2f4f4f00,
      :darkslategrey        => 0x2f4f4f00,
      :darkturquoise        => 0x00ced100,
      :darkviolet           => 0x9400d300,
      :deeppink             => 0xff149300,
      :deepskyblue          => 0x00bfff00,
      :dimgray              => 0x69696900,
      :dimgrey              => 0x69696900,
      :dodgerblue           => 0x1e90ff00,
      :firebrick            => 0xb2222200,
      :floralwhite          => 0xfffaf000,
      :forestgreen          => 0x228b2200,
      :fuchsia              => 0xff00ff00,
      :gainsboro            => 0xdcdcdc00,
      :ghostwhite           => 0xf8f8ff00,
      :gold                 => 0xffd70000,
      :goldenrod            => 0xdaa52000,
      :gray                 => 0x80808000,
      :grey                 => 0x80808000,
      :green                => 0x00800000,
      :greenyellow          => 0xadff2f00,
      :honeydew             => 0xf0fff000,
      :hotpink              => 0xff69b400,
      :indianred            => 0xcd5c5c00,
      :indigo               => 0x4b008200,
      :ivory                => 0xfffff000,
      :khaki                => 0xf0e68c00,
      :lavender             => 0xe6e6fa00,
      :lavenderblush        => 0xfff0f500,
      :lawngreen            => 0x7cfc0000,
      :lemonchiffon         => 0xfffacd00,
      :lightblue            => 0xadd8e600,
      :lightcoral           => 0xf0808000,
      :lightcyan            => 0xe0ffff00,
      :lightgoldenrodyellow => 0xfafad200,
      :lightgray            => 0xd3d3d300,
      :lightgrey            => 0xd3d3d300,
      :lightgreen           => 0x90ee9000,
      :lightpink            => 0xffb6c100,
      :lightsalmon          => 0xffa07a00,
      :lightseagreen        => 0x20b2aa00,
      :lightskyblue         => 0x87cefa00,
      :lightslategray       => 0x77889900,
      :lightslategrey       => 0x77889900,
      :lightsteelblue       => 0xb0c4de00,
      :lightyellow          => 0xffffe000,
      :lime                 => 0x00ff0000,
      :limegreen            => 0x32cd3200,
      :linen                => 0xfaf0e600,
      :magenta              => 0xff00ff00,
      :maroon               => 0x80000000,
      :mediumaquamarine     => 0x66cdaa00,
      :mediumblue           => 0x0000cd00,
      :mediumorchid         => 0xba55d300,
      :mediumpurple         => 0x9370d800,
      :mediumseagreen       => 0x3cb37100,
      :mediumslateblue      => 0x7b68ee00,
      :mediumspringgreen    => 0x00fa9a00,
      :mediumturquoise      => 0x48d1cc00,
      :mediumvioletred      => 0xc7158500,
      :midnightblue         => 0x19197000,
      :mintcream            => 0xf5fffa00,
      :mistyrose            => 0xffe4e100,
      :moccasin             => 0xffe4b500,
      :navajowhite          => 0xffdead00,
      :navy                 => 0x00008000,
      :oldlace              => 0xfdf5e600,
      :olive                => 0x80800000,
      :olivedrab            => 0x6b8e2300,
      :orange               => 0xffa50000,
      :orangered            => 0xff450000,
      :orchid               => 0xda70d600,
      :palegoldenrod        => 0xeee8aa00,
      :palegreen            => 0x98fb9800,
      :paleturquoise        => 0xafeeee00,
      :palevioletred        => 0xd8709300,
      :papayawhip           => 0xffefd500,
      :peachpuff            => 0xffdab900,
      :peru                 => 0xcd853f00,
      :pink                 => 0xffc0cb00,
      :plum                 => 0xdda0dd00,
      :powderblue           => 0xb0e0e600,
      :purple               => 0x80008000,
      :red                  => 0xff000000,
      :rosybrown            => 0xbc8f8f00,
      :royalblue            => 0x4169e100,
      :saddlebrown          => 0x8b451300,
      :salmon               => 0xfa807200,
      :sandybrown           => 0xf4a46000,
      :seagreen             => 0x2e8b5700,
      :seashell             => 0xfff5ee00,
      :sienna               => 0xa0522d00,
      :silver               => 0xc0c0c000,
      :skyblue              => 0x87ceeb00,
      :slateblue            => 0x6a5acd00,
      :slategray            => 0x70809000,
      :slategrey            => 0x70809000,
      :snow                 => 0xfffafa00,
      :springgreen          => 0x00ff7f00,
      :steelblue            => 0x4682b400,
      :tan                  => 0xd2b48c00,
      :teal                 => 0x00808000,
      :thistle              => 0xd8bfd800,
      :tomato               => 0xff634700,
      :turquoise            => 0x40e0d000,
      :violet               => 0xee82ee00,
      :wheat                => 0xf5deb300,
      :white                => 0xffffff00,
      :whitesmoke           => 0xf5f5f500,
      :yellow               => 0xffff0000,
      :yellowgreen          => 0x9acd3200
    }

    # Gets a color value based on a HTML color name.
    #
    # The color name is flexible. E.g. <tt>'yellowgreen'</tt>, <tt>'Yellow
    # green'</tt>, <tt>'YellowGreen'</tt>, <tt>'YELLOW_GREEN'</tt> and
    # <tt>:yellow_green</tt> will all return the same color value.
    #
    # You can include a opacity level in the color name (e.g. <tt>'red @
    # 0.5'</tt>) or give an explicit opacity value as second argument. If no
    # opacity value is given, the color will be fully opaque.
    #
    # @param [Symbol, String] color_name The color name. It may include an
    #   opacity specifier like <tt>@ 0.8</tt> to set the color's opacity.
    # @param [Integer] opacity The opacity value for the color between 0 and
    #   255. Overrides any opacity value given in the color name.
    # @return [Integer] The color value.
    # @raise [ChunkyPNG::Exception] If the color name was not recognized.
    def html_color(color_name, opacity = nil)
      if color_name.to_s =~ HTML_COLOR_REGEXP
        opacity ||= $2 ? ($2.to_f * 255.0).round : 0xff
        base_color_name = $1.gsub(/[^a-z]+/i, '').downcase.to_sym
        return PREDEFINED_COLORS[base_color_name] | opacity if PREDEFINED_COLORS.has_key?(base_color_name)
      end
      raise ArgumentError, "Unknown color name #{color_name}!"
    end

    # @return [Integer] Black pixel/color
    BLACK = rgb(  0,   0,   0)

    # @return [Integer] White pixel/color
    WHITE = rgb(255, 255, 255)

    # @return [Integer] Fully transparent pixel/color
    TRANSPARENT = rgba(0, 0, 0, 0)

    ####################################################################
    # STATIC UTILITY METHODS
    ####################################################################

    # Returns the number of sample values per pixel.
    # @param [Integer] color_mode The color mode being used.
    # @return [Integer] The number of sample values per pixel.
    def samples_per_pixel(color_mode)
      case color_mode
        when ChunkyPNG::COLOR_INDEXED;         1
        when ChunkyPNG::COLOR_TRUECOLOR;       3
        when ChunkyPNG::COLOR_TRUECOLOR_ALPHA; 4
        when ChunkyPNG::COLOR_GRAYSCALE;       1
        when ChunkyPNG::COLOR_GRAYSCALE_ALPHA; 2
        else raise ChunkyPNG::NotSupported, "Don't know the number of samples for this colormode: #{color_mode}!"
      end
    end

    # Returns the size in bytes of a pixel when it is stored using a given
    # color mode.
    #
    # @param [Integer] color_mode The color mode in which the pixels are
    #   stored.
    # @return [Integer] The number of bytes used per pixel in a datastream.
    def pixel_bytesize(color_mode, depth = 8)
      return 1 if depth < 8
      (pixel_bitsize(color_mode, depth) + 7) >> 3
    end

    # Returns the size in bits of a pixel when it is stored using a given color
    # mode.
    #
    # @param [Integer] color_mode The color mode in which the pixels are
    #   stored.
    # @param [Integer] depth The color depth of the pixels.
    # @return [Integer] The number of bytes used per pixel in a datastream.
    def pixel_bitsize(color_mode, depth = 8)
      samples_per_pixel(color_mode) * depth
    end

    # Returns the number of bytes used per scanline.
    # @param [Integer] color_mode The color mode in which the pixels are
    #   stored.
    # @param [Integer] depth The color depth of the pixels.
    # @param [Integer] width The number of pixels per scanline.
    # @return [Integer] The number of bytes used per scanline in a datastream.
    def scanline_bytesize(color_mode, depth, width)
      ((pixel_bitsize(color_mode, depth) * width) + 7) >> 3
    end

    # Returns the number of bytes used for an image pass
    # @param [Integer] color_mode The color mode in which the pixels are
    #   stored.
    # @param [Integer] depth The color depth of the pixels.
    # @param [Integer] width The width of the image pass.
    # @param [Integer] width The height of the image pass.
    # @return [Integer] The number of bytes used per scanline in a datastream.
    def pass_bytesize(color_mode, depth, width, height)
      return 0 if width == 0 || height == 0
      (scanline_bytesize(color_mode, depth, width) + 1) * height
    end
  end
end
