color

defmodule Color do
  @named_colors %{
    "aliceblue" => "f0f8ff",
    "antiquewhite" => "faebd7",
    "aqua" => "00ffff",
    "aquamarine" => "7fffd4",
    "azure" => "f0ffff",
    "beige" => "f5f5dc",
    "bisque" => "ffe4c4",
    "rebeccapurple" => "663399",
    "beccapurple" => "663399",
    "black" => "000000",
    "blanchedalmond" => "ffebcd",
    "blue" => "0000ff",
    "blueviolet" => "8a2be2",
    "brown" => "a52a2a",
    "burlywood" => "deb887",
    "cadetblue" => "5f9ea0",
    "carnation" => "ff5ed0",
    "cayenne" => "8d0000",
    "chartreuse" => "7fff00",
    "chocolate" => "d2691e",
    "coral" => "ff7f50",
    "cornflowerblue" => "6495ed",
    "Cornsilk" => "fff8dc",
    "Crimson" => "dc143c",
    "Cyan" => "00ffff",
    "DarkBlue" => "00008b",
    "DarkCyan" => "008b8b",
    "DarkGoldenRod" => "b8860b",
    "DarkGray" => "a9a9a9",
    "DarkGrey" => "a9a9a9",
    "DarkGreen" => "006400",
    "DarkKhaki" => "bdb76b",
    "DarkMagenta" => "8b008b",
    "DarkOliveGree" => "556b2f",
    "DarkOrange" => "ff8c00",
    "DarkOrchid" => "9932cc",
    "DarkRed" => "8b0000",
    "DarkSalmon" => "e9967a",
    "DarkSeaGreen" => "8fbc8f",
    "DarkSlateBlue" => "483d8b",
    "DarkSlateGray" => "2f4f4f",
    "DarkSlateGrey" => "2f4f4f",
    "DarkTurquoise" => "00ced1",
    "DarkViolet" => "9400d3",
    "Darkorange" => "ff8c00",
    "DeepPink" => "ff1493",
    "DeepSkyBlue" => "00bfbf",
    "DimGray" => "696969",
    "DimGrey" => "696969",
    "DodgerBlue" => "1e90ff",
    "Feldspar" => "d19275",
    "FireBrick" => "b22222",
    "FloralWhite" => "fffaf0",
    "ForestGreen" => "228b22",
    "Fuchsia" => "ff00ff",
    "Gainsboro" => "dcdcdc",
    "GhostWhite" => "f8f8ff",
    "Gold" => "ffd700",
    "GoldenRod" => "daa520",
    "Gray" => "808080",
    "Grey" => "808080",
    "Green" => "008000",
    "GreenYellow" => "adff2f",
    "HoneyDew" => "f0fff0",
    "HotPink" => "ff69b4",
    "IndianRed" => "cd5c5c",
    "Indigo" => "4b0082",
    "Ivory" => "fffff0",
    "Khaki" => "f0e68c",
    "Lavender" => "e6e6fa",
    "LavenderBlush" => "fff0f5",
    "LawnGreen" => "7cfc00",
    "LemonChiffon" => "fffacd",
    "LightBlue" => "add8e6",
    "LightCoral" => "f08080",
    "LightCyan" => "e0ffff",
    "LightGoldenRodYellow" => "fafad2",
    "LightGray" => "d3d3d3",
    "LightGrey" => "d3d3d3",
    "LightGreen" => "90ee90",
    "LightPink" => "ffb6c1",
    "LightSalmon" => "ffa07a",
    "LightSeaGreen" => "20b2aa",
    "LightSkyBlue" => "87cefa",
    "LightSlateBlue" => "8470ff",
    "LightSlateGray" => "778899",
    "LightSlateGrey" => "778899",
    "LightSteelBlue" => "b0c4de",
    "LightYellow" => "ffffe0",
    "Lime" => "00ff00",
    "LimeGreen" => "32cd32",
    "Linen" => "faf0e6",
    "Magenta" => "ff00ff",
    "Maroon" => "800000",
    "MediumAquaMarine" => "66cdaa",
    "MediumBlue" => "0000cd",
    "MediumOrchid" => "ba55d3",
    "MediumPurple" => "9370db",
    "MediumSeaGreen" => "3cb371",
    "MediumSlateBlue" => "7b68ee",
    "MediumSpringGreen" => "00fa9a",
    "MediumTurquoise" => "48d1cc",
    "MediumVioletRed" => "c71585",
    "MidnightBlue" => "191970",
    "MintCream" => "f5fffa",
    "MistyRose" => "ffe4e1",
    "Moccasin" => "ffe4b5",
    "NavajoWhite" => "ffdead",
    "Navy" => "000080",
    "OldLace" => "fdf5e6",
    "Olive" => "808000",
    "OliveDrab" => "6b8e23",
    "Orange" => "ffa500",
    "OrangeRed" => "ff4500",
    "Orchid" => "da70d6",
    "PaleGoldenRod" => "eee8aa",
    "PaleGreen" => "98fb98",
    "PaleTurquoise" => "afeeee",
    "PaleVioletRed" => "db7093",
    "PapayaWhip" => "ffefd5",
    "PeachPuff" => "ffdab9",
    "Peru" => "cd853f",
    "Pink" => "ffc0cb",
    "Plum" => "dda0dd",
    "PowderBlue" => "b0e0e6",
    "Purple" => "800080",
    "Red" => "ff0000",
    "RosyBrown" => "bc8f8f",
    "RoyalBlue" => "4169e1",
    "SaddleBrown" => "8b4513",
    "Salmon" => "fa8072",
    "SandyBrown" => "f4a460",
    "SeaGreen" => "2e8b57",
    "SeaShell" => "fff5ee",
    "Sienna" => "a0522d",
    "Silver" => "c0c0c0",
    "SkyBlue" => "87ceeb",
    "SlateBlue" => "6a5acd",
    "SlateGray" => "708090",
    "SlateGrey" => "708090",
    "Snow" => "fffafa",
    "SpringGreen" => "00ff7f",
    "SteelBlue" => "4682b4",
    "Tan" => "d2b48c",
    "Teal" => "008080",
    "Thistle" => "d8bfd8",
    "Tomato" => "ff6347",
    "Turquoise" => "40e0d0",
    "Violet" => "ee82ee",
    "VioletRed" => "d02090",
    "Wheat" => "f5deb3",
    "White" => "ffffff",
    "WhiteSmoke" => "f5f5f5",
    "Yellow" => "ffff00",
    "YellowGreen" => "9acd32"
  }

  def name_to_hex(name) do
    @named_colors[name]
  end

  def hex_to_lab(hex) do
    hex |> hex_to_rgb |> rgb_to_lab
  end

  def hex_to_rgb(hex) do
    case hex |> String.replace("#", "") |> String.graphemes do
      [r, g, b] ->
        %Color.RGB{r: Integer.parse(r <> r, 16),
                   g: Integer.parse(g <> g, 16),
                   b: Integer.parse(b <> b, 16)}
      [r_1, r_2, g_1, g_2, b_1, b_2] ->
        %Color.RGB{r: Integer.parse(r_1 <> r_2, 16),
                   g: Integer.parse(g_1 <> g_2, 16),
                   b: Integer.parse(b_1 <> b_2, 16)}
    end
  end

  def rgb_to_lab(%Color.RGB{r: r, g: g, b: b} = color) do
    color |> rgb_to_xyz |> xyz_to_lab
  end

  def rgb_to_xyz(%Color.RGB{r: r, g: g, b: b}) do
    # Inverse sRGB companding. Linearizes RGB channels with respect to
    # energy.
    [r, g, b] = Enum.map([r, g, b], fn v ->
      if v > 0.04045 do
        :math.pow((v + 0.055) / 1.055, 2.4) * 100
      else
        (v / 12.92) * 100
      end
    end)

    # Convert using the RGB/XYZ matrix at:
    # http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html#WSMatrices
    %Color.XYZ{x: (r * 0.4124564 + g * 0.3575761 + b * 0.1804375),
               y: (r * 0.2126729 + g * 0.7151522 + b * 0.0721750),
               z: (r * 0.0193339 + g * 0.1191920 + b * 0.9503041)}
  end

  def xyz_to_lab(%Color.XYZ{x: x, y: y, z: z}) do
    # Calculate the ratio of the XYZ values to the reference white.
    # http://www.brucelindbloom.com/index.html?Equations.html
    xr = x / 95.047
    yr = y / 100.00
    zr = z / 108.883

    # NOTE: This should be using Rational instead of floating point values,
    # otherwise there will be discontinuities.
    # http://www.brucelindbloom.com/LContinuity.html
    epsilon = 216 / 24389.0
    kappa = 24389 / 27.0

    # And now transform
    # http://en.wikipedia.org/wiki/Lab_color_space#Forward_transformation
    # There is a brief explanation there as far as the nature of the calculations,
    # as well as a much nicer looking modeling of the algebra.
    [fx, fy, fz] = Enum.map([xr, yr, zr], fn t ->
      if t > epsilon do
        :math.pow(t, 1.0 / 3)
      else # t <= epsilon
        ((kappa * t) + 16) / 116.0
        # The 4/29 here is for when t = 0 (black). 4/29 * 116 = 16, and 16 -
        # 16 = 0, which is the correct value for L* with black.
        #   ((1.0/3)*((29.0/6)**2) * t) + (4.0/29)
      end
    end)

    %Color.Lab{L: (116 * fy) - 16,
      a: 500 * (fx - fy),
      b: 200 * (fy - fz)}
  end

  @doc """
  The Delta E (CIE94) algorithm
  http://en.wikipedia.org/wiki/Color_difference#CIE94

  There is a newer version, CIEDE2000, that uses slightly more complicated math, but addresses
  "the perceptual uniformity issue" left lingering by the CIE94 algorithm. color_1 and color_2 are
  both L*a*b* maps, rendered by hex_to_lab/1.

  Since our source is treated as sRGB, we use the "graphic arts" presets
  for k_L, k_1, and k_2

  The calculations go through LCH(ab).

  delta_E = :math.sqrt(
    ((delta_L / (k_L * s_L)) ** 2) +
    ((delta_C / (k_C * s_C)) ** 2) +
    ((delta_H / (k_H * s_H)) ** 2)
  )

  Under some circumstances in real computers, delta_H could be an
  imaginary number (it's a square root value), so we're going to treat
  this as:

  delta_E = :math.sqrt(
    ((delta_L / (k_L * s_L)) ** 2) +
    ((delta_C / (k_C * s_C)) ** 2) +
    (delta_H2 / ((k_H * s_H) ** 2)))
  )

  And not perform the square root when calculating delta_H2.

  See also http://www.brucelindbloom.com/index.html?Eqn_DeltaE_CIE94.html
  """
  def delta_e94(%Color.RGB{} = color_1, color_2), do: delta_e94(color_1 |> rgb_to_lab, color_2)
  def delta_e94(%Color.XYZ{} = color_1, color_2), do: delta_e94(color_1 |> xyz_to_lab, color_2)
  def delta_e94(%Color.Lab{} = color_1, %Color.RGB{} = color_2),
    do: delta_e94(color_1, color_2 |> rgb_to_lab)
  def delta_e94(%Color.Lab{} = color_1, %Color.XYZ{} = color_2),
    do: delta_e94(color_1, color_2 |> xyz_to_lab)
  def delta_e94(%Color.Lab{L: l_1, a: a_1, b: b_1}, %Color.Lab{L: l_2, a: a_2, b: b_2}) do
    k_1 = 0.045
    k_2 = 0.015
    k_L = 1

    k_C = k_H = 1

    delta_a = a_1 - a_2
    delta_b = b_1 - b_2

    c_1 = :math.sqrt(:math.pow(a_1, 2) + :math.pow(b_1, 2))
    c_2 = :math.sqrt(:math.pow(a_2, 2) + :math.pow(b_2, 2))

    delta_L = l_1 - l_2
    delta_C = c_1 - c_2

    delta_H2 = :math.pow(delta_a, 2) + :math.pow(delta_b, 2) - :math.pow(delta_C, 2)

    s_L = 1
    s_C = 1 + k_1 * c_1
    s_H = 1 + k_2 * c_1

    composite_L = :math.pow(delta_L / (k_L * s_L), 2)
    composite_C = :math.pow(delta_C / (k_C * s_C), 2)
    composite_H = delta_H2 / :math.pow(k_H * s_H, 2)

    :math.sqrt(composite_L + composite_C + composite_H)
  end

  def delta_e94(color_1, color_2) do
    delta_e94(name_to_hex(String.downcase(color_1)) || color_1 |> hex_to_lab,
              name_to_hex(String.downcase(color_2)) || color_2 |> hex_to_lab)
  end
end