UX: Calculate missing hover/selected colors from existing colors (#20105)

`--d-hover` is calculated to be equivalent to primary-100 in light mode, or primary-low in dark mode

`--d-selected` is calculated to be equivalent to primary-low in light mode, or primary-100 in dark mode

`lib/color_math` is introduced to provide some utilities for making these calculations.

lib/color_math.rb

@@ -0,0 +1,132 @@
+# frozen_string_literal: true
+
+module ColorMath
+  # Equivalent to dc-color-brightness() in variables.scss
+  def self.brightness(color)
+    rgb = Converters.hex_to_rgb(color)
+    (rgb[0].to_i * 299 + rgb[1].to_i * 587 + rgb[2].to_i * 114) / 1000.0
+  end
+
+  # Equivalent to dark-light-diff() in variables.scss
+  def self.dark_light_diff(adjusted_color, comparison_color, lightness, darkness)
+    if brightness(adjusted_color) < brightness(comparison_color)
+      scale_color_lightness(adjusted_color, lightness)
+    else
+      scale_color_lightness(adjusted_color, darkness)
+    end
+  end
+
+  # Equivalent to scale_color(color, lightness: ) in sass
+  def self.scale_color_lightness(color, adjustment)
+    rgb = Converters.hex_to_rgb(color)
+    h, s, l = Converters.rgb_to_hsl(*rgb)
+
+    l =
+      if adjustment > 0
+        l + (100 - l) * adjustment
+      else
+        l + l * adjustment
+      end
+
+    rgb = Converters.hsl_to_rgb(h, s, l)
+    Converters.rgb_to_hex(rgb)
+  end
+
+  module Converters
+    # Adapted from https://github.com/anilyanduri/color_math
+    #
+    # The MIT License (MIT)
+    #
+    # Copyright (c) 2016 Anil Yanduri
+    #
+    # Permission is hereby granted, free of charge, to any person obtaining a copy
+    # of this software and associated documentation files (the "Software"), to deal
+    # in the Software without restriction, including without limitation the rights
+    # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    # copies of the Software, and to permit persons to whom the Software is
+    # furnished to do so, subject to the following conditions:
+    #
+    # The above copyright notice and this permission notice shall be included in all
+    # copies or substantial portions of the Software.
+    #
+    # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+    # SOFTWARE.
+
+    def self.hex_to_rgb(color)
+      color = color.gsub(/(.)/, '\1\1') if color.length == 3
+      raise new RuntimeError("Hex color must be 6 characters") if color.length != 6
+      color.scan(/../).map { |c| c.to_i(16) }
+    end
+
+    def self.rgb_to_hex(rgb)
+      rgb.map { |c| c.to_s(16).rjust(2, "0") }.join("")
+    end
+
+    def self.rgb_to_hsl(r, g, b)
+      r /= 255.0
+      g /= 255.0
+      b /= 255.0
+      max = [r, g, b].max
+      min = [r, g, b].min
+      h = (max + min) / 2.0
+      s = (max + min) / 2.0
+      l = (max + min) / 2.0
+
+      if (max == min)
+        h = 0
+        s = 0 # achromatic
+      else
+        d = max - min
+        s = l >= 0.5 ? d / (2.0 - max - min) : d / (max + min)
+        case max
+        when r
+          h = (g - b) / d + (g < b ? 6.0 : 0)
+        when g
+          h = (b - r) / d + 2.0
+        when b
+          h = (r - g) / d + 4.0
+        end
+        h /= 6.0
+      end
+      [(h * 360).round, (s * 100).round, (l * 100).round]
+    end
+
+    def self.hsl_to_rgb(h, s, l)
+      h = h / 360.0
+      s = s / 100.0
+      l = l / 100.0
+
+      r = 0.0
+      g = 0.0
+      b = 0.0
+
+      if (s == 0.0)
+        r = l.to_f
+        g = l.to_f
+        b = l.to_f #achromatic
+      else
+        q = l < 0.5 ? l * (1 + s) : l + s - l * s
+        p = 2 * l - q
+        r = hue_to_rgb(p, q, h + 1 / 3.0)
+        g = hue_to_rgb(p, q, h)
+        b = hue_to_rgb(p, q, h - 1 / 3.0)
+      end
+
+      [(r * 255).round, (g * 255).round, (b * 255).round]
+    end
+
+    def self.hue_to_rgb(p, q, t)
+      t += 1 if (t < 0)
+      t -= 1 if (t > 1)
+      return(p + (q - p) * 6 * t) if (t < 1 / 6.0)
+      return q if (t < 1 / 2.0)
+      return(p + (q - p) * (2 / 3.0 - t) * 6) if (t < 2 / 3.0)
+      p
+    end
+  end
+end