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.

app/models/color_scheme.rb

@@ -448,13 +448,27 @@ class ColorScheme < ActiveRecord::Base
   end
 
   def resolved_colors
-    resolved = ColorScheme.base_colors.dup
-    if base_scheme_id && base_scheme_id != "Light"
-      if scheme = CUSTOM_SCHEMES[base_scheme_id.to_sym]
-        scheme.each { |name, value| resolved[name] = value }
-      end
-    end
-    colors.each { |c| resolved[c.name] = c.hex }
+    from_base = base_colors.except("hover", "selected")
+    from_db = colors.map { |c| [c.name, c.hex] }.to_h
+
+    resolved = from_base.merge(from_db)
+
+    # Equivalent to primary-100 in light mode, or primary-low in dark mode
+    resolved["hover"] ||= ColorMath.dark_light_diff(
+      resolved["primary"],
+      resolved["secondary"],
+      0.94,
+      -0.78,
+    )
+
+    # Equivalent to primary-low in light mode, or primary-100 in dark mode
+    resolved["selected"] ||= ColorMath.dark_light_diff(
+      resolved["primary"],
+      resolved["secondary"],
+      0.9,
+      -0.8,
+    )
+
     resolved
   end
 
@@ -495,8 +509,8 @@ class ColorScheme < ActiveRecord::Base
   def is_dark?
     return if colors.to_a.empty?
 
-    primary_b = brightness(colors_by_name["primary"].hex)
-    secondary_b = brightness(colors_by_name["secondary"].hex)
+    primary_b = ColorMath.brightness(resolved_colors["primary"])
+    secondary_b = ColorMath.brightness(resolved_colors["secondary"])
 
     primary_b > secondary_b
   end
@@ -504,12 +518,6 @@ class ColorScheme < ActiveRecord::Base
   def is_wcag?
     base_scheme_id&.start_with?("WCAG")
   end
-
-  # Equivalent to dc-color-brightness() in variables.scss
-  def brightness(color)
-    rgb = color.scan(/../).map { |c| c.to_i(16) }
-    (rgb[0].to_i * 299 + rgb[1].to_i * 587 + rgb[2].to_i * 114) / 1000.0
-  end
 end
 
 # == Schema Information

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

spec/lib/color_math_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+describe ColorMath do
+  describe "#brightness" do
+    it "works" do
+      expect(ColorMath.brightness("000")).to eq(0)
+      expect(ColorMath.brightness("fff")).to eq(255)
+    end
+  end
+
+  describe "#scale_color_lightness" do
+    it "works" do
+      expect(ColorMath.scale_color_lightness("000", 0.5)).to eq("808080")
+      expect(ColorMath.scale_color_lightness("fff", -0.5)).to eq("808080")
+    end
+
+    it "works with non-greyscale colors" do
+      expect(ColorMath.scale_color_lightness("f00", 0.5)).to eq("ff8080")
+    end
+  end
+
+  describe "#dark_light_diff" do
+    it "darkens by requested amount if target color is darker than comparison" do
+      expect(ColorMath.dark_light_diff("fff", "eee", 0, -0.5)).to eq("808080")
+    end
+
+    it "lightens by requested amount if target color is lighter than comparison" do
+      expect(ColorMath.dark_light_diff("000", "eee", 0.5, 0)).to eq("808080")
+    end
+  end
+end

spec/models/color_scheme_spec.rb

@@ -134,4 +134,32 @@ RSpec.describe ColorScheme do
       )
     end
   end
+
+  describe "#resolved_colors" do
+    it "merges database colors with base scheme" do
+      color_scheme = ColorScheme.new
+      color_scheme.color_scheme_colors << ColorSchemeColor.new(name: "primary", hex: "121212")
+      resolved = color_scheme.resolved_colors
+      expect(resolved["primary"]).to eq("121212")
+      expect(resolved["secondary"]).to eq(ColorScheme.base_colors["secondary"])
+    end
+
+    it "calculates 'hover' and 'selected' from existing db colors in dark mode" do
+      color_scheme = ColorScheme.new
+      color_scheme.color_scheme_colors << ColorSchemeColor.new(name: "primary", hex: "ddd")
+      color_scheme.color_scheme_colors << ColorSchemeColor.new(name: "secondary", hex: "222")
+      resolved = color_scheme.resolved_colors
+      expect(resolved["hover"]).to eq("313131")
+      expect(resolved["selected"]).to eq("2c2c2c")
+    end
+
+    it "calculates 'hover' and 'selected' from existing db colors in light mode" do
+      color_scheme = ColorScheme.new
+      color_scheme.color_scheme_colors << ColorSchemeColor.new(name: "primary", hex: "222")
+      color_scheme.color_scheme_colors << ColorSchemeColor.new(name: "secondary", hex: "fff")
+      resolved = color_scheme.resolved_colors
+      expect(resolved["hover"]).to eq("f2f2f2")
+      expect(resolved["selected"]).to eq("e9e9e9")
+    end
+  end
 end