Contrast colors with Ruby

color_helper.rb

module ColorHelper
  def hex_to_rgb(hex_color)
    # Remove the '#' character if it's there
    hex_color = hex_color.tr('#', '')

    # Split the hex color string into the RGB components
    r = hex_color[0..1].to_i(16)
    g = hex_color[2..3].to_i(16)
    b = hex_color[4..5].to_i(16)

    [r, g, b]
  end

  def rgb_to_hex(red, green, blue)
    hex = [red, green, blue].map do |color|
      color = 255 if color > 255
      color = 0 if color.negative?

      color.round.to_s(16).rjust(2, '0')
    end.join.upcase

    "##{hex}".downcase
  end

  def get_luminance(color)
    return 0 if color == 'transparent'

    r, g, b = hex_to_rgb(color)
    (0.2126 * channel_to_luminance(r)) + (0.7152 * channel_to_luminance(g)) + (0.0722 * channel_to_luminance(b))
  end

  def get_contrast(color1, color2)
    luminance1 = get_luminance(color1)
    luminance2 = get_luminance(color2)

    if luminance1 > luminance2
      (luminance1 + 0.05) / (luminance2 + 0.05)
    else
      (luminance2 + 0.05) / (luminance1 + 0.05)
    end
  end

  def mix(color1, color2, weight)
    # Parse colors and normalize
    r1, g1, b1 = hex_to_rgb(color1).map { |c| c / 255.0 }
    r2, g2, b2 = hex_to_rgb(color2).map { |c| c / 255.0 }

    # Mixing algorithm from Sass implementation
    weight1, weight2 = get_weight_for_mixing(weight)

    # Calculate final color values
    r = ((r1 * weight1) + (r2 * weight2)) * 255
    g = ((g1 * weight1) + (g2 * weight2)) * 255
    b = ((b1 * weight1) + (b2 * weight2)) * 255

    # Convert final values to a CSS RGBA color string
    rgb_to_hex(r, g, b)
  end

  def adjust_color_for_contrast(color, background_color, target_ratio)
    max_iterations = 20
    adjusted_color = color

    max_iterations.times do
      current_ratio = get_contrast(adjusted_color, background_color)
      break if current_ratio >= target_ratio

      adjustment_direction = get_luminance(adjusted_color) < 0.5 ? '#ffffff' : '#151718'
      adjusted_color = mix(adjusted_color, adjustment_direction, 0.05)
    end

    adjusted_color
  end

  private

  def get_weight_for_mixing(weight)
    normalized_weight = (weight * 2) - 1
    weight2 = (normalized_weight + 1) / 2
    weight1 = 1 - weight2

    [weight1, weight2]
  end

  def channel_to_luminance(channel)
    channel /= 255.0
    if channel <= 0.03928
      channel / 12.92
    else
      ((channel + 0.055) / 1.055)**2.4
    end
  end
end

color_helper_spec.rb

require 'rails_helper'

# the values for these test cases are based off https://github.com/ricokahler/color2k/

RSpec.describe ColorHelper do
  describe '#hex_to_rgb' do
    it 'converts a hex color to RGB' do
      pairs = [
        ['#fefce8', [254, 252, 232]],
        ['#fef9c3', [254, 249, 195]],
        ['#fde047', [253, 224, 71]],
        ['#ffe047', [255, 224, 71]],
        ['#ffff47', [255, 255, 71]],
        ['#000f47', [0, 15, 71]]
      ]

      pairs.each do |pair|
        expect(helper.hex_to_rgb(pair[0])).to eq(pair[1])
      end
    end
  end

  describe '#rgb_to_hex' do
    it 'converts RGB to a hex color' do
      pairs = [
        ['#fefce8', [254, 252, 232]],
        ['#fef9c3', [254, 249, 195]],
        ['#fde047', [253, 224, 71]],
        ['#ffe047', [255, 224, 71]],
        ['#ffff47', [255, 255, 71]],
        ['#000f47', [0, 15, 71]]
      ]

      pairs.each do |pair|
        expect(helper.rgb_to_hex(*pair[1])).to eq(pair[0])
      end
    end
  end

  describe '#get_luminance' do
    it 'gets luminance' do
      pairs = [
        ['#fefce8', 0.9651783305430648],
        ['#fef9c3', 0.9276232470289122],
        ['#fde047', 0.7464888809386645],
        ['#ffe047', 0.7502624139378438],
        ['#ffff47', 0.9323493232745587],
        ['#000f47', 0.007965800403950861]
      ]

      pairs.each do |pair|
        expect(helper.get_luminance(pair[0])).to be_within(0.0001).of(pair[1])
      end
    end
  end

  describe '#get_contrast' do
    it 'gets contrast' do
      pairs = [
        ['#fff7ed', '#082f49', 13.071630965576853],
        ['#ffedd5', '#0c4a6e', 8.25405437296673],
        ['#fed7aa', '#075985', 5.587542088216559],
        ['#fdba74', '#0369a1', 3.518444977234888],
        ['#fb923c', '#0284c7', 1.8094825558825534],
        ['#f97316', '#0ea5e9', 1.01141749353127],
        ['#ea580c', '#38bdf8', 1.66155548862194],
        ['#c2410c', '#7dd3fc', 3.1059042598363495],
        ['#9a3412', '#bae6fd', 5.506356434201567],
        ['#7c2d12', '#e0f2fe', 8.166263099299048],
        ['#431407', '#f0f9ff', 14.681391442562134]
      ]

      pairs.each do |pair|
        expect(helper.get_contrast(pair[0], pair[1])).to be_within(0.0001).of(pair[2])
      end
    end
  end

  describe '#mix' do
    it 'mixes at 0.5' do
      pairs = [
        ['#fff7ed', '#082f49', '#84939b'],
        ['#ffedd5', '#0c4a6e', '#869ca2'],
        ['#fed7aa', '#075985', '#839898'],
        ['#fdba74', '#0369a1', '#80928b'],
        ['#fb923c', '#0284c7', '#7e8b82'],
        ['#f97316', '#0ea5e9', '#848c80'],
        ['#ea580c', '#38bdf8', '#918b82'],
        ['#c2410c', '#7dd3fc', '#a08a84'],
        ['#9a3412', '#bae6fd', '#aa8d88'],
        ['#7c2d12', '#e0f2fe', '#ae9088'],
        ['#431407', '#f0f9ff', '#9a8783']
      ]

      pairs.each do |pair|
        expect(helper.mix(pair[0], pair[1], 0.5)).to eq(pair[2])
      end
    end

    it 'mixes at 0.2' do
      pairs = [
        ['#fff7ed', '#082f49', '#cecfcc'],
        ['#ffedd5', '#0c4a6e', '#ceccc0'],
        ['#fed7aa', '#075985', '#cdbea3'],
        ['#fdba74', '#0369a1', '#cbaa7d'],
        ['#fb923c', '#0284c7', '#c98f58'],
        ['#f97316', '#0ea5e9', '#ca7d40'],
        ['#ea580c', '#38bdf8', '#c66c3b'],
        ['#c2410c', '#7dd3fc', '#b45e3c'],
        ['#9a3412', '#bae6fd', '#a05841'],
        ['#7c2d12', '#e0f2fe', '#905441'],
        ['#431407', '#f0f9ff', '#664239']
      ]

      pairs.each do |pair|
        expect(helper.mix(pair[0], pair[1], 0.2)).to eq(pair[2])
      end
    end
  end

  describe '#adjust_color_for_contrast' do
    target_ratio = 3.1

    it 'adjusts a color to meet the contrast ratio against a light background' do
      color = '#ff0000'
      background_color = '#ffffff'
      adjusted_color = helper.adjust_color_for_contrast(color, background_color, target_ratio)
      ratio = helper.get_contrast(adjusted_color, background_color)

      expect(ratio).to be >= target_ratio
    end

    it 'adjusts a color to meet the contrast ratio against a dark background' do
      color = '#00ff00'
      background_color = '#000000'
      adjusted_color = helper.adjust_color_for_contrast(color, background_color, target_ratio)
      ratio = helper.get_contrast(adjusted_color, background_color)

      expect(ratio).to be >= target_ratio
    end

    it 'returns a string representation of the color' do
      color = '#00ff00'
      background_color = '#000000'
      adjusted_color = helper.adjust_color_for_contrast(color, background_color, target_ratio)

      expect(adjusted_color).to be_a(String)
    end

    it 'handles cases where the color already meets the contrast ratio' do
      color = '#000000'
      background_color = '#ffffff'
      adjusted_color = helper.adjust_color_for_contrast(color, background_color, target_ratio)
      ratio = helper.get_contrast(adjusted_color, background_color)

      expect(ratio).to be >= target_ratio
      expect(adjusted_color).to eq(color)
    end

    it 'does not modify a color that already exceeds the contrast ratio' do
      color = '#000000'
      background_color = '#ffffff'
      adjusted_color = helper.adjust_color_for_contrast(color, background_color, target_ratio)

      expect(adjusted_color).to eq(color)
    end
  end
end