A class to convert red, green, blue values into HSL

colour.rb

class Colour
  attr_reader :red, :green, :blue

  def initialize(red, green, blue)
    @red = Float(red)
    @green = Float(green)
    @blue = Float(blue)
  end

  def to_hsl
    [hue, saturation, lightness]
  end

  def hue
    (60 * Math::PI / 180) * hue_prime
  end

  def saturation
    if delta.zero?
      0
    else
      delta / (1 - ((2 * lightness) - 1)).abs
    end
  end

  def lightness
    (c_max + c_min) / 2.0
  end

  private

  def hue_prime
    if delta.zero?
      0
    elsif c_max == red_prime
      ((green_prime - blue_prime) / delta) % 6
    elsif c_max == green_prime
      ((blue_prime - red_prime) / delta) + 2
    elsif c_max == blue_prime
      ((red_prime - green_prime) / delta) + 4
    end
  end

  def delta
    c_max - c_min
  end

  def c_max
    [red_prime, green_prime, blue_prime].max
  end

  def c_min
    [red_prime, green_prime, blue_prime].min
  end

  def red_prime
    red / 255.0
  end

  def green_prime
    green / 255.0
  end

  def blue_prime
    blue / 255.0
  end
end