freedive/lib/liliform/colorhash.ex

defmodule Liliform.Colorhash do
  @moduledoc """
  Given a string returns HSL color.
  """
  alias Freedive.Features
  require Logger

  @seed "zeni"
  @default_color {0, 0, 0}

  @doc """
  Given a string returns HSL color as a CSS string.
  """
  def hsl(input) do
    {hue, saturation, lightness} = hsl(input, raw: true)
    "hsl(#{hue}, #{saturation}%, #{lightness}%)"
  end

  @doc """
  Given a string returns HSL color.
  """
  def hsl(input, raw: true) do
    if Features.enabled?(:colorhash) do
      input = String.downcase(input) <> @seed
      hash = :erlang.phash2(input, 2_147_483_647)

      hue = calculate_hue(hash)
      saturation = 80 + rem(hash, 21)
      lightness = 30 + rem(hash, 31)

      {hue, saturation, lightness}
    else
      @default_color
    end
  end

  @doc """
  Calculates hue avoiding low contrast colors.
  """
  def calculate_hue(hash) do
    base_hue = rem(hash, 360)

    # Ranges to exclude (yellow to light green, cyan)
    excluded_ranges = [
      # yellow
      {40, 80},
      # cyan
      {175, 185}
    ]

    # Adjust the hue to skip over excluded ranges
    Enum.reduce(excluded_ranges, base_hue, fn {start, stop}, acc_hue ->
      if acc_hue >= start and acc_hue <= stop, do: stop + (acc_hue - start + 1), else: acc_hue
    end)
  end
end