claritas

defmodule Claritas do
  use Bitwise

  @moduledoc """
  Claritas lighten or darken color (hexadecimal) by integer value.
  """

  @doc """
  Shift brightness (up or down) of the given hexadecimal `color` by integer value `brightness_change`.

  Parameters:

  `color`: color in hexadecimal format (#RRGGBB)

  `brightness_change`: integer value

  ## Examples

  ```
  # positive values are lightening the color
  iex> Claritas.shift("#f06d06", 30)
  {:ok, "#FF8B24"}

  # negative values are darkening the color
  iex> Claritas.shift("#f06d06", -30)
  {:ok, "#D24F00"}
  ```

  Dynamically increase/decrease brightness of your colors.

  Controller:

  ```
  def index(conn, _params) do
    shades =
      for i <- 1..20, into: [] do
        Claritas.shift("#141C5B", 10 * i)
      end

      render(conn, "index.html", shades: shades)
  end
  ```

  Template:

  ```
  <ul>
    <%= for {_, shade} <- @shades do %>
      <li><span style='background-color: <%= shade %>'><%= shade %></span></li>
    <% end %>
  </ul>
  ```

  """
  def shift(color, _) when not is_binary(color), do: {:error, "color must be a string"}

  def shift(color, brightness_change) when is_integer(brightness_change) do
    hexc =
      color
      |> String.split("#")

    # handle both cases: hex number or hex number started with hash symbol
    hex_color =
      case hexc do
        [_, hex] -> hex
        [hex] -> hex
      end

    int_color = hex_color |> to_charlist() |> List.to_integer(16)

    red = (int_color >>> 16) + brightness_change
    green = (int_color &&& 0x0000FF) + brightness_change
    blue = (int_color >>> 8 &&& 0x00FF) + brightness_change

    red = adjust(red)
    green = adjust(green)
    blue = adjust(blue)

    new_color = (green ||| blue <<< 8 ||| red <<< 16) |> Integer.to_string(16)
    {:ok, "#" <> new_color}
  end

  def shift(_, brightness_change) when not is_integer(brightness_change),
    do: {:error, "second parameter must be integer"}

  # let's ensure values belong to RGB range
  defp adjust(intensity) when intensity > 255, do: 255
  defp adjust(intensity) when intensity < 0, do: 0
  defp adjust(intensity), do: intensity
end