ggity

defmodule GGity.Theme do
  @moduledoc """
  Themes store style and other attributes of non-geom elements of a plot.

  GGity generates an embedded stylesheet for the CSS presentation
  attributes of these elements that ties the specified styles to custom
  CSS classes that are automatically generated for each type of element.

  Classes are generated irrespective of theme values, thereby allowing for
  the use of external stylesheets. To avoid overriding a style in an external
  stylesheet, set the value of the attribute(s) styled externally to `nil` in
  a plot's theme - GGity will not include a style in the embedded stylesheet
  for an attribute with a `nil` value.

  Each key of a Theme struct represents a class. Allowed types for each item include:

  * `nil` - no selector or styles are generated for that class
  * `%Element.Line{}` struct - generates styles for a line element
  * `%Element.Rect{}` struct - generates styles for a rect element
  * `%Element.Text{}` struct - generates styles for a text element
  * An integer - for non-CSS attributes of a given element, e.g.,
  `:axis_ticks_length`, which sets the value of the `length` attribute

  See the documentation for each element type for allowed keys and values. Keys conform
  to the ggplot2 API, which means that a given attribute does not necessarily
  correspond to the CSS attribute of the same name. For example, `:size` means
  CSS `stroke-width` for lines and rectangles, and CSS `font-size` for text.

  Element structs can be generated using the constructor function for that
  element type:

  ```
  import GGity.Element.Text
  alias GGity.Plot

  Plot.theme(plot, axis_text: element_text(color: "green", size: 12))
  #=> equivalent to Plot.theme(plot, axis_text: %GGity.Element.Text{color: "green", size 12})
  ```

  The following elements are supported:

  * `:text` style for all non-data text; overriden by other text attributes (`Element.Text`)
  * `:axis_line` x- and y axes lines (`Element.Line`)
  * `:axis_line_x` x axis; overrides `:axis_line` (`Element.Line`)
  * `:axis_line_y` y axis; overrides `:axis_line` (`Element.Line`)
  * `:axis_text` x- and y axis tick label text (`Element.Text`)
  * `:axis_text_x` x axis tick label text; overrides `:axis_text` (`Element.Text`)
  * `:axis_text_y` y axis tick label text; overrides `:axis_text` (`Element.Text`)
  * `:axis_ticks` x- and y axis ticks (`Element.Line`)
  * `:axis_line_x` x axis ticks; overrides `:axis_ticks` (`Element.Line`)
  * `:axis_line_y` y axis ticks; overrides `:axis_ticks` (`Element.Line`)
  * `:axis_ticks_length` length of x- and y axis ticks (`Integer` - non-CSS attribute)
  * `:axis_ticks_x_length` length of x axis tick; overrides `:axis_tick_length` (`Integer`)
  * `:axis_ticks_y_length` length of y axis tick; overrides `:axis_tick_length` (`Integer`)
  * `:axis_title` x- and y axis title text (`Element.Text`)
  * `:axis_title_x` x axis title text; overrides `:axis_title` (`Element.Text`)
  * `:axis_title_y` y axis title text; overrides `:axis_title` (`Element.Text`)
  * `:legend_key` legend key glyph box (`Element.Rect`)
  * `:legend_text` legend item text (`Element.Text`)
  * `:legend_title` legend title text (`Element.Text`)
  * `:panel_background` panel (area bounded by axes) background (`Element.Rect`)
  * `:panel_border` panel border (`Element.Line`)
  * `:panel_grid` minor and major gridlines (`Element.Line`)
  * `:panel_grid_major` major gridlines; overrides `panel_grid` (`Element.Line`)
  * `:panel_grid_minor` minor gridlines; overrides `panel_grid` (`Element.Line`)
  * `:plot_title` plot title text (`Element.Text`)
  * `:plot_background` plot (entire SVG element) background (`Element.Rect`)

  """

  import GGity.Element.{Line, Rect, Text}

  alias GGity.{Element, Theme}

  @type t() :: %__MODULE__{}

  defstruct text: element_text(family: "Helvetica, Arial, sans-serif"),
            axis_line: nil,
            axis_line_x: nil,
            axis_line_y: nil,
            axis_text: element_text(color: "#808080", size: "8px"),
            axis_text_x: element_text(color: "#808080", size: "8px", angle: 0),
            axis_text_y: nil,
            axis_ticks: element_line(color: "#000000"),
            axis_ticks_x: nil,
            axis_ticks_y: nil,
            axis_ticks_length: 2,
            axis_ticks_length_x: nil,
            axis_ticks_length_y: nil,
            axis_title: element_text(color: "#000000", size: "10px"),
            axis_title_x: nil,
            axis_title_y: nil,
            legend_key:
              element_rect(
                fill: "#EEEEEE",
                color: "#EEEEEE",
                size: 0.5,
                height: 15
              ),
            legend_text: element_text(fill: "#000000", size: "8px"),
            legend_title: element_text(fill: "#000000", size: "9px"),
            panel_background: element_rect(fill: "#EEEEEE"),
            panel_border: element_line(color: "none"),
            panel_grid: element_line(color: "#FFFFFF"),
            panel_grid_major: element_line(size: 1),
            panel_grid_minor: element_line(size: 0.5),
            plot_background: element_rect(fill: "#FFFFFF"),
            plot_title: element_text(size: "12px")

  @doc false
  @spec to_stylesheet(Theme.t() | nil, String.t()) :: iolist()
  def to_stylesheet(nil, _id), do: []

  def to_stylesheet(%Theme{} = theme, id) do
    [
      "<style type=\"text/css\"><![CDATA[",
      theme_to_stylesheet(theme, id),
      "]]></style>"
    ]
  end

  defp theme_to_stylesheet(%Theme{} = theme, id) do
    theme
    |> Map.from_struct()
    |> Enum.map(fn attribute -> generate_attribute_style(theme, attribute, id) end)
  end

  defp generate_attribute_style(theme, {attribute, element}, id) when is_struct(element) do
    "##{id} #{Element.to_css(Map.get(theme, attribute), gg_class(attribute))}"
  end

  defp generate_attribute_style(_theme_, _attribute, _id), do: ""

  defp gg_class(key) when is_atom(key) do
    attribute =
      key
      |> Atom.to_string()
      |> String.replace("_", "-")

    "gg-" <> attribute
  end
end