prawnpdf/prawn

# frozen_string_literal: true

module Prawn
  module Graphics
    # This module is used to place transparent content on the page. It has the
    # capacity for separate transparency values for stroked content and all
    # other content.
    module Transparency
      # @group Stable API

      # Set opacity.
      #
      # @example Both the fill and stroke will be at 50% opacity.
      #   pdf.transparent(0.5) do
      #     pdf.text("hello world")
      #     pdf.fill_and_stroke_circle([x, y], 25)
      #   end
      #
      # @example The fill will be at 50% opacity, but the stroke will be at 75% opacity.
      #   pdf.transparent(0.5, 0.75) do
      #     pdf.text("hello world")
      #     pdf.fill_and_stroke_circle([x, y], 25)
      #   end
      #
      # @param opacity [Number] Fill opacity. Clipped to the 0.0 to 1.0 range.
      # @param stroke_opacity [Number] Stroke opacity. Clipped to the
      #   0.0 to 1.0 range.
      # @yield
      # @return [void]
      def transparent(opacity, stroke_opacity = opacity)
        renderer.min_version(1.4)

        opacity = opacity.clamp(0.0, 1.0)
        stroke_opacity = stroke_opacity.clamp(0.0, 1.0)

        save_graphics_state
        renderer.add_content(
          "/#{opacity_dictionary_name(opacity, stroke_opacity)} gs",
        )
        yield
        restore_graphics_state
      end

      private

      def opacity_dictionary_registry
        @opacity_dictionary_registry ||= {}
      end

      def next_opacity_dictionary_id
        opacity_dictionary_registry.length + 1
      end

      def opacity_dictionary_name(opacity, stroke_opacity)
        key = "#{opacity}_#{stroke_opacity}"

        if opacity_dictionary_registry[key]
          dictionary = opacity_dictionary_registry[key][:obj]
          dictionary_name = opacity_dictionary_registry[key][:name]
        else
          dictionary = ref!(
            Type: :ExtGState,
            CA: stroke_opacity,
            ca: opacity,
          )

          dictionary_name = "Tr#{next_opacity_dictionary_id}"
          opacity_dictionary_registry[key] = {
            name: dictionary_name,
            obj: dictionary,
          }
        end

        page.ext_gstates[dictionary_name] = dictionary
        dictionary_name
      end
    end
  end
end