require "elm-compiler"

module Elm
  module Rails
    class Sprockets
      VERSION = '3'

      def self.instance
        @instance ||= new
      end

      def self.call(input)
        instance.call(input)
      end

      def self.cache_key
        instance.cache_key
      end

      attr_reader :cache_key

      def initialize(options = {})
        @cache_key = [self.class.name, VERSION, options].freeze
      end

      def call(input)
        context  = input[:environment].context_class.new(input)
        add_elm_dependencies(input[:filename], input[:load_path], context)
        context.metadata.merge(data: Elm::Compiler.compile(input[:filename]))
      end

      private

      # Add all Elm modules imported in the target file as dependencies, then
      # recursively do the same for each of those dependent modules.
      def add_elm_dependencies(filename, load_path, context)
        File.read(filename).each_line.flat_map do |line|
          # e.g. `import Quiz.QuestionStore exposing (..)`
          match = line.match(/^import\s+([^\s]+)/)

          next [] if match.nil?

          # e.g. Quiz.QuestionStore
          module_name = match.captures[0]

          # e.g. Quiz/QuestionStore
          dependency_logical_name = module_name.gsub(".", "/")

          # e.g. ~/NoRedInk/app/assets/javascripts/Quiz/QuestionStore.elm
          dependency_filepath = load_path + "/" + dependency_logical_name + ".elm"

          # If we don't find the dependency in our filesystem, assume it's because
          # it comes in through a third-party package rather than our sources.
          if File.file? dependency_filepath
            context.depend_on(dependency_logical_name)
          end
        end
      end
    end
  end
end