module ActiveSupport
  # FileUpdateChecker specifies the API used by Rails to watch files
  # and control reloading. The API depends on four methods:
  #
  # * +initialize+ which expects two parameters and one block as
  #   described below.
  #
  # * +updated?+ which returns a boolean if there were updates in
  #   the filesystem or not.
  #
  # * +execute+ which executes the given block on initialization
  #   and updates the latest watched files and timestamp.
  #
  # * +execute_if_updated+ which just executes the block if it was updated.
  #
  # After initialization, a call to +execute_if_updated+ must execute
  # the block only if there was really a change in the filesystem.
  #
  # This class is used by Rails to reload the I18n framework whenever
  # they are changed upon a new request.
  #
  #   i18n_reloader = ActiveSupport::FileUpdateChecker.new(paths) do
  #     I18n.reload!
  #   end
  #
  #   ActionDispatch::Reloader.to_prepare do
  #     i18n_reloader.execute_if_updated
  #   end
  class FileUpdateChecker
    # It accepts two parameters on initialization. The first is an array
    # of files and the second is an optional hash of directories. The hash must
    # have directories as keys and the value is an array of extensions to be
    # watched under that directory.
    #
    # This method must also receive a block that will be called once a path
    # changes. The array of files and list of directories cannot be changed
    # after FileUpdateChecker has been initialized.
    def initialize(files, dirs = {}, &block)
      @files = files.freeze
      @glob  = compile_glob(dirs)
      @block = block

      @watched    = nil
      @updated_at = nil

      @last_watched   = watched
      @last_update_at = updated_at(@last_watched)
    end

    # Check if any of the entries were updated. If so, the watched and/or
    # updated_at values are cached until the block is executed via +execute+
    # or +execute_if_updated+.
    def updated?
      current_watched = watched
      if @last_watched.size != current_watched.size
        @watched = current_watched
        true
      else
        current_updated_at = updated_at(current_watched)
        if @last_update_at < current_updated_at
          @watched    = current_watched
          @updated_at = current_updated_at
          true
        else
          false
        end
      end
    end

    # Executes the given block and updates the latest watched files and
    # timestamp.
    def execute
      @last_watched   = watched
      @last_update_at = updated_at(@last_watched)
      @block.call
    ensure
      @watched = nil
      @updated_at = nil
    end

    # Execute the block given if updated.
    def execute_if_updated
      if updated?
        yield if block_given?
        execute
        true
      else
        false
      end
    end

    private

    def watched
      @watched || begin
        all = @files.select { |f| File.exist?(f) }
        all.concat(Dir[@glob]) if @glob
        all
      end
    end

    def updated_at(paths)
      @updated_at || max_mtime(paths) || Time.at(0)
    end

    # This method returns the maximum mtime of the files in +paths+, or +nil+
    # if the array is empty.
    #
    # Files with a mtime in the future are ignored. Such abnormal situation
    # can happen for example if the user changes the clock by hand. It is
    # healthy to consider this edge case because with mtimes in the future
    # reloading is not triggered.
    def max_mtime(paths)
      time_now = Time.now
      paths.map {|path| File.mtime(path)}.reject {|mtime| time_now < mtime}.max
    end

    def compile_glob(hash)
      hash.freeze # Freeze so changes aren't accidentally pushed
      return if hash.empty?

      globs = hash.map do |key, value|
        "#{escape(key)}/**/*#{compile_ext(value)}"
      end
      "{#{globs.join(",")}}"
    end

    def escape(key)
      key.gsub(',','\,')
    end

    def compile_ext(array)
      array = Array(array)
      return if array.empty?
      ".{#{array.join(",")}}"
    end
  end
end