module SprocketsRelativeUrl
  class Processor < Sprockets::Processor
    CSS_URL_REGEXP = /
      (?<=\burl\()      # Match url\( prefix
      \s*
      (?<q>['"]?)       # Opening quote
      (?<path>[^\)]+?)  # Capture actual path
      \k<q>             # Backref for closing quote
      \s*
      (?=\))            # Match \) terminator
    /x

    def evaluate(context, *)
      data.gsub(CSS_URL_REGEXP) do
        path = Regexp.last_match[:path]

        next path if should_skip?(path)
        root_relative_path(context, path)
      end
    end

    private

    def root_relative_path(context, path)
      asset_full_path = context.pathname.parent.join(path)
      asset = context.environment.find_asset(asset_full_path)

      return path if asset.nil?

      context.asset_path(asset.logical_path)
    end

    def should_skip?(path)
      path.start_with?('/') || path.include?('\\') || URI.parse(path).absolute?
    end
  end
end