rails--rails/actionpack/lib/action_view/asset_paths.rb

require 'zlib'
require 'active_support/core_ext/file'
require 'action_controller/metal/exceptions'

module ActionView
  class AssetPaths #:nodoc:
    URI_REGEXP = %r{^[-a-z]+://|^cid:|^//}

    attr_reader :config, :controller

    def initialize(config, controller = nil)
      @config = config
      @controller = controller
    end

    # Add the extension +ext+ if not present. Return full or scheme-relative URLs otherwise untouched.
    # Prefix with <tt>/dir/</tt> if lacking a leading +/+. Account for relative URL
    # roots. Rewrite the asset path for cache-busting asset ids. Include
    # asset host, if configured, with the correct request protocol.
    #
    # When :relative (default), the protocol will be determined by the client using current protocol
    # When :request, the protocol will be the request protocol
    # Otherwise, the protocol is used (E.g. :http, :https, etc)
    def compute_public_path(source, dir, options = {})
      source = source.to_s
      return source if is_uri?(source)

      source = rewrite_extension(source, dir, options[:ext]) if options[:ext]
      source = rewrite_asset_path(source, dir, options)
      source = rewrite_relative_url_root(source, relative_url_root)
      source = rewrite_host_and_protocol(source, options[:protocol])
      source
    end

    # Return the filesystem path for the source
    def compute_source_path(source, dir, ext)
      source = rewrite_extension(source, dir, ext) if ext
      File.join(config.assets_dir, dir, source)
    end

    def is_uri?(path)
      path =~ URI_REGEXP
    end

  private

    def rewrite_extension(source, dir, ext)
      raise NotImplementedError
    end

    def rewrite_asset_path(source, path = nil)
      raise NotImplementedError
    end

    def rewrite_relative_url_root(source, relative_url_root)
      relative_url_root && !source.starts_with?("#{relative_url_root}/") ? "#{relative_url_root}#{source}" : source
    end

    def has_request?
      controller.respond_to?(:request)
    end

    def rewrite_host_and_protocol(source, protocol = nil)
      host = compute_asset_host(source)
      if host && !is_uri?(host)
        if (protocol || default_protocol) == :request && !has_request?
          host = nil
        else
          host = "#{compute_protocol(protocol)}#{host}"
        end
      end
      host ? "#{host}#{source}" : source
    end

    def compute_protocol(protocol)
      protocol ||= default_protocol
      case protocol
      when :relative
        "//"
      when :request
        unless @controller
          invalid_asset_host!("The protocol requested was :request. Consider using :relative instead.")
        end
        @controller.request.protocol
      else
        "#{protocol}://"
      end
    end

    def default_protocol
      @config.default_asset_host_protocol || (has_request? ? :request : :relative)
    end

    def invalid_asset_host!(help_message)
      raise ActionController::RoutingError, "This asset host cannot be computed without a request in scope. #{help_message}"
    end

    # Pick an asset host for this source. Returns +nil+ if no host is set,
    # the host if no wildcard is set, the host interpolated with the
    # numbers 0-3 if it contains <tt>%d</tt> (the number is the source hash mod 4),
    # or the value returned from invoking call on an object responding to call
    # (proc or otherwise).
    def compute_asset_host(source)
      if host = asset_host_config
        if host.respond_to?(:call)
          args = [source]
          arity = arity_of(host)
          if (arity > 1 || arity < -2) && !has_request?
            invalid_asset_host!("Remove the second argument to your asset_host Proc if you do not need the request, or make it optional.")
          end
          args << current_request if (arity > 1 || arity < 0) && has_request?
          host.call(*args)
        else
          (host =~ /%d/) ? host % (Zlib.crc32(source) % 4) : host
        end
      end
    end

    def relative_url_root
      config.relative_url_root || current_request.try(:script_name)
    end

    def asset_host_config
      config.asset_host
    end

    # Returns the current request if one exists.
    def current_request
      controller.request if has_request?
    end

    # Returns the arity of a callable
    def arity_of(callable)
      callable.respond_to?(:arity) ? callable.arity : callable.method(:call).arity
    end

  end
end