compute_public_path

# File actionpack/lib/action_view/helpers/asset_tag_helper.rb, line 528
        def compute_public_path(source, dir, ext = nil, include_host = true)
          has_request = @controller.respond_to?(:request)

          source_ext = File.extname(source)[1..-1]
          if ext && (source_ext.blank? || (ext != source_ext && File.exist?(File.join(ASSETS_DIR, dir, "#{source}.#{ext}"))))
            source += ".#{ext}"
          end

          unless source =~ %r{^[-a-z]+://}
            source = "/#{dir}/#{source}" unless source[0] == ?/

            source = rewrite_asset_path(source)

            if has_request && include_host
              unless source =~ %r{^#{ActionController::Base.relative_url_root}/}
                source = "#{ActionController::Base.relative_url_root}#{source}"
              end
            end
          end

          if include_host && source !~ %r{^[-a-z]+://}
            host = compute_asset_host(source)

            if has_request && !host.blank? && host !~ %r{^[-a-z]+://}
              host = "#{@controller.request.protocol}#{host}"
            end

            "#{host}#{source}"
          else
            source
          end
        end