Combres - WebForm & MVC Client-side Resource Combine Library

#region License
// Copyright 2009 Buu Nguyen (http://www.buunguyen.net/blog)
// 
// Licensed under the Apache License, Version 2.0 (the "License"); 
// you may not use this file except in compliance with the License. 
// You may obtain a copy of the License at 
// 
// http://www.apache.org/licenses/LICENSE-2.0 
// 
// Unless required by applicable law or agreed to in writing, software 
// distributed under the License is distributed on an "AS IS" BASIS, 
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
// See the License for the specific language governing permissions and 
// limitations under the License.
// 
// The latest version of this file can be found at http://combres.codeplex.com
#endregion

using System;
using System.Text.RegularExpressions;
using log4net;

namespace Combres.Filters
{
    /// <summary>
    /// Filter which modifies relative urls to absolute urls (works for both virtual directory 
    /// as well as website).
    /// 
    /// Assume the path of the CSS file is ~/content/site.css, then the following transformations
    /// are done to each url reference in the CSS file:
    /// 1. Absolute path                ->      keep as-is
    ///       /path/to/image.gif        ->      /path/to/image.gif
    /// 2. Relative from CSS location   ->      make relative to application root
    ///       path/to/image.gif         ->      /content/path/to/imag2.gif 
    ///       ../path/to/style1.css     ->      /path/to/style1.css
    /// 3. Starting from application root
    ///     * ~/path/to/style2.css      ->      /content/path/to/style2.css
    /// 
    /// See http://combres.codeplex.com/Thread/View.aspx?ThreadId=64366 for error  description.
    /// </summary>
    public class FixUrlsInCssFilter : ICombresFilter
    {
        private static readonly ILog Log = LogManager.GetLogger(
                       System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

        public string TransformSingleContent(Settings settings, Resource resource, string content)
        {
            if (resource.ParentSet.Type == ResourceType.CSS)
                return Regex.Replace(content, @"url\((?<url>.*?)\)", 
                    match => FixUrl(resource.Path, match),
                    RegexOptions.IgnoreCase | RegexOptions.Singleline | RegexOptions.ExplicitCapture);
            return content;
        }

        private static string FixUrl(string cssPath, Match match)
        {
            try
            {
                const string template = "url(\"{0}\")";
                var url = match.Groups["url"].Value.Trim('\"', '\'');
                if (url.StartsWith("/"))
                    return string.Format(template, url);
                if (url.StartsWith("~"))
                    return string.Format(template, url.ResolveUrl());

                var cssFolder = cssPath.Substring(0, cssPath.LastIndexOf("/"));
                var backFolderCount = Regex.Matches(url, @"\.\./").Count;
                for (int i = 0; i < backFolderCount; i++)
                {
                    url = url.Substring(3); // skip 1 '../'
                    cssFolder = cssFolder.Substring(0, cssFolder.LastIndexOf("/")); // move back 1 folder
                }
                return string.Format(template, (cssFolder + "/" + url).ResolveUrl());
            }
            catch (Exception ex)
            {
                // Be lenient here, only log.  After all, this is just an image in the CSS file
                // and it should't be the reason to stop loading that CSS file.
                if (Log.IsWarnEnabled) 
                    Log.Warn("Cannot fix url " + match.Value, ex);
                return match.Value;
            }
        }

        public string TransformCombinedContent(Settings settings, ResourceSet set, string content)
        {
            return content;
        }

        public string TransformMinifiedContent(Settings settings, ResourceSet set, string content)
        {
            return content;
        }
    }
}