using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Web;
 
namespace System.Text {
    public static class StringTransformer {
        static readonly string linkFormat = "<a href=\"{0}\" title=\"{0}\" target=\"{2}\" class=\"{3}\">{1}</a>";
 
        /// <summary>
        /// Replaces all URLs in a string with HTML links.
        /// </summary>
        /// <param name="text"></param>
        /// <param name="linkClasses"></param>
        /// <param name="linkTarget"></param>
        /// <returns></returns>
        public static string LinksAsHtml(this string text, string linkClasses = "external", string linkTarget = "_self") {
            var regProtocol = new Regex(@"(https?://)?");
            var regDomain   = new Regex(@"((?:[-a-zA-Z0-9]{1,63}\.)+[-a-zA-Z0-9]{2,63}|(?:[0-9]{1,3}\.){3}[0-9]{1,3})");
            var regPort     = new Regex(@"(:[0-9]{1,5})?");
            var regPath     = new Regex(@"(/[!$-/0-9:;=@_\':;!a-zA-Z\x7f-\xff]*?)?");
            var regQuery    = new Regex(@"(\?[!$-/0-9:;=@_\':;!a-zA-Z\x7f-\xff]+?)?");
            var regFragment = new Regex(@"(#[!$-/0-9:;=@_\':;!a-zA-Z\x7f-\xff]+?)?");
            var regMain = new Regex("\\b" + regProtocol + regDomain + regPort + regPath + regQuery + regFragment + "(?=[?.!,;:\"]?([\\s]|$))");
            
            var results = regMain.Matches(text);
            var list = new List<string>();
            var matches = new List<Match>();
 
            foreach (Match item in results) {
                matches.Add(item);
            }
 
            foreach (Match match in matches.OrderByDescending(m => m.Length))
            {
                if (list.Contains(match.Value) || list.Any(s => s.Contains(match.Value)))
                    continue;
                list.Add(match.Value);
 
                string full = match.Value, friendly = full;
                if (string.IsNullOrEmpty(match.Groups[1].Value))
                    full = "http://" + full;
                if (friendly.Length > 50)
                    friendly = full.Substring(0, 25) + "..." + full.Substring(full.Length-26);
 
                full = HttpUtility.HtmlEncode(full);
                friendly = HttpUtility.HtmlEncode(friendly);
 
                string link = string.Format(linkFormat, full, friendly, linkTarget, linkClasses);
 
                text = text.Replace(match.Value, link);
            }
 
            return text;
        }
    }
}