Refactor SDP parsing to use Span<T> and StringBuilder

Replaced Regex and string.Split with Span-based parsing for SDP lines and attributes. Updated ToString methods to use StringBuilder, reducing allocations and improving performance. Standardized string comparisons and improved parsing robustness. Changes applied across SDP, media format, connection, and security description classes for better efficiency and maintainability.

Author: paulomorgado

src/SIPSorcery/net/SDP/SDP.cs

@@ -17,6 +17,8 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using Polyfills;
+using SIPSorcery.Sys;
 using SIPSorceryMedia.Abstractions;
 
 namespace SIPSorcery.Net
@@ -148,7 +150,7 @@ public bool IsH264
         {
             get
             {
-                return (Rtpmap ?? "").ToUpperInvariant().Trim().StartsWith("H264");
+                return Rtpmap.AsSpan().Trim().StartsWith("H264", StringComparison.OrdinalIgnoreCase);
             }
         }
 
@@ -164,15 +166,15 @@ public bool IsMJPEG
         {
             get
             {
-                return (Rtpmap ?? "").ToUpperInvariant().Trim().StartsWith("JPEG");
+                return Rtpmap.AsSpan().Trim().StartsWith("JPEG", StringComparison.OrdinalIgnoreCase);
             }
         }
 
         public bool isH265
         {
             get
             {
-                return (Rtpmap ?? "").ToUpperInvariant().Trim().StartsWith("H265");
+                return Rtpmap.AsSpan().Trim().StartsWith("H265", StringComparison.OrdinalIgnoreCase);
             }
         }
 
@@ -194,18 +196,20 @@ public bool CheckCompatible()
 
         private static Dictionary<string, string> ParseWebRtcParameters(string input)
         {
-            var parameters = new Dictionary<string, string>();
+            var parameters = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
             if (string.IsNullOrEmpty(input))
             {
                 return parameters;
             }
 
-            foreach (var pair in input.Split(';'))
+            Span<Range> keyValueRange = stackalloc Range[3];
+            var inputSpan = input.AsSpan();
+            foreach (var pairRange in inputSpan.Split(';'))
             {
-                var keyValue = pair.Split('=');
-                if (keyValue.Length == 2)
+                var pair = inputSpan[pairRange];
+                if (pair.Split(keyValueRange, '=') == 2)
                 {
-                    parameters[keyValue[0].Trim().ToLowerInvariant()] = keyValue[1].Trim();
+                    parameters[pair[keyValueRange[0]].Trim().ToString()] = pair[keyValueRange[1]].Trim().ToString();
                 }
             }
 
@@ -314,7 +318,7 @@ public SDPAudioVideoMediaFormat(TextFormat textFormat)
         {
             Kind = SDPMediaTypesEnum.text;
             ID = textFormat.FormatID;
-            Rtpmap = null;  
+            Rtpmap = null;
             Fmtp = textFormat.Parameters;
             _isEmpty = false;
 
@@ -458,7 +462,7 @@ public static bool AreMatch(SDPAudioVideoMediaFormat format1, SDPAudioVideoMedia
             // rtpmap takes priority as well known format ID's can be overruled.
             if (format1.Rtpmap != null && format2.Rtpmap != null)
             {
-                if (string.Equals(format1.Rtpmap.Trim(), format2.Rtpmap.Trim(), StringComparison.OrdinalIgnoreCase))
+                if (format1.Rtpmap.AsSpan().Trim().Equals(format2.Rtpmap.AsSpan().Trim(), StringComparison.OrdinalIgnoreCase))
                 {
                     return true;
                 }
@@ -471,7 +475,7 @@ public static bool AreMatch(SDPAudioVideoMediaFormat format1, SDPAudioVideoMedia
                 return true;
             }
             return false;
-            
+
         }
 
         /// <summary>
@@ -558,24 +562,46 @@ public static bool TryParseRtpmap(string rtpmap, out string name, out int clockR
             }
             else
             {
-                string[] fields = rtpmap.Trim().Split('/');
+                var rtpmapSpan = rtpmap.AsSpan().Trim();
+                var nameSpan = default(ReadOnlySpan<char>);
+                var clockRateSpan = default(ReadOnlySpan<char>);
+                var channelsSpan = default(ReadOnlySpan<char>);
+                var fieldIndex = 0;
+
+                foreach (var fieldRange in rtpmapSpan.Split('/'))
+                {
+                    var field = rtpmapSpan[fieldRange].Trim();
+
+                    if (fieldIndex == 0)
+                    {
+                        nameSpan = field;
+                    }
+                    else if (fieldIndex == 1)
+                    {
+                        clockRateSpan = field;
+                    }
+                    else if (fieldIndex == 2)
+                    {
+                        channelsSpan = field;
+                        break;
+                    }
+
+                    fieldIndex++;
+                }
 
-                if (fields.Length >= 2)
+                if (fieldIndex >= 1)
                 {
-                    name = fields[0].Trim();
-                    if (!int.TryParse(fields[1].Trim(), out clockRate))
+                    if (!int.TryParse(clockRateSpan, out clockRate))
                     {
                         return false;
                     }
 
-                    if (fields.Length >= 3)
+                    if (!channelsSpan.IsEmpty && !int.TryParse(channelsSpan, out channels))
                     {
-                        if (!int.TryParse(fields[2].Trim(), out channels))
-                        {
-                            return false;
-                        }
+                        return false;
                     }
 
+                    name = nameSpan.ToString();
                     return true;
                 }
                 else
@@ -630,8 +656,8 @@ public static SDPAudioVideoMediaFormat GetFormatForName(List<SDPAudioVideoMediaF
             }
             else
             {
-                return formats.Any(x => x.Name()?.ToLower() == formatName?.ToLower()) ?
-                   formats.First(x => x.Name()?.ToLower() == formatName?.ToLower()) :
+                return formats.Any(x => string.Equals(x.Name(), formatName, StringComparison.OrdinalIgnoreCase)) ?
+                   formats.First(x => string.Equals(x.Name(), formatName, StringComparison.OrdinalIgnoreCase)) :
                    Empty;
             }
         }

src/SIPSorcery/net/SDP/SDPAudioVideoMediaFormat.cs

@@ -13,8 +13,11 @@
 // BSD 3-Clause "New" or "Revised" License, see included LICENSE.md file.
 //-----------------------------------------------------------------------------
 
+using System;
 using System.Net;
 using System.Net.Sockets;
+using Polyfills;
+using SIPSorcery.Sys;
 
 namespace SIPSorcery.Net
 {
@@ -52,16 +55,34 @@ public SDPConnectionInformation(IPAddress connectionAddress)
         public static SDPConnectionInformation ParseConnectionInformation(string connectionLine)
         {
             SDPConnectionInformation connectionInfo = new SDPConnectionInformation();
-            string[] connectionFields = connectionLine.Substring(2).Trim().Split(' ');
-            connectionInfo.ConnectionNetworkType = connectionFields[0].Trim();
-            connectionInfo.ConnectionAddressType = connectionFields[1].Trim();
-            connectionInfo.ConnectionAddress = connectionFields[2].Trim();
+            var connectionFields = connectionLine.AsSpan(2).Trim();
+            var fieldIndex = 0;
+            foreach (var fieldRange in connectionFields.Split(' '))
+            {
+                var field = connectionFields[fieldRange].Trim().ToString();
+                if (fieldIndex == 0)
+                {
+                    connectionInfo.ConnectionNetworkType = field;
+                }
+                else if (fieldIndex == 1)
+                {
+                    connectionInfo.ConnectionAddressType = field;
+                }
+                else if (fieldIndex == 2)
+                {
+                    connectionInfo.ConnectionAddress = field;
+                    break;
+                }
+
+                fieldIndex++;
+            }
+
             return connectionInfo;
         }
 
         public override string ToString()
         {
-            return "c=" + ConnectionNetworkType + " " + ConnectionAddressType + " " + ConnectionAddress + m_CRLF;
+            return $"c={ConnectionNetworkType} {ConnectionAddressType} {ConnectionAddress}{m_CRLF}";
         }
     }
 }