refactor(parser): update user agent parsing for zero allocations

Author: BenjaminAbt

.vscode/tasks.json

@@ -0,0 +1,25 @@
+{
+	"version": "2.0.0",
+	"tasks": [
+		{
+			"label": "test",
+			"type": "shell",
+			"command": "dotnet test --nologo",
+			"args": [],
+			"problemMatcher": [
+				"$msCompile"
+			],
+			"group": "build"
+		},
+		{
+			"label": "test",
+			"type": "shell",
+			"command": "dotnet test --nologo",
+			"args": [],
+			"problemMatcher": [
+				"$msCompile"
+			],
+			"group": "build"
+		}
+	]
+}

src/HttpUserAgentParser/HttpUserAgentParser.cs

@@ -14,28 +14,26 @@ public static class HttpUserAgentParser
 
 {
     /// <summary>
-    /// Parses given <param name="userAgent">user agent</param>
+    /// Parses given user agent string without allocating a copy. Prefer this overload to avoid ToString() allocations.
     /// </summary>
-    public static HttpUserAgentInformation Parse(ReadOnlySpan<char> userAgent)
+    public static HttpUserAgentInformation Parse(string userAgent)
     {
-        // prepare
-        userAgent = Cleanup(userAgent);
+        ReadOnlySpan<char> span = Cleanup(userAgent.AsSpan());
 
-        // analyze
-        if (TryGetRobot(userAgent, out string? robotName))
+        if (TryGetRobot(span, out string? robotName))
         {
-            return HttpUserAgentInformation.CreateForRobot(userAgent.ToString(), robotName);
+            return HttpUserAgentInformation.CreateForRobot(userAgent, robotName);
         }
 
-        HttpUserAgentPlatformInformation? platform = GetPlatform(userAgent);
-        string? mobileDeviceType = GetMobileDevice(userAgent);
+        HttpUserAgentPlatformInformation? platform = GetPlatform(span);
+        string? mobileDeviceType = GetMobileDevice(span);
 
-        if (TryGetBrowser(userAgent, out (string Name, string? Version)? browser))
+        if (TryGetBrowser(span, out (string Name, string? Version)? browser))
         {
-            return HttpUserAgentInformation.CreateForBrowser(userAgent.ToString(), platform, browser?.Name, browser?.Version, mobileDeviceType);
+            return HttpUserAgentInformation.CreateForBrowser(userAgent, platform, browser?.Name, browser?.Version, mobileDeviceType);
         }
 
-        return HttpUserAgentInformation.CreateForUnknown(userAgent.ToString(), platform, mobileDeviceType);
+        return HttpUserAgentInformation.CreateForUnknown(userAgent, platform, mobileDeviceType);
     }
 
     /// <summary>
@@ -105,7 +103,8 @@ public static (string Name, string? Version)? GetBrowser(ReadOnlySpan<char> user
             string? version = null;
             if (TryExtractVersion(userAgent, versionSearchStart, out Range range))
             {
-                version = userAgent.ToString();
+                // Only allocate the version substring, not the whole user agent
+                version = userAgent[range].ToString();
             }
 
             return (rule.Name, version);

src/HttpUserAgentParser/Providers/HttpUserAgentParserDefaultProvider.cs

@@ -8,7 +8,7 @@ namespace MyCSharp.HttpUserAgentParser.Providers;
 public class HttpUserAgentParserDefaultProvider : IHttpUserAgentParserProvider
 {
     /// <summary>
-    /// returns the result of <see cref="HttpUserAgentParser.Parse"/>
+    /// returns the result of <see cref="HttpUserAgentParser.Parse(string)"/>
     /// </summary>
     public HttpUserAgentInformation Parse(string userAgent)
         => HttpUserAgentParser.Parse(userAgent);