Address perf+correctness of StrLen and StrChr

Cesium.Runtime.Tests/StringFunctionTests.cs

@@ -0,0 +1,59 @@
+using System.Text;
+
+namespace Cesium.Runtime.Tests;
+
+public unsafe class StringFunctionTests
+{
+    [Theory]
+    [InlineData("Hello\0", 5)]
+    [InlineData("Goodbye\0", 7)]
+    [InlineData("Hello\0Goodbye\0", 5)]
+    [InlineData("                  \0", 18)]
+    public void StrLen(string input, int expected)
+    {
+        // TODO: If you are rich enough to procure a 2-4+ GB RAM runner,
+        // please update this test to exercise the path where the string
+        // length exceeds int.MaxLength of bytes.
+        var bytes = Encoding.UTF8.GetBytes(input);
+        fixed (byte* str = bytes)
+        {
+            var actual = StringFunctions.StrLen(str);
+
+            Assert.Equal((nuint)expected, actual);
+        }
+    }
+
+    [Fact]
+    public void StrLen_Null()
+    {
+        var actual = StringFunctions.StrLen(null);
+
+        Assert.Equal((nuint)0, actual);
+    }
+
+    [Theory]
+    [InlineData("Hello\n", 5)]
+    [InlineData("Goodbye\n", 7)]
+    [InlineData("Hello\nGoodbye\n", 5)]
+    [InlineData("                  \n", 18)]
+    public void StrChr(string input, int expectedOffset)
+    {
+        var needle = '\n';
+        var bytes = Encoding.UTF8.GetBytes(input);
+        fixed (byte* str = bytes)
+        {
+            var ptr = StringFunctions.StrChr(str, '\n');
+
+            Assert.Equal((byte)needle, *ptr);
+            Assert.Equal(expectedOffset, (int)(ptr - str));
+        }
+    }
+
+    [Fact]
+    public void StrChr_Null()
+    {
+        var actual = StringFunctions.StrChr(null, '\0');
+
+        Assert.True(actual is null);
+    }
+}

Cesium.Runtime/Cesium.Runtime.csproj

@@ -12,4 +12,8 @@
         <InternalsVisibleTo Include="Cesium.Runtime.Tests" />
     </ItemGroup>
 
+    <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+      <PackageReference Include="System.Memory" Version="4.5.5" />
+    </ItemGroup>
+
 </Project>

Cesium.Runtime/StringFunctions.cs

@@ -1,7 +1,9 @@
 #if NETSTANDARD
+using System;
 using System.Text;
 #else
 using System.Collections.Specialized;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 #endif
 
@@ -14,26 +16,8 @@ public unsafe static class StringFunctions
 {
     public static nuint StrLen(byte* str)
     {
-#if NETSTANDARD
-        if (str == null)
-        {
-            return 0;
-        }
-
-        Encoding encoding = Encoding.UTF8;
-        int byteLength = 0;
-        byte* search = str;
-        while (*search != '\0')
-        {
-            byteLength++;
-            search++;
-        }
-
-        int stringLength = encoding.GetCharCount(str, byteLength);
-        return (uint)stringLength;
-#else
-        return (uint)(Marshal.PtrToStringUTF8((nint)str)?.Length ?? 0);
-#endif
+        var offset = StrChr(str, 0);
+        return (nuint)(offset - str);
     }
     public static byte* StrCpy(byte* dest, byte* src)
     {
@@ -186,19 +170,21 @@ public static int StrNCmp(byte* lhs, byte* rhs, nuint count)
     }
     public static byte* StrChr(byte* str, int ch)
     {
-        if (str == null)
+        if (str != null)
         {
-            return null;
-        }
+            int match;
+            nuint offset = 0;
+            byte c = (byte)ch;
 
-        while (*str != 0)
-        {
-            if (*str == ch)
+            // TODO: Copy IndexOfNullByte impl. from CoreLib in a distant future
+            var span = new Span<byte>(str, int.MaxValue);
+            while ((match = span.IndexOf(c)) < 0)
             {
-                return str;
+                offset += int.MaxValue;
+                span = new Span<byte>(str + offset, int.MaxValue);
             }
 
-            str++;
+            return str + ((uint)match + offset);
         }
 
         return null;