Optimize StringBuilder.Equals(ReadOnlySpan<char>) to compare by chunk (#565)

SimonCropp · web-flow · commit fcde467c6f74 · 2026-06-29T15:30:18.000+10:00
Walk the StringBuilder's contiguous chunks and compare each with the
vectorized ReadOnlySpan.SequenceEqual, instead of indexing per char
(the indexer walks the chunk list on every access). Add multi-chunk
and mismatch test coverage.
diff --git a/assemblySize.include.md b/assemblySize.include.md
@@ -4,12 +4,12 @@
 |----------------|----------------|---------------|-----------|-----------|--------------------|---------------------|-------------|
 | netstandard2.0 |          8.0KB |       351.5KB |  +343.5KB |    +9.5KB |             +6.5KB |              +9.5KB |     +14.0KB |
 | netstandard2.1 |          8.5KB |       306.0KB |  +297.5KB |    +8.5KB |             +6.0KB |              +9.0KB |     +13.5KB |
-| net461         |          8.5KB |       350.5KB |  +342.0KB |    +9.0KB |             +6.0KB |              +9.0KB |     +13.5KB |
+| net461         |          8.5KB |       350.5KB |  +342.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +13.5KB |
 | net462         |          7.0KB |       354.0KB |  +347.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +13.5KB |
 | net47          |          7.0KB |       353.5KB |  +346.5KB |    +9.0KB |             +6.5KB |              +9.5KB |     +13.5KB |
 | net471         |          8.5KB |       353.0KB |  +344.5KB |    +9.0KB |             +6.0KB |              +9.0KB |     +13.5KB |
 | net472         |          8.5KB |       351.5KB |  +343.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +14.0KB |
-| net48          |          8.5KB |       351.5KB |  +343.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +13.5KB |
+| net48          |          8.5KB |       351.5KB |  +343.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +14.0KB |
 | net481         |          8.5KB |       351.5KB |  +343.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +14.0KB |
 | netcoreapp2.0  |          9.0KB |       327.5KB |  +318.5KB |    +9.0KB |             +6.5KB |              +9.0KB |     +13.5KB |
 | netcoreapp2.1  |          9.0KB |       308.0KB |  +299.0KB |    +9.0KB |             +6.5KB |              +9.0KB |     +14.0KB |
@@ -31,12 +31,12 @@
 |----------------|----------------|---------------|-----------|-----------|--------------------|---------------------|-------------|
 | netstandard2.0 |          8.0KB |       513.4KB |  +505.4KB |   +17.2KB |             +8.2KB |             +14.4KB |     +19.4KB |
 | netstandard2.1 |          8.5KB |       441.7KB |  +433.2KB |   +16.2KB |             +7.7KB |             +13.9KB |     +18.9KB |
-| net461         |          8.5KB |       513.5KB |  +505.0KB |   +16.7KB |             +7.7KB |             +13.9KB |     +18.9KB |
+| net461         |          8.5KB |       513.5KB |  +505.0KB |   +16.7KB |             +8.2KB |             +13.9KB |     +18.9KB |
 | net462         |          7.0KB |       517.0KB |  +510.0KB |   +16.7KB |             +8.2KB |             +13.9KB |     +18.9KB |
 | net47          |          7.0KB |       516.2KB |  +509.2KB |   +16.7KB |             +8.2KB |             +14.4KB |     +18.9KB |
 | net471         |          8.5KB |       515.4KB |  +506.9KB |   +16.7KB |             +7.7KB |             +13.9KB |     +18.9KB |
 | net472         |          8.5KB |       512.8KB |  +504.3KB |   +16.7KB |             +8.2KB |             +13.9KB |     +19.4KB |
-| net48          |          8.5KB |       512.8KB |  +504.3KB |   +16.7KB |             +8.2KB |             +13.9KB |     +18.9KB |
+| net48          |          8.5KB |       512.8KB |  +504.3KB |   +16.7KB |             +8.2KB |             +13.9KB |     +19.4KB |
 | net481         |          8.5KB |       512.8KB |  +504.3KB |   +16.7KB |             +8.2KB |             +13.9KB |     +19.4KB |
 | netcoreapp2.0  |          9.0KB |       478.9KB |  +469.9KB |   +16.7KB |             +8.2KB |             +13.9KB |     +18.9KB |
 | netcoreapp2.1  |          9.0KB |       447.4KB |  +438.4KB |   +16.7KB |             +8.2KB |             +13.9KB |     +19.4KB |
diff --git a/src/Polyfill/Polyfill_StringBuilder.cs b/src/Polyfill/Polyfill_StringBuilder.cs
@@ -19,14 +19,17 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
             return false;
         }
 
-        for (var index = 0; index < target.Length; index++)
+        // Walk the contiguous chunks and compare each with the vectorized SequenceEqual,
+        // rather than indexing the StringBuilder per char (which walks the chunk list on every access).
+        foreach (var chunk in target.GetChunks())
         {
-            var ch1 = target[index];
-            var ch2 = span[index];
-            if (ch1 != ch2)
+            var chunkSpan = chunk.Span;
+            if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
             {
                 return false;
             }
+
+            span = span.Slice(chunkSpan.Length);
         }
 
         return true;
diff --git a/src/Split/net461/Polyfill_StringBuilder.cs b/src/Split/net461/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/net462/Polyfill_StringBuilder.cs b/src/Split/net462/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/net47/Polyfill_StringBuilder.cs b/src/Split/net47/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/net471/Polyfill_StringBuilder.cs b/src/Split/net471/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/net472/Polyfill_StringBuilder.cs b/src/Split/net472/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/net48/Polyfill_StringBuilder.cs b/src/Split/net48/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/net481/Polyfill_StringBuilder.cs b/src/Split/net481/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/netcoreapp2.0/Polyfill_StringBuilder.cs b/src/Split/netcoreapp2.0/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/netstandard2.0/Polyfill_StringBuilder.cs b/src/Split/netstandard2.0/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Split/uap10.0/Polyfill_StringBuilder.cs b/src/Split/uap10.0/Polyfill_StringBuilder.cs
@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)
 		{
 			return false;
 		}
-		for (var index = 0; index < target.Length; index++)
+		foreach (var chunk in target.GetChunks())
 		{
-			var ch1 = target[index];
-			var ch2 = span[index];
-			if (ch1 != ch2)
+			var chunkSpan = chunk.Span;
+			if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))
 			{
 				return false;
 			}
+			span = span.Slice(chunkSpan.Length);
 		}
 		return true;
 	}
diff --git a/src/Tests/PolyfillTests_Memory.cs b/src/Tests/PolyfillTests_Memory.cs
@@ -527,5 +527,28 @@ public async Task StringEqualsSpan()
     {
         var builder = new StringBuilder("value");
         await Assert.That(builder.Equals("value".AsSpan())).IsTrue();
+        await Assert.That(builder.Equals("other".AsSpan())).IsFalse();
+        await Assert.That(builder.Equals("val".AsSpan())).IsFalse();
+        await Assert.That(builder.Equals("values".AsSpan())).IsFalse();
+        await Assert.That(builder.Equals("".AsSpan())).IsFalse();
+        await Assert.That(new StringBuilder().Equals("".AsSpan())).IsTrue();
+    }
+
+    [Test]
+    public async Task StringEqualsSpanMultipleChunks()
+    {
+        // A small initial capacity followed by appends past it forces the
+        // StringBuilder to span multiple chunks, exercising the chunk-walking path.
+        var builder = new StringBuilder("a", 1);
+        builder.Append("bb");
+        builder.Append("ccc");
+
+        await Assert.That(builder.Equals("abbccc".AsSpan())).IsTrue();
+        // mismatch in the first chunk
+        await Assert.That(builder.Equals("xbbccc".AsSpan())).IsFalse();
+        // mismatch in a later chunk, after the span has advanced across a chunk boundary
+        await Assert.That(builder.Equals("abbcxc".AsSpan())).IsFalse();
+        // matching prefix but wrong length
+        await Assert.That(builder.Equals("abbcc".AsSpan())).IsFalse();
     }
 }

Original file line number	Diff line number	Diff line change
`@@ -19,14 +19,17 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)`
`19`	`19`	`return false;`
`20`	`20`	`}`
`21`	`21`
`22`		`- for (var index = 0; index < target.Length; index++)`
	`22`	`+ // Walk the contiguous chunks and compare each with the vectorized SequenceEqual,`
	`23`	`+ // rather than indexing the StringBuilder per char (which walks the chunk list on every access).`
	`24`	`+ foreach (var chunk in target.GetChunks())`
`23`	`25`	`{`
`24`		`- var ch1 = target[index];`
`25`		`- var ch2 = span[index];`
`26`		`- if (ch1 != ch2)`
	`26`	`+ var chunkSpan = chunk.Span;`
	`27`	`+ if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))`
`27`	`28`	`{`
`28`	`29`	`return false;`
`29`	`30`	`}`
	`31`	`+`
	`32`	`+ span = span.Slice(chunkSpan.Length);`
`30`	`33`	`}`
`31`	`34`
`32`	`35`	`return true;`
Original file line number	Diff line number	Diff line change
`@@ -16,14 +16,14 @@ public static bool Equals(this StringBuilder target, ReadOnlySpan<char> span)`
`16`	`16`	`{`
`17`	`17`	`return false;`
`18`	`18`	`}`
`19`		`- for (var index = 0; index < target.Length; index++)`
	`19`	`+ foreach (var chunk in target.GetChunks())`
`20`	`20`	`{`
`21`		`- var ch1 = target[index];`
`22`		`- var ch2 = span[index];`
`23`		`- if (ch1 != ch2)`
	`21`	`+ var chunkSpan = chunk.Span;`
	`22`	`+ if (!chunkSpan.SequenceEqual(span.Slice(0, chunkSpan.Length)))`
`24`	`23`	`{`
`25`	`24`	`return false;`
`26`	`25`	`}`
	`26`	`+ span = span.Slice(chunkSpan.Length);`
`27`	`27`	`}`
`28`	`28`	`return true;`
`29`	`29`	`}`