Fix empty PooledString handling and add comprehensive insert tests

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lookbusy1344

Demo/Demo.cs

@@ -60,7 +60,7 @@ public static void Main()
 
 		Console.WriteLine($"Buffer dump: \"{pool.DumpBufferAsString()}\"");
 
-		var last = PooledString.Empty;
+		var last = pool.CreateEmptyString();
 		for (var i = 0; i < 100; ++i) {
 			var randomStr = RandomString(Random.Shared.Next(5, 150));
 			var pooledStr = pool.Allocate(randomStr);

PooledString.cs

@@ -26,11 +26,6 @@ public readonly record struct PooledString(UnmanagedStringPool Pool, uint Alloca
 	// NOTE this struct is technically immutable, but some methods mutate the underlying pool like SetAtPosition() and Free()
 	// It also implements IDisposable to call Free() automatically
 
-	/// <summary>
-	/// Represents an empty pooled string
-	/// </summary>
-	public static readonly PooledString Empty = new(null!, UnmanagedStringPool.EmptyStringAllocationId);
-
 	#region Public API
 
 	/// <summary>
@@ -64,13 +59,8 @@ public readonly PooledString Insert(int pos, ReadOnlySpan<char> value)
 			return this;
 		}
 
-		if (AllocationId == UnmanagedStringPool.EmptyStringAllocationId) {
-			if (pos != 0) {
-				throw new ArgumentOutOfRangeException(nameof(pos), "Cannot insert into an empty string at position other than 0");
-			}
-
-			throw new InvalidOperationException(
-				"Cannot insert into PooledString.Empty without a valid pool. Use pool.Allocate() to create a new string.");
+		if (AllocationId == UnmanagedStringPool.EmptyStringAllocationId && pos != 0) {
+			throw new ArgumentOutOfRangeException(nameof(pos), "Cannot insert into an empty string at position other than 0");
 		}
 
 		CheckDisposed();
@@ -356,8 +346,7 @@ public override readonly int GetHashCode()
 	/// </summary>
 	private readonly void CheckDisposed()
 	{
-		// Empty string is always valid, no pool needed
-		if (AllocationId != UnmanagedStringPool.EmptyStringAllocationId && Pool?.IsDisposed != false) {
+		if (Pool.IsDisposed) {
 			throw new ObjectDisposedException(nameof(PooledString));
 		}
 	}

Tests/ConcurrentAccessTests.cs

@@ -430,7 +430,7 @@ public async Task ConcurrentEmptyStringAccess_AlwaysThreadSafeAsync()
 			tasks[t] = Task.Run(() => {
 				try {
 					for (var i = 0; i < 200; i++) {
-						var empty = PooledString.Empty;
+						var empty = pool.CreateEmptyString();
 
 						// All these operations should be thread-safe on empty strings
 						var span = empty.AsSpan();

Tests/DisposalAndLifecycleTests.cs

@@ -236,7 +236,8 @@ public void PooledString_UseAfterFree_ThrowsException()
 	[Fact]
 	public void EmptyString_Dispose_DoesNotThrow()
 	{
-		var empty = PooledString.Empty;
+		using var pool = new UnmanagedStringPool(1024);
+		var empty = pool.CreateEmptyString();
 
 		empty.Free();
 		empty.Dispose();
@@ -247,7 +248,8 @@ public void EmptyString_Dispose_DoesNotThrow()
 	[Fact]
 	public void EmptyString_MultipleDispose_Safe()
 	{
-		var empty = PooledString.Empty;
+		using var pool = new UnmanagedStringPool(1024);
+		var empty = pool.CreateEmptyString();
 
 		for (var i = 0; i < 10; i++) {
 			empty.Free();
@@ -263,8 +265,9 @@ public void EmptyString_MultipleDispose_Safe()
 	[Fact]
 	public void EmptyString_AlwaysValid_EvenAfterMultipleDisposals()
 	{
-		var empty1 = PooledString.Empty;
-		var empty2 = PooledString.Empty;
+		using var pool = new UnmanagedStringPool(1024);
+		var empty1 = pool.CreateEmptyString();
+		var empty2 = pool.CreateEmptyString();
 
 		empty1.Dispose();
 		empty2.Free();

Tests/IntegerOverflowTests.cs

@@ -337,7 +337,7 @@ public void BufferMemoryCopy_LengthCalculations_HandleBoundaries()
 		foreach (var testStr in testStrings) {
 			PooledString result;
 			if (string.IsNullOrEmpty(testStr)) {
-				// Empty string case: allocate "PREFIX" directly since pool.Allocate("") returns PooledString.Empty
+				// Empty string case: allocate "PREFIX" directly since we can't insert into an empty string from a different pool
 				result = pool.Allocate("PREFIX");
 			} else {
 				var str = pool.Allocate(testStr);

Tests/PooledStringTests.cs

@@ -60,9 +60,8 @@ public void Insert_EmptyValue_ReturnsOriginal()
 	[Fact]
 	public void Insert_IntoEmptyString_WorksCorrectly()
 	{
-		var empty = pool.Allocate("");
-		// Since pool.Allocate("") returns PooledString.Empty, we need to allocate through the pool directly
-		var result = pool.Allocate("Hello");
+		var empty = pool.CreateEmptyString();
+		var result = empty.Insert(0, "Hello");
 
 		Assert.Equal("Hello", result.ToString());
 	}
@@ -80,11 +79,38 @@ public void Insert_InvalidPosition_ThrowsArgumentOutOfRangeException(int positio
 	[Fact]
 	public void Insert_IntoEmptyStringInvalidPosition_ThrowsArgumentOutOfRangeException()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 
 		Assert.Throws<ArgumentOutOfRangeException>(() => empty.Insert(1, "test"));
 	}
 
+	[Fact]
+	public void Insert_IntoAllocatedEmptyString_WorksCorrectly()
+	{
+		var empty = pool.Allocate("");
+		var result = empty.Insert(0, "World");
+
+		Assert.Equal("World", result.ToString());
+	}
+
+	[Fact]
+	public void Insert_AppendToString_WorksCorrectly()
+	{
+		var str = pool.Allocate("Hello");
+		var result = str.Insert(str.Length, " World");
+
+		Assert.Equal("Hello World", result.ToString());
+	}
+
+	[Fact]
+	public void Insert_AppendToEmptyString_WorksCorrectly()
+	{
+		var empty = pool.CreateEmptyString();
+		var result = empty.Insert(empty.Length, "Appended");
+
+		Assert.Equal("Appended", result.ToString());
+	}
+
 	#endregion
 
 	#region Replace Tests
@@ -172,6 +198,7 @@ public void Replace_EntireString_WorksCorrectly()
 
 	#endregion
 
+
 	#region Search Operations Tests
 
 	[Fact]
@@ -203,7 +230,7 @@ public void IndexOf_EmptyString_ReturnsZero()
 	[Fact]
 	public void IndexOf_InEmptyString_ReturnsCorrectly()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 
 		Assert.Equal(0, empty.IndexOf(""));
 		Assert.Equal(-1, empty.IndexOf("test"));
@@ -303,7 +330,7 @@ public void EndsWith_Incorrect_ReturnsFalse()
 	[Fact]
 	public void StringOperations_WithEmptyString_WorkCorrectly()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 
 		Assert.True(empty.Contains(""));
 		Assert.True(empty.StartsWith(""));
@@ -382,7 +409,7 @@ public void AsSpan_ValidString_ReturnsCorrectSpan()
 	[Fact]
 	public void AsSpan_EmptyString_ReturnsEmptySpan()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 		var span = empty.AsSpan();
 
 		Assert.True(span.IsEmpty);
@@ -425,8 +452,9 @@ public void Equals_DifferentContent_ReturnsFalse()
 	[Fact]
 	public void Equals_EmptyStrings_ReturnsTrue()
 	{
-		var empty1 = PooledString.Empty;
-		var empty2 = PooledString.Empty;
+		using var pool = new UnmanagedStringPool(1024);
+		var empty1 = pool.CreateEmptyString();
+		var empty2 = pool.CreateEmptyString();
 
 		Assert.True(empty1.Equals(empty2));
 		Assert.True(empty1 == empty2);
@@ -435,7 +463,7 @@ public void Equals_EmptyStrings_ReturnsTrue()
 	[Fact]
 	public void Equals_EmptyWithAllocated_WorksCorrectly()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 		var emptyAllocated = pool.Allocate("");
 		var nonEmpty = pool.Allocate("Hello");
 
@@ -501,7 +529,7 @@ public void GetHashCode_RandomStrings_MostHaveDifferentHashes()
 	[Fact]
 	public void GetHashCode_EmptyString_ReturnsZero()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 
 		Assert.Equal(0, empty.GetHashCode());
 	}
@@ -639,7 +667,7 @@ public void Length_CorrectlyReturnsStringLength()
 	[Fact]
 	public void Length_EmptyString_ReturnsZero()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 		var emptyAllocated = pool.Allocate("");
 
 		Assert.Equal(0, empty.Length);
@@ -649,7 +677,7 @@ public void Length_EmptyString_ReturnsZero()
 	[Fact]
 	public void IsEmpty_EmptyString_ReturnsTrue()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 		var emptyAllocated = pool.Allocate("");
 
 		Assert.True(empty.IsEmpty);
@@ -679,7 +707,7 @@ public void ToString_ValidString_ReturnsCorrectString()
 	[Fact]
 	public void ToString_EmptyString_ReturnsEmptyString()
 	{
-		var empty = PooledString.Empty;
+		var empty = pool.CreateEmptyString();
 
 		Assert.Equal("", empty.ToString());
 	}

Tests/UnmanagedStringPoolTests.cs

@@ -221,7 +221,7 @@ public void AsSpan_AfterPoolDisposal_ThrowsObjectDisposedException()
 	[Fact]
 	public void FreeString_EmptyString_NoEffect()
 	{
-		var emptyStr = PooledString.Empty;
+		var emptyStr = pool.CreateEmptyString();
 
 		// Should not throw or cause issues
 		emptyStr.Free();

UnmanagedStringPool.cs

@@ -144,7 +144,7 @@ public PooledString Allocate(ReadOnlySpan<char> value)
 		ObjectDisposedException.ThrowIf(IsDisposed, typeof(UnmanagedStringPool));
 
 		if (value.IsEmpty) {
-			return PooledString.Empty;
+			return CreateEmptyString();
 		}
 
 		// allocate a buffer for this string
@@ -274,7 +274,7 @@ internal PooledString Allocate(int lengthChars)
 		ObjectDisposedException.ThrowIf(IsDisposed, typeof(UnmanagedStringPool));
 
 		if (lengthChars <= 0) {
-			return PooledString.Empty;
+			return CreateEmptyString();
 		}
 
 		// Check for overflow when converting to bytes and aligning
@@ -367,6 +367,12 @@ internal void FreeString(uint id)
 		}
 	}
 
+	/// <summary>
+	/// Create an empty PooledString in this pool. Requires a pool so we know where to add if insertion occurs
+	/// </summary>
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	internal PooledString CreateEmptyString() => new(this, EmptyStringAllocationId);
+
 	#endregion
 
 	#region Private methods