[patch] Skip empty split entries in a loop, and honour TrimEntries and combined options

Semantics.Strings/SemanticString.cs

@@ -975,6 +975,10 @@ public ref struct SpanSplitEnumerator
 		private readonly char _separator;
 		private readonly StringSplitOptions _options;
 
+		// StringSplitOptions.TrimEntries, which netstandard2.0/2.1 do not name. The value is the same on
+		// every target, so a caller on .NET 5+ passing the named member is honoured everywhere.
+		private const StringSplitOptions TrimEntries = (StringSplitOptions)2;
+
 		internal SpanSplitEnumerator(ReadOnlySpan<char> span, char separator, StringSplitOptions options)
 		{
 			_remaining = span;
@@ -1000,30 +1004,36 @@ internal SpanSplitEnumerator(ReadOnlySpan<char> span, char separator, StringSpli
 		/// <returns>true if there is a next segment; otherwise, false.</returns>
 		public bool MoveNext()
 		{
-			if (_remaining.IsEmpty)
-			{
-				return false;
-			}
-
-			int separatorIndex = _remaining.IndexOf(_separator);
-			if (separatorIndex >= 0)
-			{
-				Current = _remaining[..separatorIndex];
-				_remaining = _remaining[(separatorIndex + 1)..];
-			}
-			else
-			{
-				Current = _remaining;
-				_remaining = default;
-			}
-
-			// Handle StringSplitOptions.RemoveEmptyEntries
-			if (_options == StringSplitOptions.RemoveEmptyEntries && Current.IsEmpty)
+			// A loop, not recursion: a long run of separators with RemoveEmptyEntries would otherwise
+			// take one stack frame per empty entry and overflow the stack.
+			while (!_remaining.IsEmpty)
 			{
-				return MoveNext(); // Recursively skip empty entries
+				int separatorIndex = _remaining.IndexOf(_separator);
+				if (separatorIndex >= 0)
+				{
+					Current = _remaining[..separatorIndex];
+					_remaining = _remaining[(separatorIndex + 1)..];
+				}
+				else
+				{
+					Current = _remaining;
+					_remaining = default;
+				}
+
+				if ((_options & TrimEntries) != 0)
+				{
+					Current = Current.Trim();
+				}
+
+				if ((_options & StringSplitOptions.RemoveEmptyEntries) != 0 && Current.IsEmpty)
+				{
+					continue;
+				}
+
+				return true;
 			}
 
-			return true;
+			return false;
 		}
 	}
 }

Semantics.Test/SemanticStringTests.cs

@@ -1416,6 +1416,54 @@ public void SpanSplitEnumerator_OnlySeparators_HandlesCorrectly()
 		Assert.IsTrue(parts.All(string.IsNullOrEmpty), "All parts should be empty");
 	}
 
+	private static readonly string[] expectedOnlyX = ["x"];
+	private static readonly string[] expectedAB = ["a", "b"];
+	private static readonly string[] expectedTrimmed = ["a", "b", "", "c"];
+
+	private static List<string> SplitToList(string value, StringSplitOptions options)
+	{
+		MySemanticString semantic = SemanticString<MySemanticString>.Create<MySemanticString>(value);
+		List<string> parts = [];
+		foreach (ReadOnlySpan<char> part in semantic.Split(',', options))
+		{
+			parts.Add(part.ToString());
+		}
+
+		return parts;
+	}
+
+	[TestMethod]
+	public void SpanSplitEnumerator_RemoveEmptyEntries_LongRunOfSeparators_DoesNotOverflowTheStack()
+	{
+		List<string> parts = SplitToList(new string(',', 1_000_000) + "x", StringSplitOptions.RemoveEmptyEntries);
+
+		Assert.AreSequenceEqual(expectedOnlyX, parts);
+	}
+
+	[TestMethod]
+	public void SpanSplitEnumerator_RemoveEmptyEntriesWithTrimEntries_RemovesWhitespaceEntries()
+	{
+		List<string> parts = SplitToList("a, ,b", StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+		Assert.AreSequenceEqual(expectedAB, parts);
+	}
+
+	[TestMethod]
+	public void SpanSplitEnumerator_RemoveEmptyEntriesWithTrimEntries_RemovesEmptyEntries()
+	{
+		List<string> parts = SplitToList("a,,b", StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+		Assert.AreSequenceEqual(expectedAB, parts);
+	}
+
+	[TestMethod]
+	public void SpanSplitEnumerator_TrimEntries_TrimsEachEntry()
+	{
+		List<string> parts = SplitToList(" a , b ,  ,c", StringSplitOptions.TrimEntries);
+
+		Assert.AreSequenceEqual(expectedTrimmed, parts);
+	}
+
 	[TestMethod]
 	public void ErrorMessages_ContainTypeInformation()
 	{