perf: drop the pool dance from MemorySliceLineSource.Concat (#119)

Csv/CsvReader.Engine.cs

@@ -107,13 +107,11 @@
         internal struct MemorySliceLineSource : ILineSource
         {
             private readonly ReadOnlyMemory<char> csv;
-            private readonly CsvMemoryOptions memoryOptions;
             private int position;
 
-            public MemorySliceLineSource(ReadOnlyMemory<char> csv, CsvMemoryOptions memoryOptions)
+            public MemorySliceLineSource(ReadOnlyMemory<char> csv)
             {
                 this.csv = csv;
-                this.memoryOptions = memoryOptions;
                 this.position = 0;
             }
 
@@ -164,27 +162,8 @@
 
             public MemoryText Concat(MemoryText head, string newLine, MemoryText tail, out string? combined)
             {
-                combined = null;
-
-                var separator = newLine.AsMemory();
-                var totalLength = head.Length + separator.Length + tail.Length;
-                var buffer = memoryOptions.CharArrayPool.Rent(totalLength);
-
-                try
-                {
-                    var span = buffer.AsSpan();
-                    head.Span.CopyTo(span);
-                    separator.Span.CopyTo(span.Slice(head.Length));
-                    tail.Span.CopyTo(span.Slice(head.Length + separator.Length));
-
-                    var result = new char[totalLength];
-                    span.Slice(0, totalLength).CopyTo(result);
-                    return result.AsMemory();
-                }
-                finally
-                {
-                    memoryOptions.CharArrayPool.Return(buffer);
-                }
+                combined = string.Concat(head.Span, newLine.AsSpan(), tail.Span);
+                return combined.AsMemory();
             }
         }
 
@@ -294,7 +273,7 @@
                     // case via index == RowsToSkip + 1 and skips its own multiline pass to avoid double-reading.
                     if (!skipInitialLine && options.AllowNewLineInEnclosedFieldValues)
                     {
                         var splitLine = options.Splitter.Split(line, options);
 
                         while (splitLine.Count > 0 && CsvLineSplitter.IsUnterminatedQuotedValue(splitLine[splitLine.Count - 1].AsSpan(), options))
                         {
@@ -349,7 +328,7 @@
                 var isFirstDataLineInHeaderAbsentMode = options.HeaderMode == HeaderMode.HeaderAbsent && index == (options.RowsToSkip + 1);
                 if (options.AllowNewLineInEnclosedFieldValues && !isFirstDataLineInHeaderAbsentMode)
                 {
                     var rawSplit = options.Splitter.Split(line, options);
                     while (rawSplit.Count > 0 && CsvLineSplitter.IsUnterminatedQuotedValue(rawSplit[rawSplit.Count - 1].AsSpan(), options))
                     {
                         if (!source.TryReadLine(out var nextLine, out _))
@@ -397,7 +376,7 @@
                     // case via index == RowsToSkip + 1 and skips its own multiline pass to avoid double-reading.
                     if (!skipInitialLine && options.AllowNewLineInEnclosedFieldValues)
                     {
                         var splitLine = options.Splitter.Split(line, options);
 
                         while (splitLine.Count > 0 && CsvLineSplitter.IsUnterminatedQuotedValue(splitLine[splitLine.Count - 1].AsSpan(), options))
                         {
@@ -453,7 +432,7 @@
                 var isFirstDataLineInHeaderAbsentMode = options.HeaderMode == HeaderMode.HeaderAbsent && index == (options.RowsToSkip + 1);
                 if (options.AllowNewLineInEnclosedFieldValues && !isFirstDataLineInHeaderAbsentMode)
                 {
                     var rawSplit = options.Splitter.Split(line, options);
                     while (rawSplit.Count > 0 && CsvLineSplitter.IsUnterminatedQuotedValue(rawSplit[rawSplit.Count - 1].AsSpan(), options))
                     {
                         var (nextOk, nextLine, _) = await source.TryReadLineAsync(ct).ConfigureAwait(false);

Csv/CsvReader.cs

@@ -158,7 +158,7 @@ public static CsvBufferWriter CreateBufferWriter(ReadOnlySpan<string> headers, c
         }
 
         private static IEnumerable<ICsvLineSpan> ReadFromMemoryOptimizedImpl(ReadOnlyMemory<char> csv, CsvOptions options, CsvMemoryOptions memoryOptions)
-            => Enumerate<MemorySliceLineSource, OptimizedRowFactory, ReadLineSpanOptimized>(new MemorySliceLineSource(csv, memoryOptions), new OptimizedRowFactory(memoryOptions), options);
+            => Enumerate<MemorySliceLineSource, OptimizedRowFactory, ReadLineSpanOptimized>(new MemorySliceLineSource(csv), new OptimizedRowFactory(memoryOptions), options);
 
 #endif