S.IO.StringReader: Use ReadOnlySpan.IndexOfAny in ReadLine() for performance

src/libraries/System.IO/tests/StringReader/StringReader.CtorTests.cs

@@ -59,7 +59,7 @@ public static void ReadLine()
 
             using (StringReader sr = new StringReader(str1))
             {
-                Assert.Equal(str1, sr.ReadLine());
+                Assert.Same(str1, sr.ReadLine());
             }
             using (StringReader sr = new StringReader(str2))
             {

src/libraries/System.Private.CoreLib/src/System/IO/StringReader.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -11,7 +12,6 @@ public class StringReader : TextReader
     {
         private string? _s;
         private int _pos;
-        private int _length;
 
         public StringReader(string s)
         {
@@ -21,7 +21,6 @@ public StringReader(string s)
             }
 
             _s = s;
-            _length = s.Length;
         }
 
         public override void Close()
@@ -33,7 +32,6 @@ protected override void Dispose(bool disposing)
         {
             _s = null;
             _pos = 0;
-            _length = 0;
             base.Dispose(disposing);
         }
 
@@ -44,33 +42,40 @@ protected override void Dispose(bool disposing)
         //
         public override int Peek()
         {
-            if (_s == null)
+            string? s = _s;
+            if (s == null)
             {
-                throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+                ThrowObjectDisposedException_ReaderClosed();
             }
-            if (_pos == _length)
+
+            int pos = _pos;
+            if ((uint)pos < s.Length)
             {
-                return -1;
+                return s[pos];
             }
 
-            return _s[_pos];
+            return -1;
         }
 
         // Reads the next character from the underlying string. The returned value
         // is -1 if no further characters are available.
         //
         public override int Read()
         {
-            if (_s == null)
+            string? s = _s;
+            if (s == null)
             {
-                throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+                ThrowObjectDisposedException_ReaderClosed();
             }
-            if (_pos == _length)
+
+            int pos = _pos;
+            if ((uint)pos < s.Length)
             {
-                return -1;
+                _pos++;
+                return s[pos];
             }
 
-            return _s[_pos++];
+            return -1;
         }
 
         // Reads a block of characters. This method will read up to count
@@ -98,10 +103,10 @@ public override int Read(char[] buffer, int index, int count)
             }
             if (_s == null)
             {
-                throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+                ThrowObjectDisposedException_ReaderClosed();
             }
 
-            int n = _length - _pos;
+            int n = _s.Length - _pos;
             if (n > 0)
             {
                 if (n > count)
@@ -126,10 +131,10 @@ public override int Read(Span<char> buffer)
 
             if (_s == null)
             {
-                throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+                ThrowObjectDisposedException_ReaderClosed();
             }
 
-            int n = _length - _pos;
+            int n = _s.Length - _pos;
             if (n > 0)
             {
                 if (n > buffer.Length)
@@ -150,7 +155,7 @@ public override string ReadToEnd()
         {
             if (_s == null)
             {
-                throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+                ThrowObjectDisposedException_ReaderClosed();
             }
 
             string s;
@@ -160,10 +165,10 @@ public override string ReadToEnd()
             }
             else
             {
-                s = _s.Substring(_pos, _length - _pos);
+                s = _s.Substring(_pos);
             }
 
-            _pos = _length;
+            _pos = _s.Length;
             return s;
         }
 
@@ -175,38 +180,40 @@ public override string ReadToEnd()
         //
         public override string? ReadLine()
         {
-            if (_s == null)
+            string? s = _s;
+            if (s == null)
             {
-                throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+                ThrowObjectDisposedException_ReaderClosed();
             }
+            var pos = _pos;
+            if ((uint)pos >= (uint)s.Length)
+                return null;
 
-            int i = _pos;
-            while (i < _length)
+            ReadOnlySpan<char> remaining = s.AsSpan(pos);
+            int foundLineLength = remaining.IndexOfAny('\r', '\n');
+            if (foundLineLength >= 0)
             {
-                char ch = _s[i];
-                if (ch == '\r' || ch == '\n')
+                string result = s.Substring(pos, foundLineLength);
+
+                char ch = remaining[foundLineLength];
+                pos += foundLineLength + 1;
+                if (ch == '\r')
                 {
-                    string result = _s.Substring(_pos, i - _pos);
-                    _pos = i + 1;
-                    if (ch == '\r' && _pos < _length && _s[_pos] == '\n')
+                    if ((uint)pos < s.Length && s[pos] == '\n')
                     {
-                        _pos++;
+                        pos++;
                     }
-
-                    return result;
                 }
+                _pos = pos;
 
-                i++;
+                return result;
             }
-
-            if (i > _pos)
+            else
             {
-                string result = _s.Substring(_pos, i - _pos);
-                _pos = i;
+                string result = s.Substring(pos);
+                _pos = s.Length;
                 return result;
             }
-
-            return null;
         }
 
         #region Task based Async APIs
@@ -264,5 +271,11 @@ public override ValueTask<int> ReadAsync(Memory<char> buffer, CancellationToken
             cancellationToken.IsCancellationRequested ? ValueTask.FromCanceled<int>(cancellationToken) :
             new ValueTask<int>(Read(buffer.Span));
         #endregion
+
+        [DoesNotReturn]
+        private static void ThrowObjectDisposedException_ReaderClosed()
+        {
+            throw new ObjectDisposedException(null, SR.ObjectDisposed_ReaderClosed);
+        }
     }
 }