Optimize EnsureCapacity

Author: Joy-less

src/LinkDotNet.StringBuilder/IntegerSpanList.cs

@@ -31,15 +31,15 @@ public void Add(int value)
     {
         if (count >= buffer.Length)
         {
-            Grow();
+            EnsureCapacity();
         }
 
         buffer[count] = value;
         count++;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private void Grow(int capacity = 0)
+    private void EnsureCapacity(int capacity = 0)
     {
         var currentSize = buffer.Length;
         var newSize = capacity > 0 ? capacity : currentSize * 2;

src/LinkDotNet.StringBuilder/ValueStringBuilder.Append.cs

@@ -20,7 +20,7 @@ public unsafe void Append(bool value)
 
         if (newSize > buffer.Length)
         {
-            Grow(newSize);
+            EnsureCapacity(newSize);
         }
 
         fixed (char* dest = &buffer[bufferPosition])
@@ -67,7 +67,7 @@ public void Append(scoped ReadOnlySpan<char> str)
         var newSize = str.Length + bufferPosition;
         if (newSize > buffer.Length)
         {
-            Grow(newSize);
+            EnsureCapacity(newSize);
         }
 
         ref var strRef = ref MemoryMarshal.GetReference(str);
@@ -111,7 +111,7 @@ public void Append(char value)
         var newSize = bufferPosition + 1;
         if (newSize > buffer.Length)
         {
-            Grow(newSize);
+            EnsureCapacity(newSize);
         }
 
         buffer[bufferPosition] = value;
@@ -163,7 +163,7 @@ public Span<char> AppendSpan(int length)
         var origPos = bufferPosition;
         if (origPos > buffer.Length - length)
         {
-            Grow(length);
+            EnsureCapacity(length);
         }
 
         bufferPosition = origPos + length;
@@ -177,7 +177,7 @@ private void AppendSpanFormattable<T>(T value, ReadOnlySpan<char> format = defau
         var newSize = bufferSize + bufferPosition;
         if (newSize >= Capacity)
         {
-            Grow(newSize);
+            EnsureCapacity(newSize);
         }
 
         if (!value.TryFormat(buffer[bufferPosition..], out var written, format, null))

src/LinkDotNet.StringBuilder/ValueStringBuilder.Helper.cs

@@ -0,0 +1,14 @@
+namespace LinkDotNet.StringBuilder;
+
+public ref partial struct ValueStringBuilder
+{
+    /// <summary>
+    /// Finds the smallest power of 2 which is greater than or equal to <paramref name="minimum"/>.
+    /// </summary>
+    /// <param name="minimum">The value the result should be greater than or equal to.</param>
+    /// <returns>The smallest power of 2 >= <paramref name="minimum"/>.</returns>
+    internal static int FindSmallestPowerOf2Above(int minimum)
+    {
+        return 1 << (int)Math.Ceiling(Math.Log2(minimum));
+    }
+}

src/LinkDotNet.StringBuilder/ValueStringBuilder.Insert.cs

@@ -45,7 +45,7 @@ public void Insert(int index, scoped ReadOnlySpan<char> value)
         var newLength = bufferPosition + value.Length;
         if (newLength > buffer.Length)
         {
-            Grow(newLength);
+            EnsureCapacity(newLength);
         }
 
         bufferPosition = newLength;
@@ -79,7 +79,7 @@ private void InsertSpanFormattable<T>(int index, T value, scoped ReadOnlySpan<ch
             var newLength = bufferPosition + written;
             if (newLength > buffer.Length)
             {
-                Grow(newLength);
+                EnsureCapacity(newLength);
             }
 
             bufferPosition = newLength;

src/LinkDotNet.StringBuilder/ValueStringBuilder.cs

@@ -20,15 +20,12 @@ namespace LinkDotNet.StringBuilder;
     private char[]? arrayFromPool;
 
     /// <summary>
-    /// Initializes a new instance of the <see cref="ValueStringBuilder"/> struct.
+    /// Initializes a new instance of the <see cref="ValueStringBuilder"/> struct using a rented buffer of capacity 32.
     /// </summary>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public ValueStringBuilder()
     {
-        bufferPosition = 0;
-        buffer = default;
-        arrayFromPool = null;
-        Grow(32);
+        EnsureCapacity(32);
     }
 
     /// <summary>
@@ -41,9 +38,7 @@ public ValueStringBuilder()
 #endif
     public ValueStringBuilder(Span<char> initialBuffer)
     {
-        bufferPosition = 0;
         buffer = initialBuffer;
-        arrayFromPool = null;
     }
 
     /// <summary>
@@ -63,7 +58,7 @@ public ValueStringBuilder(ReadOnlySpan<char> initialText)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public ValueStringBuilder(int initialCapacity)
     {
-        Grow(initialCapacity);
+        EnsureCapacity(initialCapacity);
     }
 
     /// <summary>
@@ -173,19 +168,41 @@ public readonly string ToString(Range range)
     public void Clear() => bufferPosition = 0;
 
     /// <summary>
-    /// Ensures that the builder has at least <paramref name="newCapacity"/> amount of capacity.
+    /// Ensures the builder's buffer size is at least <paramref name="newCapacity"/>, renting a larger buffer if not.
     /// </summary>
     /// <param name="newCapacity">New capacity for the builder.</param>
     /// <remarks>
-    /// If <paramref name="newCapacity"/> is smaller or equal to <see cref="Length"/> nothing will be done.
+    /// If <see cref="Length"/> is already &gt;= <paramref name="newCapacity"/>, nothing is done.
     /// </remarks>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public void EnsureCapacity(int newCapacity)
     {
-        if (newCapacity > Length)
+        if (Length >= newCapacity)
         {
-            Grow(newCapacity);
+            return;
         }
+
+        int newSize = FindSmallestPowerOf2Above(newCapacity);
+
+        Span<char> rented = ArrayPool<char>.Shared.Rent(newSize);
+
+        if (bufferPosition > 0)
+        {
+            ref char sourceRef = ref MemoryMarshal.GetReference(buffer);
+            ref char destinationRef = ref MemoryMarshal.GetReference(rented);
+
+            Unsafe.CopyBlock(
+                ref Unsafe.As<char, byte>(ref destinationRef),
+                ref Unsafe.As<char, byte>(ref sourceRef),
+                (uint)bufferPosition * sizeof(char));
+        }
+
+        if (arrayFromPool is not null)
+        {
+            ArrayPool<char>.Shared.Return(arrayFromPool);
+        }
+
+        buffer = rented;
     }
 
     /// <summary>
@@ -301,36 +318,4 @@ public void Dispose()
     /// </summary>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public readonly void Reverse() => buffer[..bufferPosition].Reverse();
-
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private void Grow(int capacity = 0)
-    {
-        var size = buffer.Length == 0 ? 8 : buffer.Length;
-
-        while (size < capacity)
-        {
-            size *= 2;
-        }
-
-        var rented = ArrayPool<char>.Shared.Rent(size);
-
-        if (bufferPosition > 0)
-        {
-            ref var sourceRef = ref MemoryMarshal.GetReference(buffer);
-            ref var destinationRef = ref MemoryMarshal.GetReference(rented.AsSpan());
-
-            Unsafe.CopyBlock(
-                ref Unsafe.As<char, byte>(ref destinationRef),
-                ref Unsafe.As<char, byte>(ref sourceRef),
-                (uint)(bufferPosition * sizeof(char)));
-        }
-
-        var oldBufferFromPool = arrayFromPool;
-        buffer = arrayFromPool = rented;
-
-        if (oldBufferFromPool is not null)
-        {
-            ArrayPool<char>.Shared.Return(oldBufferFromPool);
-        }
-    }
 }

src/LinkDotNet.StringBuilder/ValueStringBuilderExtensions.cs

@@ -25,7 +25,7 @@ public static System.Text.StringBuilder ToStringBuilder(this ValueStringBuilder
     /// <param name="builder">The builder from which the new instance is derived.</param>
     /// <returns>A new <see cref="ValueStringBuilder"/> instance with the string represented by this builder.</returns>
     /// <exception cref="ArgumentNullException">Throws if <paramref name="builder"/> is null.</exception>
-    public static ValueStringBuilder ToValueStringBuilder(this System.Text.StringBuilder? builder)
+    public static ValueStringBuilder ToValueStringBuilder(this System.Text.StringBuilder builder)
     {
         ArgumentNullException.ThrowIfNull(builder);
 

tests/LinkDotNet.StringBuilder.UnitTests/ValueStringBuilderExtensionsTests.cs

@@ -31,7 +31,7 @@ public void ShouldThrowWhenStringBuilderNull()
     {
         System.Text.StringBuilder? sb = null;
 
-        Action act = () => sb.ToValueStringBuilder();
+        Action act = () => sb!.ToValueStringBuilder();
 
         act.ShouldThrow<ArgumentNullException>();
     }