Change AllowedCharsBitmap back to a struct.

I also experimented with having a fixed uint[] field inside the struct, but this actually ended up having worse performance than a proper uint[] array reference since it defeated some of the JITter's optimizations.

src/Microsoft.Framework.WebEncoders/AllowedCharsBitmap.cs

@@ -6,14 +6,15 @@ using System.Diagnostics;
 
 namespace Microsoft.Framework.WebEncoders
 {
-    internal class AllowedCharsBitmap
+    internal struct AllowedCharsBitmap
     {
         private const int ALLOWED_CHARS_BITMAP_LENGTH = 0x10000 / (8 * sizeof(uint));
-        private uint[] _allowedCharsBitmap;
+        private readonly uint[] _allowedCharsBitmap;
 
-        public AllowedCharsBitmap()
+        private AllowedCharsBitmap(uint[] allowedCharsBitmap)
         {
-            _allowedCharsBitmap = new uint[ALLOWED_CHARS_BITMAP_LENGTH];
+            Debug.Assert(allowedCharsBitmap != null);
+            _allowedCharsBitmap = allowedCharsBitmap;
         }
 
         // Marks a character as allowed (can be returned unencoded)
@@ -34,9 +35,13 @@ namespace Microsoft.Framework.WebEncoders
         // Creates a deep copy of this bitmap
         public AllowedCharsBitmap Clone()
         {
-            var retVal = new AllowedCharsBitmap();
+            return new AllowedCharsBitmap((uint[])_allowedCharsBitmap.Clone());
-            retVal._allowedCharsBitmap = (uint[])this._allowedCharsBitmap.Clone();
+        }
-            return retVal;
+
+        // should be called in place of the ctor
+        public static AllowedCharsBitmap CreateNew()
+        {
+            return new AllowedCharsBitmap(new uint[ALLOWED_CHARS_BITMAP_LENGTH]);
         }
 
         // Marks a character as forbidden (must be returned encoded)
@@ -47,7 +52,7 @@ namespace Microsoft.Framework.WebEncoders
             int offset = (int)(codePoint & 0x1FU);
             _allowedCharsBitmap[index] &= ~(0x1U << offset);
         }
-        
+
         public void ForbidUndefinedCharacters()
         {
             // Forbid codepoints which aren't mapped to characters or which are otherwise always disallowed

src/Microsoft.Framework.WebEncoders/CodePointFilter.cs

@@ -19,7 +19,7 @@ namespace Microsoft.Framework.WebEncoders
         /// </summary>
         public CodePointFilter()
         {
-            _allowedCharsBitmap = new AllowedCharsBitmap();
+            _allowedCharsBitmap = AllowedCharsBitmap.CreateNew();
         }
 
         /// <summary>
@@ -34,7 +34,7 @@ namespace Microsoft.Framework.WebEncoders
             }
             else
             {
-                _allowedCharsBitmap = new AllowedCharsBitmap();
+                _allowedCharsBitmap = AllowedCharsBitmap.CreateNew();
                 AllowFilter(other);
             }
         }
@@ -45,7 +45,7 @@ namespace Microsoft.Framework.WebEncoders
         /// </summary>
         public CodePointFilter(params UnicodeRange[] allowedRanges)
         {
-            _allowedCharsBitmap = new AllowedCharsBitmap();
+            _allowedCharsBitmap = AllowedCharsBitmap.CreateNew();
             AllowRanges(allowedRanges);
         }
 

test/Microsoft.Framework.WebEncoders.Tests/AllowedCharsBitmapTests.cs

@@ -12,7 +12,7 @@ namespace Microsoft.Framework.WebEncoders
         public void Ctor_EmptyByDefault()
         {
             // Act
-            var bitmap = new AllowedCharsBitmap();
+            var bitmap = AllowedCharsBitmap.CreateNew();
 
             // Assert
             for (int i = 0; i <= Char.MaxValue; i++)
@@ -25,7 +25,7 @@ namespace Microsoft.Framework.WebEncoders
         public void Allow_Forbid_ZigZag()
         {
             // Arrange
-            var bitmap = new AllowedCharsBitmap();
+            var bitmap = AllowedCharsBitmap.CreateNew();
 
             // Act
             // The only chars which are allowed are those whose code points are multiples of 3 or 7
@@ -58,7 +58,7 @@ namespace Microsoft.Framework.WebEncoders
         public void Clear_ForbidsEverything()
         {
             // Arrange
-            var bitmap = new AllowedCharsBitmap();
+            var bitmap = AllowedCharsBitmap.CreateNew();
             for (int i = 1; i <= Char.MaxValue; i++)
             {
                 bitmap.AllowCharacter((char)i);
@@ -78,7 +78,7 @@ namespace Microsoft.Framework.WebEncoders
         public void Clone_MakesDeepCopy()
         {
             // Arrange
-            var originalBitmap = new AllowedCharsBitmap();
+            var originalBitmap = AllowedCharsBitmap.CreateNew();
             originalBitmap.AllowCharacter('x');
 
             // Act
@@ -99,7 +99,7 @@ namespace Microsoft.Framework.WebEncoders
             // We only allow odd-numbered characters in this test so that
             // we can validate that we properly merged the two bitmaps together
             // rather than simply overwriting the target.
-            var bitmap = new AllowedCharsBitmap();
+            var bitmap = AllowedCharsBitmap.CreateNew();
             for (int i = 1; i <= Char.MaxValue; i += 2)
             {
                 bitmap.AllowCharacter((char)i);