Fix BlobBuilder.ToArray (#11376)

src/Compilers/Core/CodeAnalysisTest/PEWriter/BlobTests.cs

@@ -183,8 +183,10 @@ public void GetChunks_DestructingEnum()
             }
         }
 
+
+
         [Fact]
-        public void ToArray()
+        public void ToArray1()
         {
             var builder = new BlobBuilder(16);
 
@@ -222,6 +224,43 @@ public void ToArray()
             AssertEx.Equal(new byte[] { 0xdd, 0xcc, 0xbb, 0xaa }, builder.ToArray(13, 4));
         }
 
+        [Fact]
+        public void ToArray2()
+        {
+            var builder = new BlobBuilder(16);
+
+            AssertEx.Equal(new byte[] { }, builder.ToArray(0, 0));
+
+            for (int i = 0; i < 34; i++)
+            {
+                builder.WriteByte((byte)i);
+            }
+
+            AssertEx.Equal(new byte[]
+            {
+                0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
+                0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F,
+                0x20, 0x21
+            }, builder.ToArray());
+
+            AssertEx.Equal(new byte[]
+            {
+                0x0E, 0x0F,
+                0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F,
+                0x20, 0x21
+            }, builder.ToArray(0x0e, 20));
+
+            AssertEx.Equal(new byte[] { 0x0E }, builder.ToArray(0x0e, 1));
+            AssertEx.Equal(new byte[] { 0x0E, 0x0F }, builder.ToArray(0x0e, 2));
+            AssertEx.Equal(new byte[] { 0x0E, 0x0F, 0x10 }, builder.ToArray(0x0e, 3));
+            AssertEx.Equal(new byte[] { 0x0E, 0x0F, 0x10, 0x11 }, builder.ToArray(0x0e, 4));
+
+            AssertEx.Equal(new byte[] { 0x1E }, builder.ToArray(0x1e, 1));
+            AssertEx.Equal(new byte[] { 0x1E, 0x1F }, builder.ToArray(0x1e, 2));
+            AssertEx.Equal(new byte[] { 0x1E, 0x1F, 0x20 }, builder.ToArray(0x1e, 3));
+            AssertEx.Equal(new byte[] { 0x1E, 0x1F, 0x20, 0x21 }, builder.ToArray(0x1e, 4));
+        }
+
         [Fact]
         public void ToArray_Errors()
         {

src/Compilers/Core/Portable/PEWriter/BlobBuilder.cs

@@ -279,29 +279,33 @@ public byte[] ToArray(int start, int byteCount)
 
             var result = new byte[byteCount];
 
-            int chunkStartPosition = 0;
-            int resultOffset = 0;
+            int chunkStart = 0;
+            int bufferStart = start;
+            int bufferEnd = start + byteCount;
             foreach (var chunk in GetChunks())
             {
-                int chunkEndPosition = chunkStartPosition + chunk.Length;
+                int chunkEnd = chunkStart + chunk.Length;
+                Debug.Assert(bufferStart >= chunkStart);
 
-                if (chunkEndPosition > start)
+                if (chunkEnd > bufferStart)
                 {
-                    int bytesToCopy = Math.Min(chunk.Length, result.Length - resultOffset);
-                    if (bytesToCopy == 0)
+                    int bytesToCopy = Math.Min(bufferEnd, chunkEnd) - bufferStart;
+                    Debug.Assert(bytesToCopy >= 0);
+
+                    Array.Copy(chunk._buffer, bufferStart - chunkStart, result, bufferStart - start, bytesToCopy);
+                    bufferStart += bytesToCopy;
+
+                    if (bufferStart == bufferEnd)
                     {
                         break;
                     }
-
-                    Array.Copy(chunk._buffer, Math.Max(start - chunkStartPosition, 0), result, resultOffset, bytesToCopy);
-
-                    resultOffset += bytesToCopy;
                 }
 
-                chunkStartPosition = chunkEndPosition;
+                chunkStart = chunkEnd;
             }
 
-            Debug.Assert(resultOffset == result.Length);
+            Debug.Assert(bufferStart == bufferEnd);
+
             return result;
         }