DirectBuffer - Support ArraySegment for underlying array (#878)

* DirectBuffer - Support ArraySegment for backing storage

* fix comments

Author: guigzzz

csharp/sbe-dll/DirectBuffer.cs

@@ -64,6 +64,25 @@ public DirectBuffer(byte* pBuffer, int bufferLength, BufferOverflowDelegate buff
             Wrap(pBuffer, bufferLength);
         }
 
+        /// <summary>
+        /// Attach a view to a buffer owned by external code
+        /// </summary>
+        /// <param name="buffer">byte buffer</param>
+        public DirectBuffer(ArraySegment<byte> buffer) : this(buffer, null)
+        {
+        }
+
+        /// <summary>
+        /// Attach a view to a buffer owned by external code
+        /// </summary>
+        /// <param name="buffer">byte buffer</param>
+        /// <param name="bufferOverflow">delegate to allow reallocation of buffer</param>
+        public DirectBuffer(ArraySegment<byte> buffer, BufferOverflowDelegate bufferOverflow)
+        {
+            this.bufferOverflow = bufferOverflow;
+            Wrap(buffer);
+        }
+
         /// <summary>
         /// Creates a DirectBuffer that can later be wrapped
         /// </summary>
@@ -114,6 +133,24 @@ public void Wrap(byte* pBuffer, int bufferLength)
             _needToFreeGCHandle = false;
         }
 
+        /// <summary>
+        /// Recycles an existing <see cref="DirectBuffer"/> from a byte buffer owned by external code
+        /// </summary>
+        /// <param name="buffer">buffer of bytes</param>
+        public void Wrap(ArraySegment<byte> buffer)
+        {
+            if (buffer == null) throw new ArgumentNullException(nameof(buffer));
+
+            FreeGCHandle();
+
+            // pin the buffer so it does not get moved around by GC, this is required since we use pointers
+            _pinnedGCHandle = GCHandle.Alloc(buffer.Array, GCHandleType.Pinned);
+            _needToFreeGCHandle = true;
+
+            _pBuffer = ((byte*)_pinnedGCHandle.AddrOfPinnedObject().ToPointer()) + buffer.Offset;
+            _capacity = buffer.Count;
+        }
+
         /// <summary>
         /// Capacity of the underlying buffer
         /// </summary>

csharp/sbe-tests/DirectBufferTests.cs

@@ -66,6 +66,40 @@ public void ConstructFromNativeBuffer()
             }
         }
 
+        [TestMethod]
+        public void ConstructFromByteArray()
+        {
+            var buffer = new byte[16];
+
+            const int value = 5;
+            const int index = 0;
+            
+            using (var directBuffer = new DirectBuffer(buffer))
+            {
+                directBuffer.Int64PutLittleEndian(index, value);
+                Assert.AreEqual(value, buffer[index]);
+            }
+        }
+
+        [TestMethod]
+        public void ConstructFromArraySegment()
+        {
+            const int value = 5;
+            const int index = 0;
+            const int offset = 512;
+            const int size = 1024;
+
+            var bigBuffer = new byte[size];
+            var buffer = new ArraySegment<byte>(bigBuffer, offset, size - offset);
+            
+            using (var directBuffer = new DirectBuffer(buffer))
+            {
+                directBuffer.Int64PutLittleEndian(index, value);
+                Assert.AreEqual(value, bigBuffer[offset + index]);
+                Assert.AreEqual(value, buffer.AsSpan<byte>()[index]);
+            }
+        }
+
         [TestMethod]
         public void Recycle()
         {