Added external hashing library.

external/Common/hash.h

@@ -0,0 +1,28 @@
+// //////////////////////////////////////////////////////////
+// hash.h
+// Copyright (c) 2014,2015 Stephan Brumme. All rights reserved.
+// see http://create.stephan-brumme.com/disclaimer.html
+//
+
+#pragma once
+
+#include <string>
+
+/// abstract base class
+class Hash
+{
+public:
+  /// compute hash of a memory block
+  virtual std::string operator()(const void* data, size_t numBytes) = 0;
+  /// compute hash of a string, excluding final zero
+  virtual std::string operator()(const std::string& text) = 0;
+
+  /// add arbitrary number of bytes
+  virtual void add(const void* data, size_t numBytes) = 0;
+
+  /// return latest hash as hex characters
+  virtual std::string getHash() = 0;
+
+  /// restart
+  virtual void reset() = 0;
+};