Dumped the old implementation. Started with a more simple approach.

external/Hash/crc32.h

@@ -0,0 +1,69 @@
+// //////////////////////////////////////////////////////////
+// crc32.h
+// Copyright (c) 2014,2015 Stephan Brumme. All rights reserved.
+// see http://create.stephan-brumme.com/disclaimer.html
+//
+
+#pragma once
+
+//#include "hash.h"
+#include <string>
+
+// define fixed size integer types
+#ifdef _MSC_VER
+// Windows
+typedef unsigned __int8  uint8_t;
+typedef unsigned __int32 uint32_t;
+#else
+// GCC
+#include <stdint.h>
+#endif
+
+
+/// compute CRC32 hash, based on Intel's Slicing-by-8 algorithm
+/** Usage:
+    CRC32 crc32;
+    std::string myHash  = crc32("Hello World");     // std::string
+    std::string myHash2 = crc32("How are you", 11); // arbitrary data, 11 bytes
+
+    // or in a streaming fashion:
+
+    CRC32 crc32;
+    while (more data available)
+      crc32.add(pointer to fresh data, number of new bytes);
+    std::string myHash3 = crc32.getHash();
+
+    Note:
+    You can find code for the faster Slicing-by-16 algorithm on my website, too:
+    http://create.stephan-brumme.com/crc32/
+    Its unrolled version is about twice as fast but its look-up table doubled in size as well.
+  */
+class CRC32 //: public Hash
+{
+public:
+  /// hash is 4 bytes long
+  enum { HashBytes = 4 };
+
+  /// same as reset()
+  CRC32();
+
+  /// compute CRC32 of a memory block
+  std::string operator()(const void* data, size_t numBytes);
+  /// compute CRC32 of a string, excluding final zero
+  std::string operator()(const std::string& text);
+
+  /// add arbitrary number of bytes
+  void add(const void* data, size_t numBytes);
+
+  /// return latest hash as 8 hex characters
+  std::string getHash();
+  /// return latest hash as bytes
+  void        getHash(unsigned char buffer[HashBytes]);
+
+  /// restart
+  void reset();
+
+private:
+  /// hash
+  uint32_t m_hash;
+};