base::MD5Clone() clones MD5Context.

base/md5.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MD5_H_
 #define BASE_MD5_H_
 
 #include "base/base_export.h"
 #include "base/strings/string_piece.h"
 
@@ skipping 37 matching lines @@
 
 // Initializes the given MD5 context structure for subsequent calls to
 // MD5Update().
 BASE_EXPORT void MD5Init(MD5Context* context);
 
 // For the given buffer of |data| as a StringPiece, updates the given MD5
 // context with the sum of the data. You can call this any number of times
 // during the computation, except that MD5Init() must have been called first.
 BASE_EXPORT void MD5Update(MD5Context* context, const StringPiece& data);
 
+// Clone the state of the given context.  This can be used to generate an
+// intermediate digest without affecting the original context.

[awong, 2014/03/25 20:00:41]

This comment is a little bit confusing. Can you motivate with an example of how
to use and how it differs?

Also, what about adding a MD5IncrementalFinal() or something? That seems more
friendly than cloning, finalizing, and discarding cloned version...

+BASE_EXPORT void MD5Clone(MD5Context* clone_context,
+                          MD5Context* original_context);
+
 // Finalizes the MD5 operation and fills the buffer with the digest.
 BASE_EXPORT void MD5Final(MD5Digest* digest, MD5Context* context);
 
 // Converts a digest into human-readable hexadecimal.
 BASE_EXPORT std::string MD5DigestToBase16(const MD5Digest& digest);
 
 // Returns the MD5 (in hexadecimal) of a string.
 BASE_EXPORT std::string MD5String(const StringPiece& str);
 
 }  // namespace base
 
 #endif  // BASE_MD5_H_