base: Use a simple tricky to remove a temporary variable in MD5DigestToBase16.

base/md5.cc

 // 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.
 
 // The original file was copied from sqlite, and was in the public domain.
 
 /*
  * This code implements the MD5 message-digest algorithm.
  * The algorithm is due to Ron Rivest.  This code was
  * written by Colin Plumb in 1993, no copyright is claimed.
@@ skipping 258 matching lines @@
   memcpy(&context_copy, context, sizeof(context_copy));
   MD5Final(digest, &context_copy);
 }
 
 std::string MD5DigestToBase16(const MD5Digest& digest) {
   static char const zEncode[] = "0123456789abcdef";
 
   std::string ret;
   ret.resize(32);
 
-  int j = 0;
-  for (int i = 0; i < 16; i++) {
+  for (int i = 0, j = 0; i < 16; i++, j += 2) {
     int a = digest.a[i];
-    ret[j++] = zEncode[(a >> 4) & 0xf];
-    ret[j++] = zEncode[a & 0xf];
+    ret[j] = zEncode[(a >> 4) & 0xf];
+    ret[j + 1] = zEncode[a & 0xf];
   }
   return ret;
 }
 
 void MD5Sum(const void* data, size_t length, MD5Digest* digest) {
   MD5Context ctx;
   MD5Init(&ctx);
   MD5Update(&ctx, StringPiece(reinterpret_cast<const char*>(data), length));
   MD5Final(digest, &ctx);
 }
 
 std::string MD5String(const StringPiece& str) {
   MD5Digest digest;
   MD5Sum(str.data(), str.length(), &digest);
   return MD5DigestToBase16(digest);
 }
 
 }  // namespace base