base::MD5IntermediateFinal() generates digest non-destructively.

base/md5.cc

Index: base/md5.cc
diff --git a/base/md5.cc b/base/md5.cc
index 754994c0e562b45b2bc4b90cc2a7c612d14ec5a7..c3299c2ae109f773eab344cfa0ebcd26ae11fdc5 100644
--- a/base/md5.cc
+++ b/base/md5.cc
@@ -260,6 +260,15 @@ void MD5Final(MD5Digest* digest, MD5Context* context) {
         memset(ctx, 0, sizeof(*ctx));    /* In case it's sensitive */
 }
 
+/*
+ * Clone the context and use the clone to generate the digest.

[awong, 2014/03/31 20:22:19]

This should be an impl comment, not a function comment. Also explain why cloning
is necessary.

[Scott Hess - ex-Googler, 2014/03/31 20:48:26]

I'll try, but the commenting style is breaking my brain.  [Dear lord!  It was
copied from sqlite?]  WDYT about simply having no comment at all?

[awong, 2014/03/31 21:04:32]

I think it's critical to understand the MD5Final mutates the MD5Context* and
that this memcpy is "good enough" to fix things.

What about
/* MD5Final mutates the MD5Context*. Make a copy for generating the intermediate
value. */

[Scott Hess - ex-Googler, 2014/03/31 21:29:59]

Done.

+ */
+void MD5IntermediateFinal(MD5Digest* digest, const MD5Context* context) {
+  MD5Context context_copy;
+  memcpy(&context_copy, context, sizeof(context_copy));
+  MD5Final(digest, &context_copy);
+}
+
 std::string MD5DigestToBase16(const MD5Digest& digest) {
   static char const zEncode[] = "0123456789abcdef";