SERVER-15194 Refactor base64::decode Implementation

* Existing check for length as multiple of 4 as-is
* Added check for non-base64 characters on input
* Added check for terminators ('=') midstream
  Implicitly in positions 0 and 1 via non-base64 check
  Explicitly in positions 2 and 3 via "done" check.

Moved "Alphabet" class into cpp file in anon namespace
as it's an implementation detail and shouldn't be used
by outside classes.

Added base64::validate() method to accomodate BSON's
isBase64String() check.

1 files changed, 1 insertions, 7 deletions

diff --git a/src/mongo/bson/json.cpp b/src/mongo/bson/json.cpp
index d6a94ea08eb..647accf31ba 100644
--- a/src/mongo/bson/json.cpp
+++ b/src/mongo/bson/json.cpp
@@ -1280,13 +1280,7 @@ bool JParse::isHexString(StringData str) const {
 
 bool JParse::isBase64String(StringData str) const {
     MONGO_JSON_DEBUG("str: " << str);
-    std::size_t i;
-    for (i = 0; i < str.size(); i++) {
-        if (!match(str[i], base64::chars)) {
-            return false;
-        }
-    }
-    return true;
+    return base64::validate(str);
 }
 
 bool JParse::isArray() {