Well, if you really want to go apples-to-oranges and specialize as much as possible for the sequentially increasing case we're dealing with here, you could just

Well, if you really want to go apples-to-oranges and specialize as much as possible for the sequentially increasing case we're dealing with here, you could just do your arithmetic directly on the ASCII string itself, complete with a little ripple-carry loop...I bet that'd be fast. If I'm remembering correctly, I think x86 actually has dedicated instructions for ASCII arithmetic (though they're probably so unoptimized in a modern microarchitecture it'd probably be faster to avoid them).