One of the fun things about writing graphics code is that you get better bugs. In normal programming, bugs are mostly frustrating: they get in your way, make things harder, corrupt data you painstakingly created, or crash the program and interrupt what you're doing. But a lot of bugs in graphics code either look really cool or have no real visible effect.
At the moment I'm not writing graphics code. I'm writing an almost-Scheme compiler in itself. But I just created a really bizarre bug.
Here's a little bit of the assembly output from the compiler:
epacse__4:
# compute desired %esp on return in %ebx and push it
lea 4(%esp,%edx,4), %ebx
...
movl (htgnel_gnirts__2), %eax
Where did htgnel_gnirts__2 come from? Well, it's _string_length
spelled backwards, followed by _2. And epacse is escape spelled
backwards. I accidentally created a bug that spells names
backwards. That's almost as funny as some of my graphics-code bugs.
How this happened requires a little bit of explanation. In Scheme, as in most Lisps, adding items to the beginning of a list is fast and safe, but adding onto the end of a list is either slow and bug-prone, safe but extremely slow, or very verbose and therefore bug-prone and hard to maintain. But reversing a list is relatively fast. So I wrote a function that looked like this:
(define (stringlist->string stringlist)
(list->string (reverse (stringlist->string-2 stringlist 0))))
Because I thought stringlist->string-2 was going to have to build up a list of all the characters backwards, and then I was going to have to reverse it.
When stringlist->string-2 turned out to be able to build up the list
of characters in the right order --- by adding them backwards --- I
forgot to take out the reverse.