What I don't understand is the following line:
prndindex = (prndindex+1)&0xff;
The difference is that the AND is a binary operator — i.e., it takes two arguments:
A & B
&A
x = 3 + &y;
But here "+" is the binary operator. (You can't have two binary operators in a row; that would be like "3 + * 2", which is nonsensical.) This is similar to "3 + -2": the negative here is a unary operator, the "+" here is the binary operator "add"
It's essentially the same[0] as `prndindex = (prndinex+1) % 256`, except bitwise AND takes less effort to compute.
Modulo is essentially[1]: `mod = a - b * (a / b)` unless the compiler can manage to use bitwise shifting/masks instead.
Back when Doom was popular, the bitmask would have been much more efficient.
[0] http://stackoverflow.com/q/3072665/2967113 [1] I'm not 100% positive that's how it's implemented with an IDIV instruction.
This difference means that the AND is still faster than the MOD when used on signed integers, if the compiler is not smart enough to prove that the input will never be negative.
For example, with gcc 4.7.3 the resulting assembly is
movl %edi, %edx
sarl $31, %edx
shrl $24, %edx
leal (%rdi, %rdx), %eax
andl $255, %eax
subl %edx, %eax
> there are two operators (one taking the address of something, the result of which is a pointer, another taking a bitwise AND); they both use the same symbol "&".
Don't you just love C? ;-) (On a serious note, I've been meaning to learn it for some time, but didn't yet get down to the nitty-gritty.)