4 ms·

I read the original erroneous code, the mistake and attempted to write a correct solution. I ended up with '(2048 - (size % 2048)) + size', the same expression

I read the original erroneous code, the mistake and attempted to write a correct solution. I ended up with '(2048 - (size % 2048)) + size', the same expression as the original :/. Us humans just don't do well with edge cases in ranges/bounds. Another seemingly simple task which is easy to trip over is : determining if a time interval overlaps another.

morsch 12y ago
His corrected version padded_size := (size + 2047) & ~2047 may be correct or it may not be, I can't really tell quickly because it's so oblique to me. Combining decimal and binary arithmetic like that is a bit too old school for me -- of course I still use it on occasion but I try to avoid it. I guess it's second nature to game devs. What's wrong with padded_size := ceil(size / 2048) * 2048 apart from the fact that it's much much slower but still really really fast in what I can only assume is a part of the source where performance doesn't matter, anyway.
- innocenat 12y ago
  padded_size := ceil(size / 2048) * 2048 ... may be wrong if size is large that can't be store in mantissa of double/float. Assuming C (or equivalent), I usually do this: padded_size = ((size + 2047) / 2048) * 2048;
  - toolslive 12y ago
    this is generic. You can also take into account that 2048 is a power of 2. So you can shift right and then left again.
    - innocenat 12y ago
      All relevant compilers do that automatically.
- drv 12y ago
  The corrected version is more recognizable to me as "round"; I believe this is a fairly common pattern in code that needs to round or align to powers of two. However, I'll agree that it is perhaps overly clever. I recently wrote some code that used this pattern, except I had remembered it wrong and wrote something like: padded_size := (size + 2047) & ~2048 (2048 instead of 2047.) Luckily it was caught in code review, but it's worth being a bit more explicit; our solution (aside from fixing the number) was to add ASSERT(padded_size % 2048 == 0) so that it was clear what the code was trying to do.
mikeryan 12y ago
I think my default code these for cases where like this where an option is to "do nothing" or "return 0" is to always to have a first line like: if (dont_have_to_do_anything) return;

morsch 12y ago

His corrected version padded_size := (size + 2047) & ~2047 may be correct or it may not be, I can't really tell quickly because it's so oblique to me. Combining decimal and binary arithmetic like that is a bit too old school for me -- of course I still use it on occasion but I try to avoid it. I guess it's second nature to game devs. What's wrong with padded_size := ceil(size / 2048) * 2048 apart from the fact that it's much much slower but still really really fast in what I can only assume is a part of the source where performance doesn't matter, anyway.

innocenat 12y ago
padded_size := ceil(size / 2048) * 2048 ... may be wrong if size is large that can't be store in mantissa of double/float. Assuming C (or equivalent), I usually do this: padded_size = ((size + 2047) / 2048) * 2048;
- toolslive 12y ago
  this is generic. You can also take into account that 2048 is a power of 2. So you can shift right and then left again.
  - innocenat 12y ago
    All relevant compilers do that automatically.
drv 12y ago
The corrected version is more recognizable to me as "round"; I believe this is a fairly common pattern in code that needs to round or align to powers of two. However, I'll agree that it is perhaps overly clever. I recently wrote some code that used this pattern, except I had remembered it wrong and wrote something like: padded_size := (size + 2047) & ~2048 (2048 instead of 2047.) Luckily it was caught in code review, but it's worth being a bit more explicit; our solution (aside from fixing the number) was to add ASSERT(padded_size % 2048 == 0) so that it was clear what the code was trying to do.

mikeryan 12y ago

I think my default code these for cases where like this where an option is to "do nothing" or "return 0" is to always to have a first line like: if (dont_have_to_do_anything) return;