Trouble with gmp_xor on 32 bit

Question

On a 64-bit machine

(1 ^ 2489596804)

gives -1805370491 in response. However, when I do this on a 32-bit PHP machine, I get 2489596805 in response.

So, I tried to get it working on the 32-bit machine using gmp_strval(gmp_xor(1,2489596804)), but it also yields 2489596805. So what's up, and how can I get the right response?

Update: I just noticed that the 64-bit result is what you get from subtracting the 32-bit max from it (4294967295).

Answer 1

It's the same result, simply interpreted differently:

0x94643B85 =  2489596805 (unsigned 32-bit integer)
0x94643B85 = -1805370491 (signed 32-bit integer)

The reason you see a negative value on your 64-bit machine is that you're interpreting that result as a signed 32-bit integer.

If you really want to convert between the two interpretations, I think you'll have to do this yourself. A simple way to do that would be something like this:

<?php
function toSigned($value) {
  if ($value <= 2147483647)
    return $value;
  return $value - 4294967295 + 1;
}

function toUnsigned($value) {
  if ($value >= 0)
    return $value;
  return $value + 4294967295 - 1;
}