How to reverse bits of a byte?

Question

For example, in PHP, how would I reverse the bits of the byte 11011111 to 11111011?

Answer 1

The quickest way, but also the one requiring more space is a lookup, whereby each possible value of a byte (256 if you go for the whole range), is associated with its "reversed" equivalent.

If you only have a few such bytes to handle, bit-wise operators will do but that will be slower, maybe something like:

function reverseBits($in) {
  $out = 0;

  if ($in & 0x01) $out |= 0x80;
  if ($in & 0x02) $out |= 0x40;
  if ($in & 0x04) $out |= 0x20;
  if ($in & 0x08) $out |= 0x10;
  if ($in & 0x10) $out |= 0x08;
  if ($in & 0x20) $out |= 0x04;
  if ($in & 0x40) $out |= 0x02;
  if ($in & 0x80) $out |= 0x01;

  return $out;
}