Bitwise operation C

Question

I have this an unsigned int (uint16_t): 391 I want to flip all the bits around from right to left. That is to say, the rightmost bit becomes the leftmost bit.

I have tried x = 391 << sizeof(uint16_t) This seems not to work.

The desired output is: x = 57728

My current output is: x = 1564

Answer 1

There is no C bitwise operator that does that, you have to implement it.

One way is to define a look-up table for some suitable amount of bits (8, or 4) and then chunk the value up and flip each chunk, then combine them together again.

Sounds like fun, let's code:

#include <stdint.h>

uint16_t flip(uint16_t x)
{
  const uint16_t nibbles[] = { 0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15 };

  uint16_t out = 0;
  for (int i = 0; i < 4; ++i)
  {
    const uint16_t here = nibbles[(x >> (4 * i)) & 15];
    out |= here << (4 * (3 - i));
  }
  return out;
}