pal2ham2.c

# include <stdio.h>
# include <inttypes.h>

uint_least16_t binrev (uint_least16_t n)
{
	uint_least16_t r = 0;
	while (n)
	{
		r <<= 1;
		r |= n & 1;
		n >>= 1;
	}
	return r;
}

int main (void)
{
	uint_least16_t n;
	scanf ("%" SCNuLEAST16, & n);

	uint_least16_t k = 1;
	uint_least32_t m = 1;
	uint_least8_t p = 0;
	_Bool odd = 1;
  
	while (m < n)
	{
		m += k;
		odd = m < n;
		k <<= 1;
		m += k;
		p++;
	}
  
	n -= k - 1;

	printf ("%" PRIuLEAST64 "\n", ((uint_least64_t) n << p) + binrev (n >> odd));
}