src/lib/primes.c

	primes.c revision cb63aae771b2cfe51259de09dbff5431739dcc55
#include "lib.h"
#include "primes.h"

static const unsigned int primes[] = {
#define PRIME_SKIP_COUNT 3
    17,
    37,
    67,
    131,
    257, /* next from 2^8 */
    521,
    1031,
    2053,
    4099,
    8209,
    16411,
    32771,
    65537, /* next from 2^16 */
    131101,
    262147,
    524309,
    1048583,
    2097169,
    4194319,
    8388617,
    16777259, /* next from 2^24 */
    33554467,
    67108879,
    134217757,
    268435459,
    536870923,
    1073741827,
    2147483659,
    4294967291 /* previous from 2^32 */
};

static const unsigned int primes_count = N_ELEMENTS(primes);

unsigned int primes_closest(unsigned int num)
{
    unsigned int i;

    for (i = 31; i > PRIME_SKIP_COUNT; i--) {
        if ((num & (1 << i)) != 0)
            return primes[i - PRIME_SKIP_COUNT];
    }
    return primes[0];
}