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1.0 Paper Introduction
Fast Primality Testing for Integers That Fit into a Machine Word (Forisek & Jancina, 2015)1 introduces a deterministic primality test for 32-bit and 64-bit integers.
The algorithm works in three steps (Forisek & Jancina, 2015):
- Use trial division to check that n is relatively prime to 210.
- Compute a hash value h(n) in constant time.
- Use a precomputed lookup table to determine primality
The precomputed lookup table identifies if the input n is a strong probable-prime with base b (b-SPRP):
1.1 C Code
(Forisek & Jancina, 2015) provide C code for the 32-bit case, that needs 512 bytes of memory:
#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>
uint16_t bases[]={15591,2018,166,7429,8064,16045,10503,4399,1949,1295,2776,3620,560,3128,5212,
2657,2300,2021,4652,1471,9336,4018,2398,20462,10277,8028,2213,6219,620,3763,4852,5012,3185,
1333,6227,5298,1074,2391,5113,7061,803,1269,3875,422,751,580,4729,10239,746,2951,556,2206,
3778,481,1522,3476,481,2487,3266,5633,488,3373,6441,3344,17,15105,1490,4154,2036,1882,1813,
467,3307,14042,6371,658,1005,903,737,1887,7447,1888,2848,1784,7559,3400,951,13969,4304,177,41,
19875,3110,13221,8726,571,7043,6943,1199,352,6435,165,1169,3315,978,233,3003,2562,2994,10587,
10030,2377,1902,5354,4447,1555,263,27027,2283,305,669,1912,601,6186,429,1930,14873,1784,1661,
524,3577,236,2360,6146,2850,55637,1753,4178,8466,222,2579,2743,2031,2226,2276,374,2132,813,
23788,1610,4422,5159,1725,3597,3366,14336,579,165,1375,10018,12616,9816,1371,536,1867,10864,
857,2206,5788,434,8085,17618,727,3639,1595,4944,2129,2029,8195,8344,6232,9183,8126,1870,3296,
7455,8947,25017,541,19115,368,566,5674,411,522,1027,8215,2050,6544,10049,614,774,2333,3007,
35201,4706,1152,1785,1028,1540,3743,493,4474,2521,26845,8354,864,18915,5465,2447,42,4511,
1660,166,1249,6259,2553,304,272,7286,73,6554,899,2816,5197,13330,7054,2818,3199,811,922,350,
7514,4452,3449,2663,4708,418,1621,1171,3471,88,11345,412,1559,194};
bool is_SPRP(uint32_t n, uint32_t a)
{
uint32_t d = n-1, s = 0;
while ((d&1)==0) ++s, d>>=1;
uint64_t cur = 1, pw = d;
while (pw) {
if (pw & 1) cur = (cur*a) % n;
a = ((uint64_t)a*a) % n;
pw >>= 1;
}
if (cur == 1) return true;
for (uint32_t r=0; r<s; r++) {
if (cur == n-1) return true;
cur = (cur*cur) % n;
}
return false;
}
bool is_prime(uint32_t x) {
if (x==2 || x==3 || x==5 || x==7) return true;
if (x%2==0 || x%3==0 || x%5==0 || x%7==0) return false;
if (x<121) return (x>1);
uint64_t h = x;
h = ((h >> 16) ^ h) * 0x45d9f3b;
h = ((h >> 16) ^ h) * 0x45d9f3b;
h = ((h >> 16) ^ h) & 255;
return is_SPRP(x,bases[h]);
}
int main()
{
for(uint32_t i = 0 ;i < 10; i++)
{
if(is_prime(i))
{
printf("Found prime: %u\n",i);
}
}
return 0;
}Note however, there exist much faster algorithms as implemented in (MachinePrime, 2026)2. One may SPRP, Single Shot rabin miller in C or in Rust.