Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Fix primality test for very small numbers #102

Open
wants to merge 5 commits into
base: main
Choose a base branch
from
Open
Show file tree
Hide file tree
Changes from 1 commit
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 2 additions & 0 deletions src/pprods.h
Original file line number Diff line number Diff line change
Expand Up @@ -386,4 +386,6 @@ static const uint64_t pprods[] = {
0xaf157a9999de1f, // 14887*14891*14897*14923
0xb1110edec9a5e7, // 14929*14939*14947*14951
};

static const uint32_t pprods_max_prime = 14951;
#endif // PPRODS_H
19 changes: 14 additions & 5 deletions src/primetest.h
Original file line number Diff line number Diff line change
Expand Up @@ -141,9 +141,10 @@ static int is_prime_bpsw(const mpz_t n)
int min_pprods = 5, n_pprods = sizeof(pprods) / sizeof(*pprods);
mpz_t b, d;

/* Discard even numbers. */
if (!mpz_tstbit(n, 0) && mpz_cmp_ui(n, 2))
return 0;
/* Discard even and non-positive numbers. */
if (!mpz_tstbit(n, 0) || n->_mp_size < 1) {
return mpz_cmp_ui(n, 2) ? 0 : 2;
}

/* Adjust the number of GCDs to compute with products of small primes
* if bit length of n is less than 1024. As the computational cost of
Expand All @@ -155,8 +156,16 @@ static int is_prime_bpsw(const mpz_t n)
}

for (i = 0; i < n_pprods; i++) {
if (mpn_gcd_1(n->_mp_d, n->_mp_size, pprods[i]) != 1)
return 0;
if (mpn_gcd_1(n->_mp_d, n->_mp_size, pprods[i]) != 1) {
if (n->_mp_size > 1 || n->_mp_d[0] > pprods_max_prime) {
return 0;
} else if (n->_mp_d[0] < 8) {
/* Special case for primes 3, 5, 7. */
return (1 << n->_mp_d[0]) & 0xac ? 2 : 0;
} else {
break;
}
}
}

mpz_init2(b, n->_mp_size * sizeof(n->_mp_d[0]) * 8);
Expand Down
4 changes: 3 additions & 1 deletion tools/gen_pprods.py
Original file line number Diff line number Diff line change
Expand Up @@ -73,6 +73,8 @@ def print_pprods_h(prods, factors):
static const uint64_t pprods[] = {
%s
};

static const uint32_t pprods_max_prime = %d;
#endif // PPRODS_H
"""
tmpl = tmpl.strip()
Expand All @@ -81,7 +83,7 @@ def print_pprods_h(prods, factors):
factors_str = "*".join(map(str, factors[i]))
extra_spaces = " " * (16 - (pr.bit_length() + 3) // 4)
prods_str += tab + "0x%x, %s// %s\n" % (pr, extra_spaces, factors_str)
print(tmpl % (prods_str.rstrip(),))
print(tmpl % (prods_str.rstrip(), factors[-1][-1]))


n = int(sys.argv[1])
Expand Down