void
zn_array_invert (ulong* res, const ulong* op, size_t n, const zn_mod_t mod)
{
ZNP_ASSERT (n >= 1);
// for now assume input is monic
ZNP_ASSERT (op[0] == 1);
if (n == 1)
{
res[0] = 1;
return;
}
size_t half = (n + 1) / 2; // ceil(n / 2)
// recursively obtain the first half of the output
zn_array_invert (res, op, half, mod);
// extend to second half of the output
if (mod->m & 1)
zn_array_invert_extend_fft (res + half, res, op, half, n - half, mod);
else
zn_array_invert_extend (res + half, res, op, half, n - half, mod);
}
/*
Tests zn_array_invert() for a given series length and modulus.
*/
int
testcase_zn_array_invert (size_t n, const zn_mod_t mod)
{
ulong* op = (ulong*) malloc (sizeof (ulong) * n);
ulong* res = (ulong*) malloc (sizeof (ulong) * n);
ulong* check = (ulong*) malloc (sizeof (ulong) * (2 * n - 1));
// make up random input poly
size_t i;
op[0] = 1;
for (i = 1; i < n; i++)
op[i] = random_ulong (mod->m);
// compute inverse
zn_array_invert (res, op, n, mod);
// multiply by original series and check we get 1
ref_zn_array_mul (check, op, n, res, n, mod);
int success = (check[0] == 1);
for (i = 1; i < n; i++)
success = success && (check[i] == 0);
free (check);
free (res);
free (op);
return success;
}
