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/*
Copyright (C) 2018, Daniel Schultz
This file is part of FLINT.
FLINT is free software: you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License (LGPL) as published
by the Free Software Foundation; either version 3 of the License, or
(at your option) any later version. See <https://www.gnu.org/licenses/>.
*/
#include "gmpcompat.h"
#include "ulong_extras.h"
#include "fmpz.h"
ulong
n_powmod2_fmpz_preinv(ulong a, const fmpz_t exp, ulong n, ulong ninv)
{
flint_bitcnt_t i, bits;
ulong x, norm;
FLINT_ASSERT(n != 0);
FLINT_ASSERT(a < n);
FLINT_ASSERT(fmpz_sgn(exp) >= 0);
if (fmpz_is_zero(exp))
{
/* anything modulo 1 is 0 */
return n == 1 ? 0 : 1;
}
if (a == 0)
return 0;
norm = flint_clz(n);
a <<= norm;
n <<= norm;
bits = fmpz_bits(exp);
i = 0;
while (i < bits && fmpz_tstbit(exp, i) == 0)
{
a = n_mulmod_preinv(a, a, n, ninv, norm);
i++;
}
x = a;
i++;
while (i < bits)
{
a = n_mulmod_preinv(a, a, n, ninv, norm);
if (fmpz_tstbit(exp, i) != 0) {
x = n_mulmod_preinv(x, a, n, ninv, norm);
}
i++;
}
return x >> norm;
}