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/*
Copyright (C) 2009, 2013, 2016 William Hart
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 "ulong_extras.h"
ulong
n_powmod2_ui_preinv(ulong a, ulong exp, ulong n, ulong ninv)
{
ulong x, norm;
FLINT_ASSERT(n != 0);
if (exp == 0)
{
/* anything modulo 1 is 0 */
return n == 1 ? 0 : 1;
}
if (a == 0)
return 0;
if (a >= n)
a = n_mod2_preinv(a, n, ninv);
norm = flint_clz(n);
a <<= norm;
n <<= norm;
while ((exp & 1) == 0)
{
a = n_mulmod_preinv(a, a, n, ninv, norm);
exp >>= 1;
}
x = a;
while (exp >>= 1)
{
a = n_mulmod_preinv(a, a, n, ninv, norm);
if (exp & 1)
x = n_mulmod_preinv(x, a, n, ninv, norm);
}
return x >> norm;
}