pqc/external/flint-2.4.3/arith/cyclotomic_polynomial.c

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2014-05-18 22:03:37 +00:00
/*=============================================================================
This file is part of FLINT.
FLINT is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
FLINT is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with FLINT; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
=============================================================================*/
/******************************************************************************
Copyright (C) 2011 Fredrik Johansson
******************************************************************************/
#include "arith.h"
void
_arith_cyclotomic_polynomial(fmpz * a, ulong n, mp_ptr factors,
slong num_factors, ulong phi)
{
slong i, k;
int small;
ulong D;
D = phi / 2;
/* Phi_p(x) = 1 + x + x^2 + ... + x^{p-1} */
if (num_factors == 1)
{
for (i = 0; i <= D; i++)
fmpz_one(a + i);
return;
}
/* Phi_{2n}(x) = Phi_n(-x)*/
if (factors[0] == UWORD(2))
{
_arith_cyclotomic_polynomial(a, n / 2, factors + 1,
num_factors - 1, phi);
for (i = 1; i <= D; i += 2)
fmpz_neg(a + i, a + i);
return;
}
fmpz_one(a);
for (i = 1; i <= D; i++)
fmpz_zero(a + i);
/* Coefficients are guaranteed not to overflow an fmpz */
small = (num_factors == 2) || /* Always +1/0/-1*/
(n < WORD(10163195)) || /* At most 27 bits */
(FLINT_BITS == 64 && n < WORD(169828113)); /* At most 60 bits */
/* Iterate over all divisors of n */
for (k = 0; k < (WORD(1) << num_factors); k++)
{
int mu;
ulong d;
mu = (num_factors & 1) ? -1 : 1;
d = WORD(1);
for (i = 0; i < num_factors; i++)
{
if ((k >> i) & 1)
{
d *= factors[i];
mu = -mu;
}
}
/* Multiply by (x^d - 1)^{\mu(n/d)} */
if (small)
{
if (mu == 1)
for (i = D; i >= d; i--) a[i] -= a[i - d];
else
for (i = d; i <= D; i++) a[i] += a[i - d];
}
else
{
if (mu == 1)
for (i = D; i >= d; i--) fmpz_sub(a + i, a + i, a + i - d);
else
for (i = d; i <= D; i++) fmpz_add(a + i, a + i, a + i - d);
}
}
}
void
arith_cyclotomic_polynomial(fmpz_poly_t poly, ulong n)
{
n_factor_t factors;
slong i, j;
ulong s, phi;
if (n <= 2)
{
if (n == 0)
{
fmpz_poly_set_ui(poly, UWORD(1));
}
else
{
fmpz_poly_fit_length(poly, 2);
fmpz_set_si(poly->coeffs, (n == 1) ? WORD(-1) : WORD(1));
fmpz_set_si(poly->coeffs + 1, WORD(1));
_fmpz_poly_set_length(poly, 2);
}
return;
}
/* Write n = q * s where q is squarefree, compute the factors of q,
and compute phi(s) which determines the degree of the polynomial. */
n_factor_init(&factors);
n_factor(&factors, n, 1);
s = phi = UWORD(1);
for (i = 0; i < factors.num; i++)
{
phi *= factors.p[i] - 1;
while (factors.exp[i] > 1)
{
s *= factors.p[i];
factors.exp[i]--;
}
}
fmpz_poly_fit_length(poly, phi * s + 1);
/* Evaluate lower half of Phi_s(x) */
_arith_cyclotomic_polynomial(poly->coeffs, n / s,
factors.p, factors.num, phi);
/* Palindromic extension */
for (i = 0; i < (phi + 1) / 2; i++)
fmpz_set(poly->coeffs + phi - i, poly->coeffs + i);
/* Stretch */
if (s != 1)
{
for (i = phi; i > 0; i--)
{
fmpz_set(poly->coeffs + i*s, poly->coeffs + i);
for (j = 1; j < s; j++)
fmpz_zero(poly->coeffs + i*s - j);
}
}
_fmpz_poly_set_length(poly, phi * s + 1);
}