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

/*=============================================================================

    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

    Inspired by a Sage implementation written by William Stein.

******************************************************************************/

#include <math.h>
#include "arith.h"


/* Bound coefficients using (x + u)^(2^n) and the binomial
   coefficients. TODO: this is about 2x too large... */
static slong __bound_prec(ulong n)
{
    slong i;
    double u, N;

    N = UWORD(1) << n;

    /* u = (sum of square roots)^(2^n) */
    u = 0;
    for (i = 0; i < n; i++)
        u += sqrt(n_nth_prime(1 + i));
    u = N * log(u) * 1.44269504088897;

    /* Central binomial coefficient C(N,N/2) < 2^N / sqrt(3*N/2) */
    u += N - 0.5*(n-1) - 0.792481250360578;  /* log(sqrt(3)) */

    return u;
}

void arith_swinnerton_dyer_polynomial(fmpz_poly_t poly, ulong n)
{
    fmpz *square_roots, *T, *tmp1, *tmp2, *tmp3;
    fmpz_t one;
    slong i, j, k, N;
    slong prec;

    if (n == 0)
    {
        fmpz_poly_zero(poly);
        fmpz_poly_set_coeff_ui(poly, 1, UWORD(1));
        return;
    }

    N = WORD(1) << n;

    prec = __bound_prec(n);
    /* flint_printf("prec: %wd\n", prec); */

    fmpz_poly_fit_length(poly, N + 1);
    T = poly->coeffs;

    fmpz_init(one);
    fmpz_one(one);
    fmpz_mul_2exp(one, one, prec);

    square_roots = _fmpz_vec_init(n);
    tmp1 = flint_malloc((N/2 + 1) * sizeof(fmpz));
    tmp2 = flint_malloc((N/2 + 1) * sizeof(fmpz));
    tmp3 = _fmpz_vec_init(N);

    for (i = 0; i < n; i++)
    {
        fmpz_set_ui(square_roots + i, n_nth_prime(i + 1));
        fmpz_mul_2exp(square_roots + i, square_roots + i, 2 * prec);
        fmpz_sqrt(square_roots + i, square_roots + i);
    }

    /* Build linear factors */
    for (i = 0; i < N; i++)
    {
        fmpz_zero(T + i);
        for (j = 0; j < n; j++)
        {
            if ((i >> j) & 1)
                fmpz_add(T + i, T + i, square_roots + j);
            else
                fmpz_sub(T + i, T + i, square_roots + j);
        }
    }

    /* For each level... */
    for (i = 0; i < n; i++)
    {
        slong stride = UWORD(1) << i;

        for (j = 0; j < N; j += 2*stride)
        {
            for (k = 0; k < stride; k++)
            {
                tmp1[k] = T[j + k];
                tmp2[k] = T[j + stride + k];
            }
            tmp1[stride] = *one;
            tmp2[stride] = *one;

            _fmpz_poly_mullow(tmp3, tmp1, stride + 1, tmp2, stride + 1, 2*stride);
            _fmpz_vec_scalar_fdiv_q_2exp(T + j, tmp3, 2*stride, prec);
        }
    }

    /* Round */
    fmpz_fdiv_q_2exp(one, one, 1);
    for (i = 0; i < N; i++)
        fmpz_add(T + i, T + i, one);

    _fmpz_vec_scalar_fdiv_q_2exp(T, T, N, prec);
    fmpz_one(T + (UWORD(1) << n));
    _fmpz_poly_set_length(poly, N + 1);

    _fmpz_vec_clear(square_roots, n);
    flint_free(tmp1);
    flint_free(tmp2);
    _fmpz_vec_clear(tmp3, UWORD(1) << n);
    fmpz_clear(one);
}