/*
    Copyright (C) 2008, 2009 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 2.1 of the License, or
    (at your option) any later version.  See <https://www.gnu.org/licenses/>.
*/

#include <gmp.h>
#include "flint.h"
#include "fmpz.h"
#include "fmpz_vec.h"
#include "fmpz_poly.h"

/* Assumes poly1 and poly2 are not length 0. */
void
_fmpz_poly_mul_classical(fmpz * res, const fmpz * poly1,
                         slong len1, const fmpz * poly2, slong len2)
{
    if (len1 == 1 && len2 == 1) /* Special case if the length of both inputs is 1 */
    {
        fmpz_mul(res, poly1, poly2);
    }
    else                        /* Ordinary case */
    {
        slong i;

        /* Set res[i] = poly1[i]*poly2[0] */
        _fmpz_vec_scalar_mul_fmpz(res, poly1, len1, poly2);

        /* Set res[i+len1-1] = in1[len1-1]*in2[i] */
        _fmpz_vec_scalar_mul_fmpz(res + len1, poly2 + 1, len2 - 1,
                                  poly1 + len1 - 1);

        /* out[i+j] += in1[i]*in2[j] */
        for (i = 0; i < len1 - 1; i++)
            _fmpz_vec_scalar_addmul_fmpz(res + i + 1, poly2 + 1, len2 - 1,
                                         poly1 + i);
    }
}

void
fmpz_poly_mul_classical(fmpz_poly_t res,
                        const fmpz_poly_t poly1, const fmpz_poly_t poly2)
{
    slong len_out;

    if ((poly1->length == 0) || (poly2->length == 0))
    {
        fmpz_poly_zero(res);
        return;
    }

    len_out = poly1->length + poly2->length - 1;

    if (res == poly1 || res == poly2)
    {
        fmpz_poly_t temp;
        fmpz_poly_init2(temp, len_out);
        _fmpz_poly_mul_classical(temp->coeffs, poly1->coeffs, poly1->length,
                                 poly2->coeffs, poly2->length);
        fmpz_poly_swap(res, temp);
        fmpz_poly_clear(temp);
    }
    else
    {
        fmpz_poly_fit_length(res, len_out);
        _fmpz_poly_mul_classical(res->coeffs, poly1->coeffs, poly1->length,
                                 poly2->coeffs, poly2->length);
    }

    _fmpz_poly_set_length(res, len_out);
}