pqc/external/flint-2.4.3/fmpz/xgcd_partial.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) 2012 William Hart
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "ulong_extras.h"
#include "fmpz.h"
void fmpz_xgcd_partial(fmpz_t co2, fmpz_t co1,
fmpz_t r2, fmpz_t r1, const fmpz_t L)
{
fmpz_t q, r;
slong aa2, aa1, bb2, bb1, rr1, rr2, qq, bb, t1, t2, t3, i;
slong bits, bits1, bits2;
fmpz_init(q); fmpz_init(r);
fmpz_zero(co2);
fmpz_set_si(co1, -1);
while (!fmpz_is_zero(r1) && fmpz_cmp(r1, L) > 0)
{
bits2 = fmpz_bits(r2);
bits1 = fmpz_bits(r1);
bits = FLINT_MAX(bits2, bits1) - FLINT_BITS + 1;
if (bits < 0) bits = 0;
fmpz_tdiv_q_2exp(r, r2, bits);
rr2 = fmpz_get_ui(r);
fmpz_tdiv_q_2exp(r, r1, bits);
rr1 = fmpz_get_ui(r);
fmpz_tdiv_q_2exp(r, L, bits);
bb = fmpz_get_ui(r);
aa2 = 0; aa1 = 1;
bb2 = 1; bb1 = 0;
for (i = 0; rr1 != 0 && rr1 > bb; i++)
{
qq = rr2 / rr1;
t1 = rr2 - qq*rr1;
t2 = aa2 - qq*aa1;
t3 = bb2 - qq*bb1;
if (i & 1)
{
if (t1 < -t3 || rr1 - t1 < t2 - aa1) break;
} else
{
if (t1 < -t2 || rr1 - t1 < t3 - bb1) break;
}
rr2 = rr1; rr1 = t1;
aa2 = aa1; aa1 = t2;
bb2 = bb1; bb1 = t3;
}
if (i == 0)
{
fmpz_fdiv_qr(q, r2, r2, r1);
fmpz_swap(r2, r1);
fmpz_submul(co2, co1, q);
fmpz_swap(co2, co1);
} else
{
fmpz_mul_si(r, r2, bb2);
if (aa2 >= 0)
fmpz_addmul_ui(r, r1, aa2);
else
fmpz_submul_ui(r, r1, -aa2);
fmpz_mul_si(r1, r1, aa1);
if (bb1 >= 0)
fmpz_addmul_ui(r1, r2, bb1);
else
fmpz_submul_ui(r1, r2, -bb1);
fmpz_set(r2, r);
fmpz_mul_si(r, co2, bb2);
if (aa2 >= 0)
fmpz_addmul_ui(r, co1, aa2);
else
fmpz_submul_ui(r, co1, -aa2);
fmpz_mul_si(co1, co1, aa1);
if (bb1 >= 0)
fmpz_addmul_ui(co1, co2, bb1);
else
fmpz_submul_ui(co1, co2, -bb1);
fmpz_set(co2, r);
if (fmpz_sgn(r1) < 0) { fmpz_neg(co1, co1); fmpz_neg(r1, r1); }
if (fmpz_sgn(r2) < 0) { fmpz_neg(co2, co2); fmpz_neg(r2, r2); }
}
}
if (fmpz_sgn(r2) < 0)
{
fmpz_neg(co2, co2); fmpz_neg(co1, co1);
fmpz_neg(r2, r2);
}
fmpz_clear(q); fmpz_clear(r);
}