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142 lines
3.2 KiB
C
142 lines
3.2 KiB
C
/* mpn_udiv_w_sdiv -- implement udiv_qrnnd on machines with only signed
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division.
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Contributed by Peter L. Montgomery.
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THIS IS AN INTERNAL FUNCTION WITH A MUTABLE INTERFACE. IT IS ONLY SAFE
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TO REACH THIS FUNCTION THROUGH DOCUMENTED INTERFACES. IN FACT, IT IS
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ALMOST GUARANTEED THAT THIS FUNCTION WILL CHANGE OR DISAPPEAR IN A FUTURE
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GNU MP RELEASE.
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Copyright 1992, 1994, 1996, 2000, 2011, 2012 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of either:
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* the GNU Lesser General Public License as published by the Free
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Software Foundation; either version 3 of the License, or (at your
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option) any later version.
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or
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* the GNU General Public License as published by the Free Software
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Foundation; either version 2 of the License, or (at your option) any
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later version.
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or both in parallel, as here.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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You should have received copies of the GNU General Public License and the
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GNU Lesser General Public License along with the GNU MP Library. If not,
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see https://www.gnu.org/licenses/. */
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#include "gmp-impl.h"
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#include "longlong.h"
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mp_limb_t
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mpn_udiv_w_sdiv (mp_limb_t *rp, mp_limb_t a1, mp_limb_t a0, mp_limb_t d)
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{
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mp_limb_t q, r;
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mp_limb_t c0, c1, b1;
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ASSERT (d != 0);
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ASSERT (a1 < d);
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if ((mp_limb_signed_t) d >= 0)
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{
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if (a1 < d - a1 - (a0 >> (GMP_LIMB_BITS - 1)))
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{
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/* dividend, divisor, and quotient are nonnegative */
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sdiv_qrnnd (q, r, a1, a0, d);
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}
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else
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{
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/* Compute c1*2^32 + c0 = a1*2^32 + a0 - 2^31*d */
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sub_ddmmss (c1, c0, a1, a0, d >> 1, d << (GMP_LIMB_BITS - 1));
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/* Divide (c1*2^32 + c0) by d */
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sdiv_qrnnd (q, r, c1, c0, d);
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/* Add 2^31 to quotient */
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q += (mp_limb_t) 1 << (GMP_LIMB_BITS - 1);
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}
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}
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else
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{
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b1 = d >> 1; /* d/2, between 2^30 and 2^31 - 1 */
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c1 = a1 >> 1; /* A/2 */
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c0 = (a1 << (GMP_LIMB_BITS - 1)) + (a0 >> 1);
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if (a1 < b1) /* A < 2^32*b1, so A/2 < 2^31*b1 */
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{
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sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
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r = 2*r + (a0 & 1); /* Remainder from A/(2*b1) */
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if ((d & 1) != 0)
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{
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if (r >= q)
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r = r - q;
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else if (q - r <= d)
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{
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r = r - q + d;
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q--;
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}
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else
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{
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r = r - q + 2*d;
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q -= 2;
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}
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}
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}
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else if (c1 < b1) /* So 2^31 <= (A/2)/b1 < 2^32 */
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{
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c1 = (b1 - 1) - c1;
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c0 = ~c0; /* logical NOT */
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sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
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q = ~q; /* (A/2)/b1 */
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r = (b1 - 1) - r;
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r = 2*r + (a0 & 1); /* A/(2*b1) */
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if ((d & 1) != 0)
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{
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if (r >= q)
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r = r - q;
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else if (q - r <= d)
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{
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r = r - q + d;
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q--;
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}
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else
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{
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r = r - q + 2*d;
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q -= 2;
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}
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}
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}
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else /* Implies c1 = b1 */
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{ /* Hence a1 = d - 1 = 2*b1 - 1 */
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if (a0 >= -d)
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{
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q = -CNST_LIMB(1);
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r = a0 + d;
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}
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else
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{
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q = -CNST_LIMB(2);
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r = a0 + 2*d;
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}
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}
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}
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*rp = r;
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return q;
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}
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