151 lines
4.3 KiB
C
151 lines
4.3 KiB
C
/* mpn_divexact_1 -- mpn by limb exact division.
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THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY. THEY'RE ALMOST
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CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
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FUTURE GNU MP RELEASES.
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Copyright 2000, 2001, 2002, 2003, 2005 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 the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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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 Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/* Divide a={src,size} by d=divisor and store the quotient in q={dst,size}.
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q will only be correct if d divides a exactly.
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A separate loop is used for shift==0 because n<<BITS_PER_MP_LIMB doesn't
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give zero on all CPUs (for instance it doesn't on the x86s). This
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separate loop might run faster too, helping odd divisors.
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Possibilities:
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mpn_divexact_1c could be created, accepting and returning c. This would
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let a long calculation be done piece by piece. Currently there's no
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particular need for that, and not returning c means that a final umul can
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be skipped.
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Another use for returning c would be letting the caller know whether the
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division was in fact exact. It would work just to return the carry bit
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"c=(l>s)" and let the caller do a final umul if interested.
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When the divisor is even, the factors of two could be handled with a
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separate mpn_rshift, instead of shifting on the fly. That might be
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faster on some CPUs and would mean just the shift==0 style loop would be
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needed.
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If n<<BITS_PER_MP_LIMB gives zero on a particular CPU then the separate
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shift==0 loop is unnecessary, and could be eliminated if there's no great
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speed difference.
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It's not clear whether "/" is the best way to handle size==1. Alpha gcc
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2.95 for instance has a poor "/" and might prefer the modular method.
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Perhaps a tuned parameter should control this.
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If src[size-1] < divisor then dst[size-1] will be zero, and one divide
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step could be skipped. A test at last step for s<divisor (or ls in the
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even case) might be a good way to do that. But if this code is often
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used with small divisors then it might not be worth bothering */
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void
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mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)
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{
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mp_size_t i;
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mp_limb_t c, h, l, ls, s, s_next, inverse, dummy;
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unsigned shift;
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ASSERT (size >= 1);
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ASSERT (divisor != 0);
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ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
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ASSERT_MPN (src, size);
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ASSERT_LIMB (divisor);
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s = src[0];
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if (size == 1)
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{
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dst[0] = s / divisor;
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return;
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}
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if ((divisor & 1) == 0)
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{
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count_trailing_zeros (shift, divisor);
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divisor >>= shift;
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}
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else
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shift = 0;
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modlimb_invert (inverse, divisor);
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divisor <<= GMP_NAIL_BITS;
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if (shift != 0)
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{
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c = 0;
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i = 0;
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size--;
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do
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{
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s_next = src[i+1];
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ls = ((s >> shift) | (s_next << (GMP_NUMB_BITS-shift))) & GMP_NUMB_MASK;
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s = s_next;
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SUBC_LIMB (c, l, ls, c);
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l = (l * inverse) & GMP_NUMB_MASK;
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dst[i] = l;
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umul_ppmm (h, dummy, l, divisor);
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c += h;
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i++;
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}
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while (i < size);
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ls = s >> shift;
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l = ls - c;
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l = (l * inverse) & GMP_NUMB_MASK;
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dst[i] = l;
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}
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else
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{
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l = (s * inverse) & GMP_NUMB_MASK;
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dst[0] = l;
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i = 1;
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c = 0;
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do
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{
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umul_ppmm (h, dummy, l, divisor);
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c += h;
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s = src[i];
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SUBC_LIMB (c, l, s, c);
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l = (l * inverse) & GMP_NUMB_MASK;
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dst[i] = l;
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i++;
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}
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while (i < size);
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}
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}
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