dive_1.c
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- /* mpn_divexact_1 -- mpn by limb exact division.
- THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY. THEY'RE ALMOST
- CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
- FUTURE GNU MP RELEASES.
- Copyright 2000, 2001, 2002, 2003, 2005 Free Software Foundation, Inc.
- This file is part of the GNU MP Library.
- The GNU MP Library is free software; you can redistribute it and/or modify
- it under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 3 of the License, or (at your
- option) any later version.
- The GNU MP Library 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 Lesser General Public
- License for more details.
- You should have received a copy of the GNU Lesser General Public License
- along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
- #include "gmp.h"
- #include "gmp-impl.h"
- #include "longlong.h"
- /* Divide a={src,size} by d=divisor and store the quotient in q={dst,size}.
- q will only be correct if d divides a exactly.
- A separate loop is used for shift==0 because n<<GMP_LIMB_BITS doesn't
- give zero on all CPUs (for instance it doesn't on the x86s). This
- separate loop might run faster too, helping odd divisors.
- Possibilities:
- mpn_divexact_1c could be created, accepting and returning c. This would
- let a long calculation be done piece by piece. Currently there's no
- particular need for that, and not returning c means that a final umul can
- be skipped.
- Another use for returning c would be letting the caller know whether the
- division was in fact exact. It would work just to return the carry bit
- "c=(l>s)" and let the caller do a final umul if interested.
- When the divisor is even, the factors of two could be handled with a
- separate mpn_rshift, instead of shifting on the fly. That might be
- faster on some CPUs and would mean just the shift==0 style loop would be
- needed.
- If n<<GMP_LIMB_BITS gives zero on a particular CPU then the separate
- shift==0 loop is unnecessary, and could be eliminated if there's no great
- speed difference.
- It's not clear whether "/" is the best way to handle size==1. Alpha gcc
- 2.95 for instance has a poor "/" and might prefer the modular method.
- Perhaps a tuned parameter should control this.
- If src[size-1] < divisor then dst[size-1] will be zero, and one divide
- step could be skipped. A test at last step for s<divisor (or ls in the
- even case) might be a good way to do that. But if this code is often
- used with small divisors then it might not be worth bothering */
- void
- mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)
- {
- mp_size_t i;
- mp_limb_t c, h, l, ls, s, s_next, inverse, dummy;
- unsigned shift;
- ASSERT (size >= 1);
- ASSERT (divisor != 0);
- ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
- ASSERT_MPN (src, size);
- ASSERT_LIMB (divisor);
- s = src[0];
- if (size == 1)
- {
- dst[0] = s / divisor;
- return;
- }
- if ((divisor & 1) == 0)
- {
- count_trailing_zeros (shift, divisor);
- divisor >>= shift;
- }
- else
- shift = 0;
- binvert_limb (inverse, divisor);
- divisor <<= GMP_NAIL_BITS;
- if (shift != 0)
- {
- c = 0;
- i = 0;
- size--;
- do
- {
- s_next = src[i+1];
- ls = ((s >> shift) | (s_next << (GMP_NUMB_BITS-shift))) & GMP_NUMB_MASK;
- s = s_next;
- SUBC_LIMB (c, l, ls, c);
- l = (l * inverse) & GMP_NUMB_MASK;
- dst[i] = l;
- umul_ppmm (h, dummy, l, divisor);
- c += h;
- i++;
- }
- while (i < size);
- ls = s >> shift;
- l = ls - c;
- l = (l * inverse) & GMP_NUMB_MASK;
- dst[i] = l;
- }
- else
- {
- l = (s * inverse) & GMP_NUMB_MASK;
- dst[0] = l;
- i = 1;
- c = 0;
- do
- {
- umul_ppmm (h, dummy, l, divisor);
- c += h;
- s = src[i];
- SUBC_LIMB (c, l, s, c);
- l = (l * inverse) & GMP_NUMB_MASK;
- dst[i] = l;
- i++;
- }
- while (i < size);
- }
- }