163 lines
3.9 KiB
C
163 lines
3.9 KiB
C
/* mpn_sub -- Subtract two low-level natural-number integers.
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Copyright (C) 1991 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 General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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The GNU MP Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with the GNU MP Library; see the file COPYING. If not, write to
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the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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/* Subtract SUB_PTR/SUB_SIZE from MIN_PTR/MIN_SIZE and store the
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result (MIN_SIZE words) at DIF_PTR.
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Return 1 if min < sub (result is negative). Otherwise, return the
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negative difference between the number of words in dif and min.
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(I.e. return 0 if the result has MIN_SIZE words, -1 if it has
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MIN_SIZE - 1 words, etc.)
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Argument constraint: MIN_SIZE >= SUB_SIZE.
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The size of DIF can be calculated as MIN_SIZE + the return value. */
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mp_size
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#ifdef __STDC__
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mpn_sub (mp_ptr dif_ptr,
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mp_srcptr min_ptr, mp_size min_size,
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mp_srcptr sub_ptr, mp_size sub_size)
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#else
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mpn_sub (dif_ptr, min_ptr, min_size, sub_ptr, sub_size)
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mp_ptr dif_ptr;
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mp_srcptr min_ptr;
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mp_size min_size;
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mp_srcptr sub_ptr;
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mp_size sub_size;
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#endif
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{
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mp_limb m, s, dif;
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mp_size j;
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/* The loop counter and index J goes from some negative value to zero.
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This way the loops are faster. Need to offset the base pointers
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to take care of the negative indices. */
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j = -sub_size;
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if (j == 0)
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goto sub_finished;
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min_ptr -= j;
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sub_ptr -= j;
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dif_ptr -= j;
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/* There are two do-loops, marked NON-CARRY LOOP and CARRY LOOP that
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jump between each other. The first loop is for when the previous
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subtraction didn't produce a carry-out; the second is for the
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complementary case. */
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/* NON-CARRY LOOP */
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do
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{
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m = min_ptr[j];
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s = sub_ptr[j];
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dif = m - s;
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dif_ptr[j] = dif;
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if (dif > m)
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goto cy_loop;
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ncy_loop:
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j++;
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}
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while (j < 0);
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/* We have exhausted SUB, with no carry out. Copy remaining part of
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MIN to DIF. */
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sub_finished:
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j = sub_size - min_size;
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/* If there's no difference between the length of the operands, the
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last words might have become zero, and re-normalization is needed. */
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if (j == 0)
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goto normalize;
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min_ptr -= j;
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dif_ptr -= j;
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goto copy;
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/* CARRY LOOP */
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do
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{
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m = min_ptr[j];
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s = sub_ptr[j];
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dif = m - s - 1;
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dif_ptr[j] = dif;
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if (dif < m)
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goto ncy_loop;
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cy_loop:
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j++;
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}
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while (j < 0);
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/* We have exhausted SUB, but need to propagate carry. */
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j = sub_size - min_size;
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if (j == 0)
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return 1; /* min < sub. Flag it to the caller */
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min_ptr -= j;
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dif_ptr -= j;
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/* Propagate carry. Sooner or later the carry will cancel with a
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non-zero word, because the minuend is normalized. Considering this,
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there's no need to test the index J. */
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for (;;)
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{
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m = min_ptr[j];
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dif = m - 1;
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dif_ptr[j] = dif;
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j++;
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if (dif < m)
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break;
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}
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if (j == 0)
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goto normalize;
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copy:
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/* Don't copy the remaining words of MIN to DIF if MIN_PTR and DIF_PTR
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are equal. It would just be a no-op copying. Return 0, as the length
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of the result equals that of the minuend. */
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if (dif_ptr == min_ptr)
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return 0;
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do
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{
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dif_ptr[j] = min_ptr[j];
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j++;
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}
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while (j < 0);
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return 0;
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normalize:
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for (j = -1; j >= -min_size; j--)
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{
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if (dif_ptr[j] != 0)
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return j + 1;
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}
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return -min_size;
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}
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