source/libs/gmp/gmp-src/mpn/generic/sub_err2_n.c

   1 /* mpn_sub_err2_n -- sub_n with two error terms
   2
   3    Contributed by David Harvey.
   4
   5    THE FUNCTION IN THIS FILE IS INTERNAL WITH A MUTABLE INTERFACE.  IT IS ONLY
   6    SAFE TO REACH IT THROUGH DOCUMENTED INTERFACES.  IN FACT, IT IS ALMOST
   7    GUARANTEED THAT IT'LL CHANGE OR DISAPPEAR IN A FUTURE GNU MP RELEASE.
   8
   9 Copyright 2011 Free Software Foundation, Inc.
  10
  11 This file is part of the GNU MP Library.
  12
  13 The GNU MP Library is free software; you can redistribute it and/or modify
  14 it under the terms of either:
  15
  16   * the GNU Lesser General Public License as published by the Free
  17     Software Foundation; either version 3 of the License, or (at your
  18     option) any later version.
  19
  20 or
  21
  22   * the GNU General Public License as published by the Free Software
  23     Foundation; either version 2 of the License, or (at your option) any
  24     later version.
  25
  26 or both in parallel, as here.
  27
  28 The GNU MP Library is distributed in the hope that it will be useful, but
  29 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  30 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  31 for more details.
  32
  33 You should have received copies of the GNU General Public License and the
  34 GNU Lesser General Public License along with the GNU MP Library.  If not,
  35 see https://www.gnu.org/licenses/.  */
  36
  37 #include "gmp.h"
  38 #include "gmp-impl.h"
  39
  40 /*
  41   Computes:
  42
  43   (1) {rp,n} := {up,n} - {vp,n} (just like mpn_sub_n) with incoming borrow cy,
  44   return value is borrow out.
  45
  46   (2) Let c[i+1] = borrow from i-th limb subtraction (c[0] = cy).
  47   Computes c[1]*yp1[n-1] + ... + c[n]*yp1[0],
  48            c[1]*yp2[n-1] + ... + c[n]*yp2[0],
  49   stores two-limb results at {ep,2} and {ep+2,2} respectively.
  50
  51   Requires n >= 1.
  52
  53   None of the outputs may overlap each other or any of the inputs, except
  54   that {rp,n} may be equal to {up,n} or {vp,n}.
  55 */
  56 mp_limb_t
  57 mpn_sub_err2_n (mp_ptr rp, mp_srcptr up, mp_srcptr vp,
  58                 mp_ptr ep, mp_srcptr yp1, mp_srcptr yp2,
  59                 mp_size_t n, mp_limb_t cy)
  60 {
  61   mp_limb_t el1, eh1, el2, eh2, ul, vl, yl1, yl2, zl1, zl2, rl, sl, cy1, cy2;
  62
  63   ASSERT (n >= 1);
  64   ASSERT (MPN_SAME_OR_SEPARATE_P (rp, up, n));
  65   ASSERT (MPN_SAME_OR_SEPARATE_P (rp, vp, n));
  66   ASSERT (! MPN_OVERLAP_P (rp, n, yp1, n));
  67   ASSERT (! MPN_OVERLAP_P (rp, n, yp2, n));
  68   ASSERT (! MPN_OVERLAP_P (ep, 4, up, n));
  69   ASSERT (! MPN_OVERLAP_P (ep, 4, vp, n));
  70   ASSERT (! MPN_OVERLAP_P (ep, 4, yp1, n));
  71   ASSERT (! MPN_OVERLAP_P (ep, 4, yp2, n));
  72   ASSERT (! MPN_OVERLAP_P (ep, 4, rp, n));
  73
  74   yp1 += n - 1;
  75   yp2 += n - 1;
  76   el1 = eh1 = 0;
  77   el2 = eh2 = 0;
  78
  79   do
  80     {
  81       yl1 = *yp1--;
  82       yl2 = *yp2--;
  83       ul = *up++;
  84       vl = *vp++;
  85
  86       /* ordinary sub_n */
  87       SUBC_LIMB (cy1, sl, ul, vl);
  88       SUBC_LIMB (cy2, rl, sl, cy);
  89       cy = cy1 | cy2;
  90       *rp++ = rl;
  91
  92       /* update (eh1:el1) */
  93       zl1 = (-cy) & yl1;
  94       el1 += zl1;
  95       eh1 += el1 < zl1;
  96
  97       /* update (eh2:el2) */
  98       zl2 = (-cy) & yl2;
  99       el2 += zl2;
 100       eh2 += el2 < zl2;
 101     }
 102   while (--n);
 103
 104 #if GMP_NAIL_BITS != 0
 105   eh1 = (eh1 << GMP_NAIL_BITS) + (el1 >> GMP_NUMB_BITS);
 106   el1 &= GMP_NUMB_MASK;
 107   eh2 = (eh2 << GMP_NAIL_BITS) + (el2 >> GMP_NUMB_BITS);
 108   el2 &= GMP_NUMB_MASK;
 109 #endif
 110
 111   ep[0] = el1;
 112   ep[1] = eh1;
 113   ep[2] = el2;
 114   ep[3] = eh2;
 115
 116   return cy;
 117 }