1  /* mpn_sub_err3_n -- sub_n with three 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-impl.h"
      38  
      39  /*
      40    Computes:
      41  
      42    (1) {rp,n} := {up,n} - {vp,n} (just like mpn_sub_n) with incoming borrow cy,
      43    return value is borrow out.
      44  
      45    (2) Let c[i+1] = borrow from i-th limb subtraction (c[0] = cy).
      46    Computes c[1]*yp1[n-1] + ... + c[n]*yp1[0],
      47             c[1]*yp2[n-1] + ... + c[n]*yp2[0],
      48             c[1]*yp3[n-1] + ... + c[n]*yp3[0],
      49    stores two-limb results at {ep,2}, {ep+2,2} and {ep+4,2} 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_err3_n (mp_ptr rp, mp_srcptr up, mp_srcptr vp,
      58                  mp_ptr ep, mp_srcptr yp1, mp_srcptr yp2, mp_srcptr yp3,
      59                  mp_size_t n, mp_limb_t cy)
      60  {
      61    mp_limb_t el1, eh1, el2, eh2, el3, eh3, ul, vl, yl1, yl2, yl3, zl1, zl2, zl3, 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 (rp, n, yp3, n));
      69    ASSERT (! MPN_OVERLAP_P (ep, 6, up, n));
      70    ASSERT (! MPN_OVERLAP_P (ep, 6, vp, n));
      71    ASSERT (! MPN_OVERLAP_P (ep, 6, yp1, n));
      72    ASSERT (! MPN_OVERLAP_P (ep, 6, yp2, n));
      73    ASSERT (! MPN_OVERLAP_P (ep, 6, yp3, n));
      74    ASSERT (! MPN_OVERLAP_P (ep, 6, rp, n));
      75  
      76    yp1 += n - 1;
      77    yp2 += n - 1;
      78    yp3 += n - 1;
      79    el1 = eh1 = 0;
      80    el2 = eh2 = 0;
      81    el3 = eh3 = 0;
      82  
      83    do
      84      {
      85        yl1 = *yp1--;
      86        yl2 = *yp2--;
      87        yl3 = *yp3--;
      88        ul = *up++;
      89        vl = *vp++;
      90  
      91        /* ordinary sub_n */
      92        SUBC_LIMB (cy1, sl, ul, vl);
      93        SUBC_LIMB (cy2, rl, sl, cy);
      94        cy = cy1 | cy2;
      95        *rp++ = rl;
      96  
      97        /* update (eh1:el1) */
      98        zl1 = (-cy) & yl1;
      99        el1 += zl1;
     100        eh1 += el1 < zl1;
     101  
     102        /* update (eh2:el2) */
     103        zl2 = (-cy) & yl2;
     104        el2 += zl2;
     105        eh2 += el2 < zl2;
     106  
     107        /* update (eh3:el3) */
     108        zl3 = (-cy) & yl3;
     109        el3 += zl3;
     110        eh3 += el3 < zl3;
     111      }
     112    while (--n);
     113  
     114  #if GMP_NAIL_BITS != 0
     115    eh1 = (eh1 << GMP_NAIL_BITS) + (el1 >> GMP_NUMB_BITS);
     116    el1 &= GMP_NUMB_MASK;
     117    eh2 = (eh2 << GMP_NAIL_BITS) + (el2 >> GMP_NUMB_BITS);
     118    el2 &= GMP_NUMB_MASK;
     119    eh3 = (eh3 << GMP_NAIL_BITS) + (el3 >> GMP_NUMB_BITS);
     120    el3 &= GMP_NUMB_MASK;
     121  #endif
     122  
     123    ep[0] = el1;
     124    ep[1] = eh1;
     125    ep[2] = el2;
     126    ep[3] = eh2;
     127    ep[4] = el3;
     128    ep[5] = eh3;
     129  
     130    return cy;
     131  }