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

   1 /* mpn_redc_n.  Set rp[] <- up[]/R^n mod mp[].  Clobber up[].
   2    mp[] is n limbs; up[] is 2n limbs, the inverse ip[] is n limbs.
   3
   4    THIS IS AN INTERNAL FUNCTION WITH A MUTABLE INTERFACE.  IT IS ONLY
   5    SAFE TO REACH THIS FUNCTION THROUGH DOCUMENTED INTERFACES.
   6
   7 Copyright 2009, 2012 Free Software Foundation, Inc.
   8
   9 This file is part of the GNU MP Library.
  10
  11 The GNU MP Library is free software; you can redistribute it and/or modify
  12 it under the terms of either:
  13
  14   * the GNU Lesser General Public License as published by the Free
  15     Software Foundation; either version 3 of the License, or (at your
  16     option) any later version.
  17
  18 or
  19
  20   * the GNU General Public License as published by the Free Software
  21     Foundation; either version 2 of the License, or (at your option) any
  22     later version.
  23
  24 or both in parallel, as here.
  25
  26 The GNU MP Library is distributed in the hope that it will be useful, but
  27 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  28 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  29 for more details.
  30
  31 You should have received copies of the GNU General Public License and the
  32 GNU Lesser General Public License along with the GNU MP Library.  If not,
  33 see https://www.gnu.org/licenses/.  */
  34
  35 #include "gmp.h"
  36 #include "gmp-impl.h"
  37
  38 /*
  39   TODO
  40
  41   * We assume mpn_mulmod_bnm1 is always faster than plain mpn_mul_n (or a
  42     future mpn_mulhi) for the range we will be called.  Follow up that
  43     assumption.
  44
  45   * Decrease scratch usage.
  46
  47   * Consider removing the residue canonicalisation.
  48 */
  49
  50 void
  51 mpn_redc_n (mp_ptr rp, mp_ptr up, mp_srcptr mp, mp_size_t n, mp_srcptr ip)
  52 {
  53   mp_ptr xp, yp, scratch;
  54   mp_limb_t cy;
  55   mp_size_t rn;
  56   TMP_DECL;
  57   TMP_MARK;
  58
  59   ASSERT (n > 8);
  60
  61   rn = mpn_mulmod_bnm1_next_size (n);
  62
  63   scratch = TMP_ALLOC_LIMBS (n + rn + mpn_mulmod_bnm1_itch (rn, n, n));
  64
  65   xp = scratch;
  66   mpn_mullo_n (xp, up, ip, n);
  67
  68   yp = scratch + n;
  69   mpn_mulmod_bnm1 (yp, rn, xp, n, mp, n, scratch + n + rn);
  70
  71   ASSERT_ALWAYS (2 * n > rn);                           /* could handle this */
  72
  73   cy = mpn_sub_n (yp + rn, yp, up, 2*n - rn);           /* undo wrap around */
  74   MPN_DECR_U (yp + 2*n - rn, rn, cy);
  75
  76   cy = mpn_sub_n (rp, up + n, yp + n, n);
  77   if (cy != 0)
  78     mpn_add_n (rp, rp, mp, n);
  79
  80   TMP_FREE;
  81 }