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Annotation of OpenXM_contrib/gmp/mpn/generic/udiv_w_sdiv.c, Revision 1.1.1.1

1.1       maekawa     1: /* mpn_udiv_w_sdiv -- implement udiv_qrnnd on machines with only signed
                      2:    division.
                      3:
                      4:    Contributed by Peter L. Montgomery.
                      5:
                      6: Copyright (C) 1992, 1994, 1996 Free Software Foundation, Inc.
                      7:
                      8: This file is part of the GNU MP Library.
                      9:
                     10: The GNU MP Library is free software; you can redistribute it and/or modify
                     11: it under the terms of the GNU Library General Public License as published by
                     12: the Free Software Foundation; either version 2 of the License, or (at your
                     13: option) any later version.
                     14:
                     15: The GNU MP Library is distributed in the hope that it will be useful, but
                     16: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
                     17: or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
                     18: License for more details.
                     19:
                     20: You should have received a copy of the GNU Library General Public License
                     21: along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
                     22: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
                     23: MA 02111-1307, USA. */
                     24:
                     25: #include "gmp.h"
                     26: #include "gmp-impl.h"
                     27: #include "longlong.h"
                     28:
                     29: mp_limb_t
                     30: mpn_udiv_w_sdiv (rp, a1, a0, d)
                     31:      mp_limb_t *rp, a1, a0, d;
                     32: {
                     33:   mp_limb_t q, r;
                     34:   mp_limb_t c0, c1, b1;
                     35:
                     36:   if ((mp_limb_signed_t) d >= 0)
                     37:     {
                     38:       if (a1 < d - a1 - (a0 >> (BITS_PER_MP_LIMB - 1)))
                     39:        {
                     40:          /* dividend, divisor, and quotient are nonnegative */
                     41:          sdiv_qrnnd (q, r, a1, a0, d);
                     42:        }
                     43:       else
                     44:        {
                     45:          /* Compute c1*2^32 + c0 = a1*2^32 + a0 - 2^31*d */
                     46:          sub_ddmmss (c1, c0, a1, a0, d >> 1, d << (BITS_PER_MP_LIMB - 1));
                     47:          /* Divide (c1*2^32 + c0) by d */
                     48:          sdiv_qrnnd (q, r, c1, c0, d);
                     49:          /* Add 2^31 to quotient */
                     50:          q += (mp_limb_t) 1 << (BITS_PER_MP_LIMB - 1);
                     51:        }
                     52:     }
                     53:   else
                     54:     {
                     55:       b1 = d >> 1;                     /* d/2, between 2^30 and 2^31 - 1 */
                     56:       c1 = a1 >> 1;                    /* A/2 */
                     57:       c0 = (a1 << (BITS_PER_MP_LIMB - 1)) + (a0 >> 1);
                     58:
                     59:       if (a1 < b1)                     /* A < 2^32*b1, so A/2 < 2^31*b1 */
                     60:        {
                     61:          sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
                     62:
                     63:          r = 2*r + (a0 & 1);           /* Remainder from A/(2*b1) */
                     64:          if ((d & 1) != 0)
                     65:            {
                     66:              if (r >= q)
                     67:                r = r - q;
                     68:              else if (q - r <= d)
                     69:                {
                     70:                  r = r - q + d;
                     71:                  q--;
                     72:                }
                     73:              else
                     74:                {
                     75:                  r = r - q + 2*d;
                     76:                  q -= 2;
                     77:                }
                     78:            }
                     79:        }
                     80:       else if (c1 < b1)                        /* So 2^31 <= (A/2)/b1 < 2^32 */
                     81:        {
                     82:          c1 = (b1 - 1) - c1;
                     83:          c0 = ~c0;                     /* logical NOT */
                     84:
                     85:          sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
                     86:
                     87:          q = ~q;                       /* (A/2)/b1 */
                     88:          r = (b1 - 1) - r;
                     89:
                     90:          r = 2*r + (a0 & 1);           /* A/(2*b1) */
                     91:
                     92:          if ((d & 1) != 0)
                     93:            {
                     94:              if (r >= q)
                     95:                r = r - q;
                     96:              else if (q - r <= d)
                     97:                {
                     98:                  r = r - q + d;
                     99:                  q--;
                    100:                }
                    101:              else
                    102:                {
                    103:                  r = r - q + 2*d;
                    104:                  q -= 2;
                    105:                }
                    106:            }
                    107:        }
                    108:       else                             /* Implies c1 = b1 */
                    109:        {                               /* Hence a1 = d - 1 = 2*b1 - 1 */
                    110:          if (a0 >= -d)
                    111:            {
                    112:              q = -1;
                    113:              r = a0 + d;
                    114:            }
                    115:          else
                    116:            {
                    117:              q = -2;
                    118:              r = a0 + 2*d;
                    119:            }
                    120:        }
                    121:     }
                    122:
                    123:   *rp = r;
                    124:   return q;
                    125: }