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