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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:
1.1.1.2 maekawa 6: THIS IS AN INTERNAL FUNCTION WITH A MUTABLE INTERFACE. IT IS ONLY SAFE
7: TO REACH THIS FUNCTION THROUGH DOCUMENTED INTERFACES. IN FACT, IT IS
8: ALMOST GUARANTEED THAT THIS FUNCTION WILL CHANGE OR DISAPPEAR IN A FUTURE
9: GNU MP RELEASE.
10:
11:
1.1.1.3 ! ohara 12: Copyright 1992, 1994, 1996, 2000 Free Software Foundation, Inc.
1.1 maekawa 13:
14: This file is part of the GNU MP Library.
15:
16: The GNU MP Library is free software; you can redistribute it and/or modify
1.1.1.2 maekawa 17: it under the terms of the GNU Lesser General Public License as published by
18: the Free Software Foundation; either version 2.1 of the License, or (at your
1.1 maekawa 19: option) any later version.
20:
21: The GNU MP Library is distributed in the hope that it will be useful, but
22: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
1.1.1.2 maekawa 23: or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
1.1 maekawa 24: License for more details.
25:
1.1.1.2 maekawa 26: You should have received a copy of the GNU Lesser General Public License
1.1 maekawa 27: along with the GNU MP Library; see the file COPYING.LIB. If not, write to
28: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
29: MA 02111-1307, USA. */
30:
31: #include "gmp.h"
32: #include "gmp-impl.h"
33: #include "longlong.h"
34:
35: mp_limb_t
36: mpn_udiv_w_sdiv (rp, a1, a0, d)
37: mp_limb_t *rp, a1, a0, d;
38: {
39: mp_limb_t q, r;
40: mp_limb_t c0, c1, b1;
1.1.1.3 ! ohara 41:
! 42: ASSERT (d != 0);
! 43: ASSERT (a1 < d);
1.1 maekawa 44:
45: if ((mp_limb_signed_t) d >= 0)
46: {
47: if (a1 < d - a1 - (a0 >> (BITS_PER_MP_LIMB - 1)))
48: {
49: /* dividend, divisor, and quotient are nonnegative */
50: sdiv_qrnnd (q, r, a1, a0, d);
51: }
52: else
53: {
54: /* Compute c1*2^32 + c0 = a1*2^32 + a0 - 2^31*d */
55: sub_ddmmss (c1, c0, a1, a0, d >> 1, d << (BITS_PER_MP_LIMB - 1));
56: /* Divide (c1*2^32 + c0) by d */
57: sdiv_qrnnd (q, r, c1, c0, d);
58: /* Add 2^31 to quotient */
59: q += (mp_limb_t) 1 << (BITS_PER_MP_LIMB - 1);
60: }
61: }
62: else
63: {
64: b1 = d >> 1; /* d/2, between 2^30 and 2^31 - 1 */
65: c1 = a1 >> 1; /* A/2 */
66: c0 = (a1 << (BITS_PER_MP_LIMB - 1)) + (a0 >> 1);
67:
68: if (a1 < b1) /* A < 2^32*b1, so A/2 < 2^31*b1 */
69: {
70: sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
71:
72: r = 2*r + (a0 & 1); /* Remainder from A/(2*b1) */
73: if ((d & 1) != 0)
74: {
75: if (r >= q)
76: r = r - q;
77: else if (q - r <= d)
78: {
79: r = r - q + d;
80: q--;
81: }
82: else
83: {
84: r = r - q + 2*d;
85: q -= 2;
86: }
87: }
88: }
89: else if (c1 < b1) /* So 2^31 <= (A/2)/b1 < 2^32 */
90: {
91: c1 = (b1 - 1) - c1;
92: c0 = ~c0; /* logical NOT */
93:
94: sdiv_qrnnd (q, r, c1, c0, b1); /* (A/2) / (d/2) */
95:
96: q = ~q; /* (A/2)/b1 */
97: r = (b1 - 1) - r;
98:
99: r = 2*r + (a0 & 1); /* A/(2*b1) */
100:
101: if ((d & 1) != 0)
102: {
103: if (r >= q)
104: r = r - q;
105: else if (q - r <= d)
106: {
107: r = r - q + d;
108: q--;
109: }
110: else
111: {
112: r = r - q + 2*d;
113: q -= 2;
114: }
115: }
116: }
117: else /* Implies c1 = b1 */
118: { /* Hence a1 = d - 1 = 2*b1 - 1 */
119: if (a0 >= -d)
120: {
121: q = -1;
122: r = a0 + d;
123: }
124: else
125: {
126: q = -2;
127: r = a0 + 2*d;
128: }
129: }
130: }
131:
132: *rp = r;
133: return q;
134: }