Annotation of OpenXM_contrib/gmp/mpf/set_q.c, Revision 1.1.1.1
1.1 maekawa 1: /* mpf_set_q (mpf_t rop, mpq_t op) -- Convert the rational op to the float rop.
2:
3: Copyright (C) 1996 Free Software Foundation, Inc.
4:
5: This file is part of the GNU MP Library.
6:
7: The GNU MP Library is free software; you can redistribute it and/or modify
8: it under the terms of the GNU Library General Public License as published by
9: the Free Software Foundation; either version 2 of the License, or (at your
10: option) any later version.
11:
12: The GNU MP Library is distributed in the hope that it will be useful, but
13: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14: or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
15: License for more details.
16:
17: You should have received a copy of the GNU Library General Public License
18: along with the GNU MP Library; see the file COPYING.LIB. If not, write to
19: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
20: MA 02111-1307, USA. */
21:
22: #include "gmp.h"
23: #include "gmp-impl.h"
24: #include "longlong.h"
25:
26: /* Algorithm:
27: 1. Develop >= n bits of src.num / src.den, where n is the number of bits
28: in a double. This (partial) division will use all bits from the
29: denominator.
30: 2. Use the remainder to determine how to round the result.
31: 3. Assign the integral result to a temporary double.
32: 4. Scale the temporary double, and return the result.
33:
34: An alternative algorithm, that would be faster:
35: 0. Let n be somewhat larger than the number of significant bits in a double.
36: 1. Extract the most significant n bits of the denominator, and an equal
37: number of bits from the numerator.
38: 2. Interpret the extracted numbers as integers, call them a and b
39: respectively, and develop n bits of the fractions ((a + 1) / b) and
40: (a / (b + 1)) using mpn_divrem.
41: 3. If the computed values are identical UP TO THE POSITION WE CARE ABOUT,
42: we are done. If they are different, repeat the algorithm from step 1,
43: but first let n = n * 2.
44: 4. If we end up using all bits from the numerator and denominator, fall
45: back to the first algorithm above.
46: 5. Just to make life harder, The computation of a + 1 and b + 1 above
47: might give carry-out... Needs special handling. It might work to
48: subtract 1 in both cases instead.
49: */
50:
51: void
52: #if __STDC__
53: mpf_set_q (mpf_t r, mpq_srcptr q)
54: #else
55: mpf_set_q (r, q)
56: mpf_t r;
57: mpq_srcptr q;
58: #endif
59: {
60: mp_ptr np, dp;
61: mp_ptr rp;
62: mp_size_t nsize, dsize;
63: mp_size_t qsize, rsize;
64: mp_size_t sign_quotient;
65: unsigned normalization_steps;
66: mp_limb_t qlimb;
67: mp_ptr qp;
68: mp_size_t prec;
69: mp_exp_t exp;
70: TMP_DECL (marker);
71:
72: nsize = SIZ (&q->_mp_num);
73: dsize = SIZ (&q->_mp_den);
74:
75: if (nsize == 0)
76: {
77: SIZ (r) = 0;
78: EXP (r) = 0;
79: return;
80: }
81:
82: prec = PREC (r) + 1;
83:
84: TMP_MARK (marker);
85:
86: qp = PTR (r);
87:
88: sign_quotient = nsize ^ dsize;
89: nsize = ABS (nsize);
90: dsize = ABS (dsize);
91: np = PTR (&q->_mp_num);
92: dp = PTR (&q->_mp_den);
93:
94: exp = nsize - dsize;
95:
96: if (nsize > prec)
97: {
98: np += nsize - prec;
99: nsize = prec;
100: }
101: if (dsize > prec)
102: {
103: dp += dsize - prec;
104: dsize = prec;
105: }
106:
107: rsize = MAX (nsize, dsize);
108: rp = (mp_ptr) TMP_ALLOC ((rsize + 1) * BYTES_PER_MP_LIMB);
109:
110: count_leading_zeros (normalization_steps, dp[dsize - 1]);
111:
112: /* Normalize the denominator, i.e. make its most significant bit set by
113: shifting it NORMALIZATION_STEPS bits to the left. Also shift the
114: numerator the same number of steps (to keep the quotient the same!). */
115: if (normalization_steps != 0)
116: {
117: mp_ptr tp;
118: mp_limb_t nlimb;
119:
120: /* Shift up the denominator setting the most significant bit of
121: the most significant limb. Use temporary storage not to clobber
122: the original contents of the denominator. */
123: tp = (mp_ptr) TMP_ALLOC (dsize * BYTES_PER_MP_LIMB);
124: mpn_lshift (tp, dp, dsize, normalization_steps);
125: dp = tp;
126:
127: if (rsize != nsize)
128: {
129: MPN_ZERO (rp, rsize - nsize);
130: nlimb = mpn_lshift (rp + (rsize - nsize),
131: np, nsize, normalization_steps);
132: }
133: else
134: {
135: nlimb = mpn_lshift (rp, np, nsize, normalization_steps);
136: }
137: if (nlimb != 0)
138: {
139: rp[rsize] = nlimb;
140: rsize++;
141: exp++;
142: }
143: }
144: else
145: {
146: if (rsize != nsize)
147: {
148: MPN_ZERO (rp, rsize - nsize);
149: MPN_COPY (rp + (rsize - nsize), np, nsize);
150: }
151: else
152: {
153: MPN_COPY (rp, np, rsize);
154: }
155: }
156:
157: qlimb = mpn_divrem (qp, prec - 1 - (rsize - dsize), rp, rsize, dp, dsize);
158: qsize = prec - 1;
159: if (qlimb)
160: {
161: qp[qsize] = qlimb;
162: qsize++;
163: exp++;
164: }
165:
166: EXP (r) = exp;
167: SIZ (r) = qsize;
168:
169: TMP_FREE (marker);
170: }
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