1/*
2 * Copyright (c) 1994, 1995 Carnegie-Mellon University.
3 * All rights reserved.
4 *
5 * Author: Chris G. Demetriou
6 *
7 * Permission to use, copy, modify and distribute this software and
8 * its documentation is hereby granted, provided that both the copyright
9 * notice and this permission notice appear in all copies of the
10 * software, derivative works or modified versions, and any portions
11 * thereof, and that both notices appear in supporting documentation.
12 *
13 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
14 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
15 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
16 *
17 * Carnegie Mellon requests users of this software to return to
18 *
19 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
20 *  School of Computer Science
21 *  Carnegie Mellon University
22 *  Pittsburgh PA 15213-3890
23 *
24 * any improvements or extensions that they make and grant Carnegie the
25 * rights to redistribute these changes.
26 *
27 *	$NetBSD: modf.c,v 1.1 1995/02/10 17:50:25 cgd Exp $
28 */
29
30#include <sys/cdefs.h>
31__FBSDID("$FreeBSD$");
32
33#include <sys/types.h>
34#include <machine/ieee.h>
35#include <errno.h>
36#include <math.h>
37
38/*
39 * double modf(double val, double *iptr)
40 * returns: f and i such that |f| < 1.0, (f + i) = val, and
41 *	sign(f) == sign(i) == sign(val).
42 *
43 * Beware signedness when doing subtraction, and also operand size!
44 */
45double
46modf(val, iptr)
47	double val, *iptr;
48{
49	union doub {
50		double v;
51		struct ieee_double s;
52	} u, v;
53	u_int64_t frac;
54
55	/*
56	 * If input is Inf or NaN, return it and leave i alone.
57	 */
58	u.v = val;
59	if (u.s.dbl_exp == DBL_EXP_INFNAN)
60		return (u.v);
61
62	/*
63	 * If input can't have a fractional part, return
64	 * (appropriately signed) zero, and make i be the input.
65	 */
66	if ((int)u.s.dbl_exp - DBL_EXP_BIAS > DBL_FRACBITS - 1) {
67		*iptr = u.v;
68		v.v = 0.0;
69		v.s.dbl_sign = u.s.dbl_sign;
70		return (v.v);
71	}
72
73	/*
74	 * If |input| < 1.0, return it, and set i to the appropriately
75	 * signed zero.
76	 */
77	if (u.s.dbl_exp < DBL_EXP_BIAS) {
78		v.v = 0.0;
79		v.s.dbl_sign = u.s.dbl_sign;
80		*iptr = v.v;
81		return (u.v);
82	}
83
84	/*
85	 * There can be a fractional part of the input.
86	 * If you look at the math involved for a few seconds, it's
87	 * plain to see that the integral part is the input, with the
88	 * low (DBL_FRACBITS - (exponent - DBL_EXP_BIAS)) bits zeroed,
89	 * the fractional part is the part with the rest of the
90	 * bits zeroed.  Just zeroing the high bits to get the
91	 * fractional part would yield a fraction in need of
92	 * normalization.  Therefore, we take the easy way out, and
93	 * just use subtraction to get the fractional part.
94	 */
95	v.v = u.v;
96	/* Zero the low bits of the fraction, the sleazy way. */
97	frac = ((u_int64_t)v.s.dbl_frach << 32) + v.s.dbl_fracl;
98	frac >>= DBL_FRACBITS - (u.s.dbl_exp - DBL_EXP_BIAS);
99	frac <<= DBL_FRACBITS - (u.s.dbl_exp - DBL_EXP_BIAS);
100	v.s.dbl_fracl = frac & 0xffffffff;
101	v.s.dbl_frach = frac >> 32;
102	*iptr = v.v;
103
104	u.v -= v.v;
105	u.s.dbl_sign = v.s.dbl_sign;
106	return (u.v);
107}
108