1/*	$OpenBSD: modf.c,v 1.7 2023/08/13 06:55:37 miod Exp $	*/
2/*	$NetBSD: modf.c,v 1.1 1995/02/10 17:50:25 cgd Exp $	*/
3
4/*
5 * Copyright (c) 1994, 1995 Carnegie-Mellon University.
6 * All rights reserved.
7 *
8 * Author: Chris G. Demetriou
9 *
10 * Permission to use, copy, modify and distribute this software and
11 * its documentation is hereby granted, provided that both the copyright
12 * notice and this permission notice appear in all copies of the
13 * software, derivative works or modified versions, and any portions
14 * thereof, and that both notices appear in supporting documentation.
15 *
16 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
17 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
18 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
19 *
20 * Carnegie Mellon requests users of this software to return to
21 *
22 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
23 *  School of Computer Science
24 *  Carnegie Mellon University
25 *  Pittsburgh PA 15213-3890
26 *
27 * any improvements or extensions that they make and grant Carnegie the
28 * rights to redistribute these changes.
29 */
30
31#include <sys/types.h>
32#include <machine/ieee.h>
33#include <errno.h>
34#include <float.h>
35#include <math.h>
36
37/*
38 * double modf(double val, double *iptr)
39 * returns: f and i such that |f| < 1.0, (f + i) = val, and
40 *	sign(f) == sign(i) == sign(val).
41 *
42 * Beware signedness when doing subtraction, and also operand size!
43 */
44double
45modf(double val, double *iptr)
46{
47	union doub {
48		double v;
49		struct ieee_double s;
50	} u, v;
51	u_int64_t frac;
52
53	/*
54	 * If input is +/-Inf or NaN, return +/-0 or NaN.
55	 */
56	u.v = val;
57	if (u.s.dbl_exp == DBL_EXP_INFNAN) {
58		*iptr = u.v;
59		return (0.0 / u.v);
60	}
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
109#if	LDBL_MANT_DIG == DBL_MANT_DIG
110__strong_alias(modfl, modf);
111#endif	/* LDBL_MANT_DIG == DBL_MANT_DIG */
112