1/* $NetBSD: compat_modf_ieee754.c,v 1.6 2024/01/20 14:52:45 christos Exp $ */
2
3/*
4 * Copyright (c) 1994, 1995 Carnegie-Mellon University.
5 * All rights reserved.
6 *
7 * Author: Chris G. Demetriou
8 *
9 * Permission to use, copy, modify and distribute this software and
10 * its documentation is hereby granted, provided that both the copyright
11 * notice and this permission notice appear in all copies of the
12 * software, derivative works or modified versions, and any portions
13 * thereof, and that both notices appear in supporting documentation.
14 *
15 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
16 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
17 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
18 *
19 * Carnegie Mellon requests users of this software to return to
20 *
21 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
22 *  School of Computer Science
23 *  Carnegie Mellon University
24 *  Pittsburgh PA 15213-3890
25 *
26 * any improvements or extensions that they make and grant Carnegie the
27 * rights to redistribute these changes.
28 */
29
30#include <sys/types.h>
31#include <machine/ieee.h>
32#include <errno.h>
33#include <math.h>
34
35/*
36 * double modf(double val, double *iptr)
37 * returns: f and i such that |f| < 1.0, (f + i) = val, and
38 *	sign(f) == sign(i) == sign(val).
39 *
40 * Beware signedness when doing subtraction, and also operand size!
41 */
42double
43modf(double val, double *iptr)
44{
45	union ieee_double_u u, v;
46	u_int64_t frac;
47
48	/*
49	 * If input is +/-Inf or NaN, return +/-0 or NaN.
50	 */
51	u.dblu_d = val;
52	if (u.dblu_dbl.dbl_exp == DBL_EXP_INFNAN) {
53		*iptr = u.dblu_d;
54		return (0.0 / u.dblu_d);
55	}
56
57	/*
58	 * If input can't have a fractional part, return
59	 * (appropriately signed) zero, and make i be the input.
60	 */
61	if ((int)u.dblu_dbl.dbl_exp - DBL_EXP_BIAS > DBL_FRACBITS - 1) {
62		*iptr = u.dblu_d;
63		v.dblu_d = 0.0;
64		v.dblu_dbl.dbl_sign = u.dblu_dbl.dbl_sign;
65		return (v.dblu_d);
66	}
67
68	/*
69	 * If |input| < 1.0, return it, and set i to the appropriately
70	 * signed zero.
71	 */
72	if (u.dblu_dbl.dbl_exp < DBL_EXP_BIAS) {
73		v.dblu_d = 0.0;
74		v.dblu_dbl.dbl_sign = u.dblu_dbl.dbl_sign;
75		*iptr = v.dblu_d;
76		return (u.dblu_d);
77	}
78
79	/*
80	 * There can be a fractional part of the input.
81	 * If you look at the math involved for a few seconds, it's
82	 * plain to see that the integral part is the input, with the
83	 * low (DBL_FRACBITS - (exponent - DBL_EXP_BIAS)) bits zeroed,
84	 * the fractional part is the part with the rest of the
85	 * bits zeroed.  Just zeroing the high bits to get the
86	 * fractional part would yield a fraction in need of
87	 * normalization.  Therefore, we take the easy way out, and
88	 * just use subtraction to get the fractional part.
89	 */
90	v.dblu_d = u.dblu_d;
91	/* Zero the low bits of the fraction, the sleazy way. */
92	frac = ((u_int64_t)v.dblu_dbl.dbl_frach << 32) + v.dblu_dbl.dbl_fracl;
93	frac >>= DBL_FRACBITS - (u.dblu_dbl.dbl_exp - DBL_EXP_BIAS);
94	frac <<= DBL_FRACBITS - (u.dblu_dbl.dbl_exp - DBL_EXP_BIAS);
95	v.dblu_dbl.dbl_fracl = (unsigned int)(frac & 0xffffffffULL);
96	v.dblu_dbl.dbl_frach = (unsigned int)(frac >> 32);
97	*iptr = v.dblu_d;
98
99	u.dblu_d -= v.dblu_d;
100	u.dblu_dbl.dbl_sign = v.dblu_dbl.dbl_sign;
101	return (u.dblu_d);
102}
103