1/*
2 * CDDL HEADER START
3 *
4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
7 *
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
12 *
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
18 *
19 * CDDL HEADER END
20 */
21
22/*
23 * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
24 */
25/*
26 * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
27 * Use is subject to license terms.
28 */
29
30/*
31 * log2l(x)
32 * RETURN THE BASE 2 LOGARITHM OF X
33 *
34 * Method:
35 *	purge off 0,INF, and NaN.
36 *	n = ilogb(x)
37 *	if (n<0) n+=1
38 *	z = scalbn(x,-n)
39 *	LOG2(x) = n + (1/ln2)*log(x)
40 */
41
42#pragma weak __log2l = log2l
43
44#include "libm.h"
45#include "longdouble.h"
46
47static const long double
48	zero 	= 0.0L,
49	half	= 0.5L,
50	one	= 1.0L,
51	invln2	= 1.442695040888963407359924681001892137427e+0000L;
52
53long double
54log2l(long double x) {
55	int n;
56
57	if (x == zero || !finitel(x))
58		return (logl(x));
59	n = ilogbl(x);
60	if (n < 0)
61		n += 1;
62	x = scalbnl(x, -n);
63	if (x == half)
64		return (n - one);
65	return (n + invln2 * logl(x));
66}
67