eas_flog.c revision 7df30109963092559d3760c0661a020f9daf1030 
1/*----------------------------------------------------------------------------
2 *
3 * File:
4 * eas_flog2.c
5 *
6 * Contents and purpose:
7 * Fixed point square root
8 *
9 *
10 * Copyright (c) 2006 Sonic Network Inc.
11
12 * Licensed under the Apache License, Version 2.0 (the "License");
13 * you may not use this file except in compliance with the License.
14 * You may obtain a copy of the License at
15 *
16 *      http://www.apache.org/licenses/LICENSE-2.0
17 *
18 * Unless required by applicable law or agreed to in writing, software
19 * distributed under the License is distributed on an "AS IS" BASIS,
20 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
21 * See the License for the specific language governing permissions and
22 * limitations under the License.
23 *
24 *----------------------------------------------------------------------------
25 * Revision Control:
26 *   $Revision$
27 *   $Date$
28 *----------------------------------------------------------------------------
29*/
30
31#include "eas_types.h"
32#include "eas_math.h"
33
34#define MANTISSA_SHIFT			27
35#define MANTISSA_MASK			0x0000000f
36#define MANTISSA_LSB_SHIFT		7
37#define MANTISSA_LSB_MASK		0x000fffff
38#define LOG_EXPONENT_SHIFT		10
39#define INTERPOLATION_SHIFT		20
40#define MAX_NEGATIVE			(-2147483647-1)
41
42/* log lookup table */
43static const EAS_U16 eas_log2_table[] =
44{
45	0, 90, 174, 254, 330, 402, 470, 536,
46	599, 659, 717, 773, 827, 879, 929, 977,
47	1024
48};
49
50/*----------------------------------------------------------------------------
51 * EAS_flog2()
52 *----------------------------------------------------------------------------
53 * Purpose:
54 * Calculates the log2 of a 32-bit fixed point value
55 *
56 * Inputs:
57 * n = value of interest
58 *
59 * Outputs:
60 * returns the log2 of n
61 *
62 *----------------------------------------------------------------------------
63*/
64EAS_I32 EAS_flog2 (EAS_U32 n)
65{
66	EAS_U32 exp;
67	EAS_U32 interp;
68
69	/* check for error condition */
70	if (n == 0)
71		return MAX_NEGATIVE;
72
73	/* find exponent */
74	for (exp = 31; exp > 0; exp--)
75	{
76		/* shift until we get a 1 bit in bit 31 */
77		if ((n & (EAS_U32) MAX_NEGATIVE) != 0)
78			break;
79		n <<= 1;
80	}
81	/*lint -e{701} use shift for performance */
82	exp <<= LOG_EXPONENT_SHIFT;
83
84	/* get the least significant bits for interpolation */
85	interp = (n >> MANTISSA_LSB_SHIFT) & MANTISSA_LSB_MASK;
86
87	/* get the most significant bits for mantissa lookup */
88	n = (n >> MANTISSA_SHIFT) & MANTISSA_MASK;
89
90	/* interpolate mantissa */
91	interp = ((eas_log2_table[n+1] - eas_log2_table[n]) * interp) >> INTERPOLATION_SHIFT;
92	exp += eas_log2_table[n] + interp;
93
94	return (EAS_I32) exp;
95}
96
97