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 */
43 static 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 */
EAS_flog2(EAS_U32 n)64 EAS_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