Dave Sparks | 56c99cd | 2009-08-24 17:35:45 -0700 | [diff] [blame] | 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. |
The Android Open Source Project | 7df3010 | 2009-03-03 19:30:38 -0800 | [diff] [blame] | 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. |
Dave Sparks | 56c99cd | 2009-08-24 17:35:45 -0700 | [diff] [blame] | 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 | */ |
| 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 | |