1 /* 2 ** Copyright 2003-2010, VisualOn, Inc. 3 ** 4 ** Licensed under the Apache License, Version 2.0 (the "License"); 5 ** you may not use this file except in compliance with the License. 6 ** You may obtain a copy of the License at 7 ** 8 ** http://www.apache.org/licenses/LICENSE-2.0 9 ** 10 ** Unless required by applicable law or agreed to in writing, software 11 ** distributed under the License is distributed on an "AS IS" BASIS, 12 ** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 ** See the License for the specific language governing permissions and 14 ** limitations under the License. 15 */ 16 17 /*********************************************************************** 18 * * 19 * File : log2.c * 20 * Purpose : Computes log2(L_x) * 21 * * 22 ************************************************************************/ 23 24 #include "log2.h" 25 /******************************************************************************** 26 * INCLUDE FILES 27 *********************************************************************************/ 28 #include "typedef.h" 29 #include "basic_op.h" 30 31 /********************************************************************************* 32 * LOCAL VARIABLES AND TABLES 33 **********************************************************************************/ 34 #include "log2_tab.h" /* Table for Log2() */ 35 36 /************************************************************************* 37 * 38 * FUNCTION: Log2_norm() 39 * 40 * PURPOSE: Computes log2(L_x, exp), where L_x is positive and 41 * normalized, and exp is the normalisation exponent 42 * If L_x is negative or zero, the result is 0. 43 * 44 * DESCRIPTION: 45 * The function Log2(L_x) is approximated by a table and linear 46 * interpolation. The following steps are used to compute Log2(L_x) 47 * 48 * 1- exponent = 30-norm_exponent 49 * 2- i = bit25-b31 of L_x; 32<=i<=63 (because of normalization). 50 * 3- a = bit10-b24 51 * 4- i -=32 52 * 5- fraction = table[i]<<16 - (table[i] - table[i+1]) * a * 2 53 * 54 *************************************************************************/ 55 56 void Log2_norm ( 57 Word32 L_x, /* (i) : input value (normalized) */ 58 Word16 exp, /* (i) : norm_l (L_x) */ 59 Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */ 60 Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */ 61 ) 62 { 63 Word16 i, a, tmp; 64 Word32 L_y; 65 if (L_x <= (Word32) 0) 66 { 67 *exponent = 0; 68 *fraction = 0; 69 return; 70 } 71 *exponent = (30 - exp); 72 L_x = (L_x >> 9); 73 i = extract_h (L_x); /* Extract b25-b31 */ 74 L_x = (L_x >> 1); 75 a = (Word16)(L_x); /* Extract b10-b24 of fraction */ 76 a = (Word16)(a & (Word16)0x7fff); 77 i -= 32; 78 L_y = L_deposit_h (table[i]); /* table[i] << 16 */ 79 tmp = vo_sub(table[i], table[i + 1]); /* table[i] - table[i+1] */ 80 L_y = vo_L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */ 81 *fraction = extract_h (L_y); 82 83 return; 84 } 85 86 /************************************************************************* 87 * 88 * FUNCTION: Log2() 89 * 90 * PURPOSE: Computes log2(L_x), where L_x is positive. 91 * If L_x is negative or zero, the result is 0. 92 * 93 * DESCRIPTION: 94 * normalizes L_x and then calls Log2_norm(). 95 * 96 *************************************************************************/ 97 98 void Log2 ( 99 Word32 L_x, /* (i) : input value */ 100 Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */ 101 Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */ 102 ) 103 { 104 Word16 exp; 105 106 exp = norm_l(L_x); 107 Log2_norm ((L_x << exp), exp, exponent, fraction); 108 } 109 110 111 112
