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annotate libtwamr/log2.c @ 480:332397bc80aa

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doc/AMR-library-API: document public const data items
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 23:18:17 +0000
parents bfe74a9edd5a
children
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1 /*
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2 ********************************************************************************
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3 *
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4 * GSM AMR-NB speech codec R98 Version 7.6.0 December 12, 2001
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5 * R99 Version 3.3.0
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6 * REL-4 Version 4.1.0
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7 *
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8 ********************************************************************************
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9 *
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10 * File : log2.c
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11 * Purpose : Computes log2(L_x)
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12 *
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13 ********************************************************************************
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14 */
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15 /*
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16 ********************************************************************************
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17 * MODULE INCLUDE FILE AND VERSION ID
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18 ********************************************************************************
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19 */
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20 #include "namespace.h"
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21 #include "log2.h"
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22
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23 /*
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24 ********************************************************************************
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25 * INCLUDE FILES
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26 ********************************************************************************
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27 */
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28 #include "typedef.h"
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29 #include "basic_op.h"
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30 #include "no_count.h"
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31
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32 /*
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33 ********************************************************************************
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34 * LOCAL VARIABLES AND TABLES
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35 ********************************************************************************
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36 */
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37 #include "log2.tab" /* Table for Log2() */
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38
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39 /*
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40 ********************************************************************************
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41 * PUBLIC PROGRAM CODE
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42 ********************************************************************************
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43 */
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44
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45 /*************************************************************************
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46 *
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47 * FUNCTION: Log2_norm()
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48 *
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49 * PURPOSE: Computes log2(L_x, exp), where L_x is positive and
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50 * normalized, and exp is the normalisation exponent
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51 * If L_x is negative or zero, the result is 0.
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52 *
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53 * DESCRIPTION:
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54 * The function Log2(L_x) is approximated by a table and linear
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55 * interpolation. The following steps are used to compute Log2(L_x)
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56 *
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57 * 1- exponent = 30-norm_exponent
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58 * 2- i = bit25-b31 of L_x; 32<=i<=63 (because of normalization).
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59 * 3- a = bit10-b24
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60 * 4- i -=32
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61 * 5- fraction = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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62 *
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63 *************************************************************************/
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64 void Log2_norm (
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65 Word32 L_x, /* (i) : input value (normalized) */
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66 Word16 exp, /* (i) : norm_l (L_x) */
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67 Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */
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68 Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */
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69 )
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70 {
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71 Word16 i, a, tmp;
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72 Word32 L_y;
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73
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74 test ();
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75 if (L_x <= (Word32) 0)
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76 {
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77 *exponent = 0; move16 ();
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78 *fraction = 0; move16 ();
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79 return;
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80 }
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81
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82 *exponent = sub (30, exp); move16 ();
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83
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84 L_x = L_shr (L_x, 9);
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85 i = extract_h (L_x); /* Extract b25-b31 */
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86 L_x = L_shr (L_x, 1);
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87 a = extract_l (L_x); /* Extract b10-b24 of fraction */
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88 a = a & (Word16) 0x7fff; logic16 ();
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89
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90 i = sub (i, 32);
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91
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92 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
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93 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */
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94 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
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95
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96 *fraction = extract_h (L_y);move16 ();
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97
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98 return;
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99 }
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100
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101 /*************************************************************************
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102 *
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103 * FUNCTION: Log2()
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104 *
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105 * PURPOSE: Computes log2(L_x), where L_x is positive.
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106 * If L_x is negative or zero, the result is 0.
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107 *
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108 * DESCRIPTION:
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109 * normalizes L_x and then calls Log2_norm().
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110 *
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111 *************************************************************************/
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112 void Log2 (
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113 Word32 L_x, /* (i) : input value */
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114 Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */
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115 Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */
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116 )
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117 {
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118 Word16 exp;
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119
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120 exp = norm_l (L_x);
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121 Log2_norm (L_shl (L_x, exp), exp, exponent, fraction);
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122 }