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annotate libtwamr/sqrt_l.c @ 478:936a08cc73ce

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doc/AMR-library-API: describe the decoder
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 19 May 2024 21:32:31 +0000
parents 8dfb7cbe6b59
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 : sqrt_l.c
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11 * Purpose : Computes sqrt(L_x), where L_x is positive.
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12 * : If L_var is negative or zero, the result is 0
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13 * Description :
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14 * The function sqrt(L_x) is approximated by a table and linear
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15 * interpolation. The square root is computed using the
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16 * following steps:
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17 * 1- Normalization of L_x.
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18 * 2- If exponent is even then shift right once.
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19 * 3- exponent = exponent/2
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20 * 4- i = bit25-b31 of L_x; 16<=i<=63 because of normalization.
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21 * 5- a = bit10-b24
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22 * 6- i -=16
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23 * 7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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24 * 8- return L_y and exponent so caller can do denormalization
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25 *
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26 ********************************************************************************
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27 */
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28 /*
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29 ********************************************************************************
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30 * MODULE INCLUDE FILE AND VERSION ID
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31 ********************************************************************************
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32 */
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33 #include "namespace.h"
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34 #include "sqrt_l.h"
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35
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36 /*
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37 ********************************************************************************
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38 * INCLUDE FILES
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39 ********************************************************************************
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40 */
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41 #include "typedef.h"
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42 #include "basic_op.h"
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43 #include "no_count.h"
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44
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45 /*
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46 ********************************************************************************
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47 * LOCAL VARIABLES AND TABLES
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48 ********************************************************************************
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49 */
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50 #include "sqrt_l.tab" /* Table for sqrt_l_exp() */
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51
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52 /*
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53 ********************************************************************************
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54 * PUBLIC PROGRAM CODE
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55 ********************************************************************************
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56 */
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57
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58 Word32 sqrt_l_exp (/* o : output value, Q31 */
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59 Word32 L_x, /* i : input value, Q31 */
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60 Word16 *exp /* o : right shift to be applied to result, Q1 */
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61 )
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62 {
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63 /*
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64 y = sqrt(x)
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65
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66 x = f * 2^-e, 0.5 <= f < 1 (normalization)
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67
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68 y = sqrt(f) * 2^(-e/2)
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69
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70 a) e = 2k --> y = sqrt(f) * 2^-k (k = e div 2,
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71 0.707 <= sqrt(f) < 1)
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72 b) e = 2k+1 --> y = sqrt(f/2) * 2^-k (k = e div 2,
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73 0.5 <= sqrt(f/2) < 0.707)
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74 */
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75
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76
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77 Word16 e, i, a, tmp;
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78 Word32 L_y;
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79
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80 test ();
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81 if (L_x <= (Word32) 0)
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82 {
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83 *exp = 0; move16 ();
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84 return (Word32) 0;
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85 }
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86
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87 e = norm_l (L_x) & 0xFFFE; logic16 (); /* get next lower EVEN norm. exp */
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88 L_x = L_shl (L_x, e); /* L_x is normalized to [0.25..1) */
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89 *exp = e; move16 (); /* return 2*exponent (or Q1) */
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90
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91 L_x = L_shr (L_x, 9);
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92 i = extract_h (L_x); /* Extract b25-b31, 16 <= i <= 63 because
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93 of normalization */
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94 L_x = L_shr (L_x, 1);
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95 a = extract_l (L_x); /* Extract b10-b24 */
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96 a = a & (Word16) 0x7fff; logic16 ();
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97
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98 i = sub (i, 16); /* 0 <= i <= 47 */
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99
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100 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
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101 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1]) */
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102 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
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103
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104 /* L_y = L_shr (L_y, *exp); */ /* denormalization done by caller */
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105
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106 return (L_y);
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107 }