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annotate libtwamr/inv_sqrt.c @ 431:f0496507d409

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libtwamr: implement amr_dhf_subst_efr()
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 08 May 2024 00:27:51 +0000
parents 07f936338de1
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 : inv_sqrt.c
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11 * Purpose : Computes 1/sqrt(L_x), where L_x is positive.
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12 * : If L_x is negative or zero,
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13 * : the result is 1 (3fff ffff).
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14 * Description :
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15 * The function 1/sqrt(L_x) is approximated by a table and linear
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16 * interpolation. The inverse square root is computed using the
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17 * following steps:
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18 * 1- Normalization of L_x.
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19 * 2- If (30-exponent) is even then shift right once.
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20 * 3- exponent = (30-exponent)/2 +1
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21 * 4- i = bit25-b31 of L_x; 16<=i<=63 because of normalization.
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22 * 5- a = bit10-b24
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23 * 6- i -=16
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24 * 7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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25 * 8- L_y >>= exponent
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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 ********************************************************************************
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31 * MODULE INCLUDE FILE AND VERSION ID
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32 ********************************************************************************
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33 */
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34 #include "namespace.h"
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35 #include "inv_sqrt.h"
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36
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37 /*
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38 ********************************************************************************
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39 * INCLUDE FILES
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40 ********************************************************************************
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41 */
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42 #include "typedef.h"
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43 #include "basic_op.h"
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44 #include "no_count.h"
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45
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46 /*
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47 ********************************************************************************
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48 * LOCAL VARIABLES AND TABLES
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49 ********************************************************************************
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50 */
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51 #include "inv_sqrt.tab" /* Table for inv_sqrt() */
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52
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53 /*
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54 ********************************************************************************
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55 * PUBLIC PROGRAM CODE
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56 ********************************************************************************
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57 */
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58
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59 Word32 Inv_sqrt ( /* (o) : output value */
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60 Word32 L_x /* (i) : input value */
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61 )
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62 {
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63 Word16 exp, i, a, tmp;
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64 Word32 L_y;
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65
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66 test ();
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67 if (L_x <= (Word32) 0)
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68 return ((Word32) 0x3fffffffL);
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69
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70 exp = norm_l (L_x);
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71 L_x = L_shl (L_x, exp); /* L_x is normalize */
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72
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73 exp = sub (30, exp);
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74 test (); logic16 ();
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75 if ((exp & 1) == 0) /* If exponent even -> shift right */
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76 {
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77 L_x = L_shr (L_x, 1);
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78 }
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79 exp = shr (exp, 1);
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80 exp = add (exp, 1);
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81
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82 L_x = L_shr (L_x, 9);
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83 i = extract_h (L_x); /* Extract b25-b31 */
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84 L_x = L_shr (L_x, 1);
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85 a = extract_l (L_x); /* Extract b10-b24 */
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86 a = a & (Word16) 0x7fff; logic16 ();
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87
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88 i = sub (i, 16);
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89
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90 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
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91 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1]) */
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92 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
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93
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94 L_y = L_shr (L_y, exp); /* denormalization */
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95
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96 return (L_y);
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97 }