FreeCalypso > hg > gsm-codec-lib
annotate libtwamr/inv_sqrt.c @ 472:d80ccb3c3970
gsmefr-dlcap-sync: use DHF datum in the library
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Tue, 14 May 2024 07:30:48 +0000 |
parents | 07f936338de1 |
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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 } |