FreeCalypso > hg > gsm-codec-lib

annotate libgsmefr/inv_sqrt.c @ 282:9ee8ad3d4d30

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frtest: rm gsmfr-hand-test and gsmfr-max-out utils These hack programs were never properly documented and were written only as part of a debug chase, in pursuit of a bug that ultimately turned out to be in our then-hacky patch to osmo-bts-sysmo, before beginning of proper patches in Osmocom. These hack programs need to be dropped from the present sw package because they depend on old libgsm, and we are eliminating that dependency.
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 14 Apr 2024 05:44:47 +0000
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children
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1 /*************************************************************************
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2 *
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3 * FUNCTION: Inv_sqrt
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4 *
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5 * PURPOSE: Computes 1/sqrt(L_x), where L_x is positive.
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6 * If L_x is negative or zero, the result is 1 (3fff ffff).
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7 *
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8 * DESCRIPTION:
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9 * The function 1/sqrt(L_x) is approximated by a table and linear
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10 * interpolation. The inverse square root is computed using the
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11 * following steps:
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12 * 1- Normalization of L_x.
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13 * 2- If (30-exponent) is even then shift right once.
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14 * 3- exponent = (30-exponent)/2 +1
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15 * 4- i = bit25-b31 of L_x; 16<=i<=63 because of normalization.
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16 * 5- a = bit10-b24
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17 * 6- i -=16
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18 * 7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
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19 * 8- L_y >>= exponent
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20 *
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21 *************************************************************************/
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22
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23 #include "gsm_efr.h"
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24 #include "typedef.h"
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25 #include "namespace.h"
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26 #include "basic_op.h"
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27 #include "no_count.h"
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28 #include "sig_proc.h"
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30 #include "inv_sqrt.tab" /* Table for inv_sqrt() */
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32 Word32 Inv_sqrt ( /* (o) : output value */
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33 Word32 L_x /* (i) : input value */
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34 )
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35 {
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36 Word16 exp, i, a, tmp;
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37 Word32 L_y;
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39 test ();
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40 if (L_x <= (Word32) 0)
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41 return ((Word32) 0x3fffffffL);
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43 exp = norm_l (L_x);
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44 L_x = L_shl (L_x, exp); /* L_x is normalize */
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46 exp = sub (30, exp);
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47 test (); logic16 ();
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48 if ((exp & 1) == 0) /* If exponent even -> shift right */
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49 {
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50 L_x = L_shr (L_x, 1);
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51 }
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52 exp = shr (exp, 1);
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53 exp = add (exp, 1);
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55 L_x = L_shr (L_x, 9);
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56 i = extract_h (L_x); /* Extract b25-b31 */
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57 L_x = L_shr (L_x, 1);
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58 a = extract_l (L_x); /* Extract b10-b24 */
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59 a = a & (Word16) 0x7fff; logic16 ();
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61 i = sub (i, 16);
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63 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
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64 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1]) */
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65 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
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67 L_y = L_shr (L_y, exp); /* denormalization */
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69 return (L_y);
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70 }