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annotate libgsmefr/inv_sqrt.c @ 585:3c6bf0d26ee7 default tip

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TW-TS-005 reader: fix maximum line length bug TW-TS-005 section 4.1 states: The maximum allowed length of each line is 80 characters, not including the OS-specific newline encoding. The implementation of this line length limit in the TW-TS-005 hex file reader function in the present suite was wrong, such that lines of the full maximum length could not be read. Fix it. Note that this bug affects comment lines too, not just actual RTP payloads. Neither Annex A nor Annex B features an RTP payload format that goes to the maximum of 40 bytes, but if a comment line goes to the maximum allowed length of 80 characters not including the terminating newline, the bug will be triggered, necessitating the present fix.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 25 Feb 2025 07:49:28 +0000
parents 2f0828ba0725
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 }