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comparison libgsmefr/pow2.c @ 53:49dd1ac8e75b

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libgsmefr: import most *.c files from ETSI source
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 25 Nov 2022 16:18:21 +0000
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children 33714b36841a
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52:988fd7ff514f 53:49dd1ac8e75b
1 /*************************************************************************
2 *
3 * FUNCTION: Pow2()
4 *
5 * PURPOSE: computes L_x = pow(2.0, exponent.fraction)
6 *
7 * DESCRIPTION:
8 * The function Pow2(L_x) is approximated by a table and linear
9 * interpolation.
10 * 1- i = bit10-b15 of fraction, 0 <= i <= 31
11 * 2- a = bit0-b9 of fraction
12 * 3- L_x = table[i]<<16 - (table[i] - table[i+1]) * a * 2
13 * 4- L_x = L_x >> (30-exponent) (with rounding)
14 *
15 *************************************************************************/
16
17 #include "typedef.h"
18 #include "basic_op.h"
19 #include "count.h"
20
21 #include "pow2.tab" /* Table for Pow2() */
22
23 Word32 Pow2 ( /* (o) : result (range: 0<=val<=0x7fffffff) */
24 Word16 exponent, /* (i) : Integer part. (range: 0<=val<=30) */
25 Word16 fraction /* (i) : Fractional part. (range: 0.0<=val<1.0) */
26 )
27 {
28 Word16 exp, i, a, tmp;
29 Word32 L_x;
30
31 L_x = L_mult (fraction, 32);/* L_x = fraction<<6 */
32 i = extract_h (L_x); /* Extract b10-b16 of fraction */
33 L_x = L_shr (L_x, 1);
34 a = extract_l (L_x); /* Extract b0-b9 of fraction */
35 a = a & (Word16) 0x7fff; logic16 ();
36
37 L_x = L_deposit_h (table[i]); /* table[i] << 16 */
38 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */
39 L_x = L_msu (L_x, tmp, a); /* L_x -= tmp*a*2 */
40
41 exp = sub (30, exponent);
42 L_x = L_shr_r (L_x, exp);
43
44 return (L_x);
45 }