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comparison libgsmefr/log2.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
parents
children 68f772469ba7
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52:988fd7ff514f 53:49dd1ac8e75b
1 /*************************************************************************
2 *
3 * FUNCTION: Log2()
4 *
5 * PURPOSE: Computes log2(L_x), where L_x is positive.
6 * If L_x is negative or zero, the result is 0.
7 *
8 * DESCRIPTION:
9 * The function Log2(L_x) is approximated by a table and linear
10 * interpolation. The following steps are used to compute Log2(L_x)
11 *
12 * 1- Normalization of L_x.
13 * 2- exponent = 30-exponent
14 * 3- i = bit25-b31 of L_x; 32<=i<=63 (because of normalization).
15 * 4- a = bit10-b24
16 * 5- i -=32
17 * 6- fraction = table[i]<<16 - (table[i] - table[i+1]) * a * 2
18 *
19 *************************************************************************/
20
21 #include "typedef.h"
22 #include "basic_op.h"
23 #include "count.h"
24
25 #include "log2.tab" /* Table for Log2() */
26
27 void Log2 (
28 Word32 L_x, /* (i) : input value */
29 Word16 *exponent, /* (o) : Integer part of Log2. (range: 0<=val<=30) */
30 Word16 *fraction /* (o) : Fractional part of Log2. (range: 0<=val<1) */
31 )
32 {
33 Word16 exp, i, a, tmp;
34 Word32 L_y;
35
36 test ();
37 if (L_x <= (Word32) 0)
38 {
39 *exponent = 0; move16 ();
40 *fraction = 0; move16 ();
41 return;
42 }
43 exp = norm_l (L_x);
44 L_x = L_shl (L_x, exp); /* L_x is normalized */
45
46 *exponent = sub (30, exp); move16 ();
47
48 L_x = L_shr (L_x, 9);
49 i = extract_h (L_x); /* Extract b25-b31 */
50 L_x = L_shr (L_x, 1);
51 a = extract_l (L_x); /* Extract b10-b24 of fraction */
52 a = a & (Word16) 0x7fff; logic16 ();
53
54 i = sub (i, 32);
55
56 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
57 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */
58 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
59
60 *fraction = extract_h (L_y);move16 ();
61
62 return;
63 }