--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmefr/log2.c	Fri Nov 25 16:18:21 2022 +0000
@@ -0,0 +1,63 @@
+/*************************************************************************
+ *
+ *   FUNCTION:   Log2()
+ *
+ *   PURPOSE:   Computes log2(L_x),  where   L_x is positive.
+ *              If L_x is negative or zero, the result is 0.
+ *
+ *   DESCRIPTION:
+ *        The function Log2(L_x) is approximated by a table and linear
+ *        interpolation. The following steps are used to compute Log2(L_x)
+ *
+ *           1- Normalization of L_x.
+ *           2- exponent = 30-exponent
+ *           3- i = bit25-b31 of L_x;  32<=i<=63  (because of normalization).
+ *           4- a = bit10-b24
+ *           5- i -=32
+ *           6- fraction = table[i]<<16 - (table[i] - table[i+1]) * a * 2
+ *
+ *************************************************************************/
+
+#include "typedef.h"
+#include "basic_op.h"
+#include "count.h"
+
+#include "log2.tab"     /* Table for Log2() */
+
+void Log2 (
+    Word32 L_x,         /* (i) : input value                                 */
+    Word16 *exponent,   /* (o) : Integer part of Log2.   (range: 0<=val<=30) */
+    Word16 *fraction    /* (o) : Fractional part of Log2. (range: 0<=val<1) */
+)
+{
+    Word16 exp, i, a, tmp;
+    Word32 L_y;
+
+    test (); 
+    if (L_x <= (Word32) 0)
+    {
+        *exponent = 0;          move16 (); 
+        *fraction = 0;          move16 (); 
+        return;
+    }
+    exp = norm_l (L_x);
+    L_x = L_shl (L_x, exp);     /* L_x is normalized */
+
+    *exponent = sub (30, exp);  move16 (); 
+
+    L_x = L_shr (L_x, 9);
+    i = extract_h (L_x);        /* Extract b25-b31 */
+    L_x = L_shr (L_x, 1);
+    a = extract_l (L_x);        /* Extract b10-b24 of fraction */
+    a = a & (Word16) 0x7fff;    logic16 (); 
+
+    i = sub (i, 32);
+
+    L_y = L_deposit_h (table[i]);       /* table[i] << 16        */
+    tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1] */
+    L_y = L_msu (L_y, tmp, a);  /* L_y -= tmp*a*2        */
+
+    *fraction = extract_h (L_y);move16 (); 
+
+    return;
+}