--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmefr/inv_sqrt.c	Fri Nov 25 16:18:21 2022 +0000
@@ -0,0 +1,67 @@
+/*************************************************************************
+ *
+ *  FUNCTION:   Inv_sqrt
+ *
+ *  PURPOSE:   Computes 1/sqrt(L_x),  where  L_x is positive.
+ *             If L_x is negative or zero, the result is 1 (3fff ffff).
+ *
+ *  DESCRIPTION:
+ *       The function 1/sqrt(L_x) is approximated by a table and linear
+ *       interpolation. The inverse square root is computed using the
+ *       following steps:
+ *          1- Normalization of L_x.
+ *          2- If (30-exponent) is even then shift right once.
+ *          3- exponent = (30-exponent)/2  +1
+ *          4- i = bit25-b31 of L_x;  16<=i<=63  because of normalization.
+ *          5- a = bit10-b24
+ *          6- i -=16
+ *          7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
+ *          8- L_y >>= exponent
+ *
+ *************************************************************************/
+
+#include "typedef.h"
+#include "basic_op.h"
+#include "count.h"
+
+#include "inv_sqrt.tab" /* Table for inv_sqrt() */
+
+Word32 Inv_sqrt (       /* (o) : output value   */
+    Word32 L_x          /* (i) : input value    */
+)
+{
+    Word16 exp, i, a, tmp;
+    Word32 L_y;
+
+    test (); 
+    if (L_x <= (Word32) 0)
+        return ((Word32) 0x3fffffffL);
+
+    exp = norm_l (L_x);
+    L_x = L_shl (L_x, exp);     /* L_x is normalize */
+
+    exp = sub (30, exp);
+    test (); logic16 (); 
+    if ((exp & 1) == 0)         /* If exponent even -> shift right */
+    {
+        L_x = L_shr (L_x, 1);
+    }
+    exp = shr (exp, 1);
+    exp = add (exp, 1);
+
+    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 */
+    a = a & (Word16) 0x7fff;    logic16 (); 
+
+    i = sub (i, 16);
+
+    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         */
+
+    L_y = L_shr (L_y, exp);     /* denormalization */
+
+    return (L_y);
+}