+
−/*
+
−********************************************************************************
+
−*
+
−*      GSM AMR-NB speech codec   R98   Version 7.6.0   December 12, 2001
+
−*                                R99   Version 3.3.0                
+
−*                                REL-4 Version 4.1.0                
+
−*
+
−********************************************************************************
+
−*
+
−*      File             : inv_sqrt.c
+
−*      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
+
−*
+
−********************************************************************************
+
−*/
+
−/*
+
−********************************************************************************
+
−*                         MODULE INCLUDE FILE AND VERSION ID
+
−********************************************************************************
+
−*/
+
−#include "namespace.h"
+
−#include "inv_sqrt.h"
+
−
+
−/*
+
−********************************************************************************
+
−*                         INCLUDE FILES
+
−********************************************************************************
+
−*/
+
−#include "typedef.h"
+
−#include "basic_op.h"
+
−#include "no_count.h"
+
−
+
−/*
+
−********************************************************************************
+
−*                         LOCAL VARIABLES AND TABLES
+
−********************************************************************************
+
−*/
+
−#include "inv_sqrt.tab" /* Table for inv_sqrt() */
+
−
+
−/*
+
−********************************************************************************
+
−*                         PUBLIC PROGRAM CODE
+
−********************************************************************************
+
−*/
+
−
+
−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);
+
−}