inv_sqrt.c

来自「Trolltech公司发布的图形界面操作系统。可在qt-embedded-2.3」· C语言 代码 · 共 98 行

/*
********************************************************************************
*
*      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 "inv_sqrt.h"
const char inv_sqrt_id[] = "@(#)$Id $" inv_sqrt_h;

/*
********************************************************************************
*                         INCLUDE FILES
********************************************************************************
*/
#include "typedef.h"
#include "basic_op.h"
#include "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);
}