RTL271_1/amrnb/sqrt_l.c

/*
********************************************************************************
*
*      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             : sqrt_l.c
*      Purpose          : Computes sqrt(L_x),  where  L_x is positive.
*                       : If L_var is negative or zero, the result is 0
*      Description      :
*            The function sqrt(L_x) is approximated by a table and linear
*            interpolation. The square root is computed using the
*            following steps:
*                1- Normalization of L_x.
*                2- If exponent is even then shift right once.
*                3- exponent = exponent/2
*                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- return L_y and exponent so caller can do denormalization
*
********************************************************************************
*/
/*
********************************************************************************
*                         MODULE INCLUDE FILE AND VERSION ID
********************************************************************************
*/
#include "sqrt_l.h"
const char sqrt_l_id[] = "@(#)$Id $" sqrt_l_h;

/*
********************************************************************************
*                         INCLUDE FILES
********************************************************************************
*/
#include "typedef.h"
#include "basic_op.h"
#include "count.h"

/*
********************************************************************************
*                         LOCAL VARIABLES AND TABLES
********************************************************************************
*/
#include "sqrt_l.tab" /* Table for sqrt_l_exp() */

/*
********************************************************************************
*                         PUBLIC PROGRAM CODE
********************************************************************************
*/

Word32 sqrt_l_exp (/* o : output value,                          Q31 */
    Word32 L_x,    /* i : input value,                           Q31 */
    Word16 *exp    /* o : right shift to be applied to result,   Q1  */
)
{
    /*
          y = sqrt(x)

          x = f * 2^-e,   0.5 <= f < 1   (normalization)

          y = sqrt(f) * 2^(-e/2)  

          a) e = 2k   --> y = sqrt(f)   * 2^-k  (k = e div 2,
                                                 0.707 <= sqrt(f) < 1)
          b) e = 2k+1 --> y = sqrt(f/2) * 2^-k  (k = e div 2,
                                                 0.5 <= sqrt(f/2) < 0.707)
     */
    
    
    Word16 e, i, a, tmp;
    Word32 L_y;

    test (); 
    if (L_x <= (Word32) 0)
    {
        *exp = 0;               move16 ();
        return (Word32) 0;
    }

    e = norm_l_ex (L_x) & 0xFFFE;  logic16 (); /* get next lower EVEN norm. exp  */
    L_x = L_shl_ex (L_x, e);                   /* L_x is normalized to [0.25..1) */
    *exp = e;                   move16 ();  /* return 2*exponent (or Q1)      */

    L_x = L_shr_ex (L_x, 9);
    i = extract_h_ex (L_x);            /* Extract b25-b31, 16 <= i <= 63 because
                                       of normalization                       */
    L_x = L_shr_ex (L_x, 1);   
    a = extract_l_ex (L_x);            /* Extract b10-b24                        */
    a = a & (Word16) 0x7fff;    logic16 (); 

    i = sub_ex (i, 16);                /* 0 <= i <= 47                           */

    L_y = L_deposit_h_ex (table[i]);           /* table[i] << 16                 */
    tmp = sub_ex (table[i], table[i + 1]);     /* table[i] - table[i+1])         */
    L_y = L_msu_ex (L_y, tmp, a);              /* L_y -= tmp*a*2                 */
       
    /* L_y = L_shr_ex (L_y, *exp); */          /* denormalization done by caller */

    return (L_y);
}