RTL271_1/evrcc/code/inv_sqrt.c

/**********************************************************************
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/*======================================================================*/
/*     Enhanced Variable Rate Codec - Bit-Exact C Specification         */
/*     Copyright (C) 1997-1998 Telecommunications Industry Association. */
/*     All rights reserved.                                             */
/*----------------------------------------------------------------------*/
/* Note:  Reproduction and use of this software for the design and      */
/*     development of North American Wideband CDMA Digital              */
/*     Cellular Telephony Standards is authorized by the TIA.           */
/*     The TIA does not authorize the use of this software for any      */
/*     other purpose.                                                   */
/*                                                                      */
/*     The availability of this software does not provide any license   */
/*     by implication, estoppel, or otherwise under any patent rights   */
/*     of TIA member companies or others covering any use of the        */
/*     contents herein.                                                 */
/*                                                                      */
/*     Any copies of this software or derivative works must include     */
/*     this and all other proprietary notices.                          */
/*======================================================================*/
/* 
 |                                                                           |
 |   Function Name : Inv_sqrt                                                |
 |                                                                           |
 |       Compute 1/sqrt(L_x).                                                |
 |       L_x is positive.                                                    |
 |                                                                           |
 |       if L_x is negative or zero, result is 1 (3fff ffff).                |
 |---------------------------------------------------------------------------|
 |  Algorithm:                                                               |
 |                                                                           |
 |   The function 1/sqrt(L_x) is approximated by a table and linear          |
 |   interpolation.                                                          |
 |                                                                           |
 |   1- Normalization of L_x.                                                |
 |   2- If (30-exponant) is even then shift right once.                      |
 |   3- exponant = (30-exponant)/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 >>= exponant                                                     |
 |___________________________________________________________________________|
*/

#include "typedefs.h"
//#include "mathevrc.h"
#include "dsp_math.h"
#include "mathadv.h"

#include "inv_sqrt.dat"         /* Table for inv_sqrt() */

Longword Inv_sqrt(                /* (o) Q30 : output value   (range: 0<=val<1)           */
                   Longword L_x   /* (i) Q0  : input value    (range: 0<=val<=7fffffff)   */
)
{
    Shortword exp, i, a, tmp;
    Longword L_y;

    if (L_x <= (Longword) 0)
        return ((Longword) 0x3fffffff);

	exp = norm_l(L_x);
	L_x = L_shl(L_x, exp);		/* L_x is normalize */

	exp = sub(30, exp);

	if ((exp & 1) == 0)			/* If exponant 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 & (Shortword) 0x7fff;

	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);
}