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/*
* IBM Accurate Mathematical Library
* written by International Business Machines Corp.
* Copyright (C) 2001-2013 Free Software Foundation, Inc.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
/****************************************************************************/
/* MODULE_NAME:mpsqrt.c */
/* */
/* FUNCTION:mpsqrt */
/* fastiroot */
/* */
/* FILES NEEDED:endian.h mpa.h mpsqrt.h */
/* mpa.c */
/* Multi-Precision square root function subroutine for precision p >= 4. */
/* The relative error is bounded by 3.501*r**(1-p), where r=2**24. */
/* */
/****************************************************************************/
#include "endian.h"
#include "mpa.h"
#ifndef SECTION
# define SECTION
#endif
#include "mpsqrt.h"
/****************************************************************************/
/* Multi-Precision square root function subroutine for precision p >= 4. */
/* The relative error is bounded by 3.501*r**(1-p), where r=2**24. */
/* Routine receives two pointers to Multi Precision numbers: */
/* x (left argument) and y (next argument). Routine also receives precision */
/* p as integer. Routine computes sqrt(*x) and stores result in *y */
/****************************************************************************/
static double fastiroot(double);
void
SECTION
__mpsqrt(mp_no *x, mp_no *y, int p) {
int i,m,ey;
double dx,dy;
static const mp_no
mphalf = {0,{1.0,8388608.0 /* 2^23 */}},
mp3halfs = {1,{1.0,1.0,8388608.0 /* 2^23 */}};
mp_no mpxn,mpz,mpu,mpt1,mpt2;
ey=EX/2; __cpy(x,&mpxn,p); mpxn.e -= (ey+ey);
__mp_dbl(&mpxn,&dx,p); dy=fastiroot(dx); __dbl_mp(dy,&mpu,p);
__mul(&mpxn,&mphalf,&mpz,p);
m=__mpsqrt_mp[p];
for (i=0; i<m; i++) {
__sqr(&mpu,&mpt1,p);
__mul(&mpt1,&mpz,&mpt2,p);
__sub(&mp3halfs,&mpt2,&mpt1,p);
__mul(&mpu,&mpt1,&mpt2,p);
__cpy(&mpt2,&mpu,p);
}
__mul(&mpxn,&mpu,y,p); EY += ey;
return;
}
/***********************************************************/
/* Compute a double precision approximation for 1/sqrt(x) */
/* with the relative error bounded by 2**-51. */
/***********************************************************/
static double
SECTION
fastiroot(double x) {
union {int i[2]; double d;} p,q;
double y,z, t;
int n;
static const double c0 = 0.99674, c1 = -0.53380, c2 = 0.45472, c3 = -0.21553;
p.d = x;
p.i[HIGH_HALF] = (p.i[HIGH_HALF] & 0x3FFFFFFF ) | 0x3FE00000 ;
q.d = x;
y = p.d;
z = y -1.0;
n = (q.i[HIGH_HALF] - p.i[HIGH_HALF])>>1;
z = ((c3*z + c2)*z + c1)*z + c0; /* 2**-7 */
z = z*(1.5 - 0.5*y*z*z); /* 2**-14 */
p.d = z*(1.5 - 0.5*y*z*z); /* 2**-28 */
p.i[HIGH_HALF] -= n;
t = x*p.d;
return p.d*(1.5 - 0.5*p.d*t);
}
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