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/*
* IBM Accurate Mathematical Library
* written by International Business Machines Corp.
* Copyright (C) 2001-2017 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, HALFRAD}};
static const mp_no mp3halfs = {1, {1.0, 1.0, HALFRAD}};
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;
}
/***********************************************************/
/* 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;
static const double 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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