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/* sqrt.c
*
* Square root
*
*
*
* SYNOPSIS:
*
* double x, y, sqrt();
*
* y = sqrt( x );
*
*
*
* DESCRIPTION:
*
* Returns the square root of x.
*
* Range reduction involves isolating the power of two of the
* argument and using a polynomial approximation to obtain
* a rough value for the square root. Then Heron's iteration
* is used three times to converge to an accurate value.
*
*
*
* ACCURACY:
*
*
* Relative error:
* arithmetic domain # trials peak rms
* DEC 0, 10 60000 2.1e-17 7.9e-18
* IEEE 0,1.7e308 30000 1.7e-16 6.3e-17
*
*
* ERROR MESSAGES:
*
* message condition value returned
* sqrt domain x < 0 0.0
*
*/
/*
Cephes Math Library Release 2.8: June, 2000
Copyright 1984, 1987, 1988, 2000 by Stephen L. Moshier
*/
#include "mconf.h"
#ifdef ANSIPROT
extern double frexp ( double, int * );
extern double ldexp ( double, int );
#else
double frexp(), ldexp();
#endif
extern double SQRT2; /* SQRT2 = 1.41421356237309504880 */
double sqrt(x)
double x;
{
int e;
#ifndef UNK
short *q;
#endif
double z, w;
if( x <= 0.0 )
{
if( x < 0.0 )
mtherr( "sqrt", DOMAIN );
return( 0.0 );
}
w = x;
/* separate exponent and significand */
#ifdef UNK
z = frexp( x, &e );
#endif
#ifdef DEC
q = (short *)&x;
e = ((*q >> 7) & 0377) - 0200;
*q &= 0177;
*q |= 040000;
z = x;
#endif
/* Note, frexp and ldexp are used in order to
* handle denormal numbers properly.
*/
#ifdef IBMPC
z = frexp( x, &e );
q = (short *)&x;
q += 3;
/*
e = ((*q >> 4) & 0x0fff) - 0x3fe;
*q &= 0x000f;
*q |= 0x3fe0;
z = x;
*/
#endif
#ifdef MIEEE
z = frexp( x, &e );
q = (short *)&x;
/*
e = ((*q >> 4) & 0x0fff) - 0x3fe;
*q &= 0x000f;
*q |= 0x3fe0;
z = x;
*/
#endif
/* approximate square root of number between 0.5 and 1
* relative error of approximation = 7.47e-3
*/
x = 4.173075996388649989089E-1 + 5.9016206709064458299663E-1 * z;
/* adjust for odd powers of 2 */
if( (e & 1) != 0 )
x *= SQRT2;
/* re-insert exponent */
#ifdef UNK
x = ldexp( x, (e >> 1) );
#endif
#ifdef DEC
*q += ((e >> 1) & 0377) << 7;
*q &= 077777;
#endif
#ifdef IBMPC
x = ldexp( x, (e >> 1) );
/*
*q += ((e >>1) & 0x7ff) << 4;
*q &= 077777;
*/
#endif
#ifdef MIEEE
x = ldexp( x, (e >> 1) );
/*
*q += ((e >>1) & 0x7ff) << 4;
*q &= 077777;
*/
#endif
/* Newton iterations: */
#ifdef UNK
x = 0.5*(x + w/x);
x = 0.5*(x + w/x);
x = 0.5*(x + w/x);
#endif
/* Note, assume the square root cannot be denormal,
* so it is safe to use integer exponent operations here.
*/
#ifdef DEC
x += w/x;
*q -= 0200;
x += w/x;
*q -= 0200;
x += w/x;
*q -= 0200;
#endif
#ifdef IBMPC
x += w/x;
*q -= 0x10;
x += w/x;
*q -= 0x10;
x += w/x;
*q -= 0x10;
#endif
#ifdef MIEEE
x += w/x;
*q -= 0x10;
x += w/x;
*q -= 0x10;
x += w/x;
*q -= 0x10;
#endif
return(x);
}
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