s_rint.c

/*
 * @(#)s_rint.c 5.1 93/09/24
 *
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 *
 * rint(x)
 * Return x rounded to integral value according to the prevailing
 * rounding mode.
 * Method:
 *      Using floating addition.
 * Exception:
 *      Inexact flag raised if x not equal to rint(x).
 */

#include <math.h>
#include "s_rint.h"

static const double
TWO52[2]={
	4.50359962737049600000e+15,  /* 0x43300000, 0x00000000 */
	-4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
};

double rint(double x)
{
	int32_t i0,j0,sx;
	uint32_t i,i1;
	double w=0.0,t;
	EXTRACT_WORDS(i0,i1,x);
	sx = (i0>>31)&1;
	j0 = ((i0>>20)&0x7ff)-0x3ff;
	if(j0<20) {
		if(j0<0) {
			if(((i0&0x7fffffff)|i1)==0) return x;
			i1 |= (i0&0x0fffff);
			i0 &= 0xfffe0000;
			i0 |= ((i1|-i1)>>12)&0x80000;
			SET_HIGH_WORD(x,i0);
			w = TWO52[sx]+x;
			t =  w-TWO52[sx];
			GET_HIGH_WORD(i0,t);
			SET_HIGH_WORD(t,(i0&0x7fffffff)|(sx<<31));
			return t;
		} else {
			i = (0x000fffff)>>j0;
			if(((i0&i)|i1)==0) return x; /* x is integral */
			i>>=1;
			if(((i0&i)|i1)!=0) {
				/*
				 * Some bit is set after the 0.5 bit.  To avoid the
				 * possibility of errors from double rounding in
				 * w = TWO52[sx]+x, adjust the 0.25 bit to a lower
				 * guard bit.  We do this for all j0<=51.  The
				 * adjustment is trickiest for j0==18 and j0==19
				 * since then it spans the word boundary.
				 */
				if(j0==19) i1 = 0x40000000; else
					if(j0==18) i1 = 0x80000000; else
						i0 = (i0&(~i))|((0x20000)>>j0);
			}
		}
	} else if (j0>51) {
		if(j0==0x400) return x+x;       /* inf or NaN */
		else return x;                  /* x is integral */
	} else {
		i = ((uint32_t)(0xffffffff))>>(j0-20);
		if((i1&i)==0) return x;         /* x is integral */
		i>>=1;
		if((i1&i)!=0) i1 = (i1&(~i))|((0x40000000)>>(j0-20));
	}
	INSERT_WORDS(x,i0,i1);
	*(volatile double *)&w = TWO52[sx]+x;   /* clip any extra precision */
	return w-TWO52[sx];
}