e_remainder.c

/* @(#)e_remainder.c 1.3 95/01/18 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 */

/* __ieee754_remainder(x,p)
 * Return :                  
 *      returns  x REM p  =  x - [x/p]*p as if in infinite 
 *      precise arithmetic, where [x/p] is the (infinite bit) 
 *      integer nearest x/p (in half way case choose the even one).
 * Method : 
 *      Based on fmod() return x-[x/p]chopped*p exactlp.
 */

#include "fdlibm.h"

static const double zero = 0.0;

double __ieee754_remainder(double x, double p)
{
        int hx,hp;
        unsigned sx,lx,lp;
        double p_half;

        __getHI(hx,x);          /* high word of x */
        __getLO(lx,x);          /* low  word of x */
        __getHI(hp,p);          /* high word of p */
        __getLO(lp,p);          /* low  word of p */
        sx = hx&0x80000000;
        hp &= 0x7fffffff;
        hx &= 0x7fffffff;

    /* purge off exception values */
        if((hp|lp)==0) return (x*p)/(x*p);      /* p = 0 */
        if((hx>=0x7ff00000)||                   /* x not finite */
          ((hp>=0x7ff00000)&&                   /* p is NaN */
          (((hp-0x7ff00000)|lp)!=0)))
            return (x*p)/(x*p);


        if (hp<=0x7fdfffff) x = __ieee754_fmod(x,p+p);  /* now x < 2p */
        if (((hx-hp)|(lx-lp))==0) return zero*x;
        x  = fabs(x);
        p  = fabs(p);
        if (hp<0x00200000) {
            if(x+x>p) {
                x-=p;
                if(x+x>=p) x -= p;
            }
        } else {
            p_half = 0.5*p;
            if(x>p_half) {
                x-=p;
                if(x>=p_half) x -= p;
            }
        }
        __getHI(hx,x);
        __setHI(x,hx ^ sx);
        return x;
}