s_nextafter.c

/* @(#)s_nextafter.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.
 * ====================================================
 */

/* IEEE functions
 *      nextafter(x,y)
 *      return the next machine floating-point number of x in the
 *      direction toward y.
 *   Special cases:
 */

#include "fdlibm.h"

double nextafter(double x, double y)
{
        int     hx,hy,ix,iy;
        unsigned lx,ly;

        __getHI(hx,x);          /* high word of x */
        __getLO(lx,x);          /* low  word of x */
        __getHI(hy,y);          /* high word of y */
        __getLO(ly,y);          /* low  word of y */
        ix = hx&0x7fffffff;             /* |x| */
        iy = hy&0x7fffffff;             /* |y| */

        if(((ix>=0x7ff00000)&&((ix-0x7ff00000)|lx)!=0) ||   /* x is nan */ 
           ((iy>=0x7ff00000)&&((iy-0x7ff00000)|ly)!=0))     /* y is nan */ 
           return x+y;                          
        if(x==y) return x;              /* x=y, return x */
        if((ix|lx)==0) {                        /* x == 0 */
            __setHILO(x, hy&0x80000000U,1); /* return +-minsubnormal */
            y = x*x;
            if(y==x) return y; else return x;   /* raise underflow flag */
        } 
        if(hx>=0) {                             /* x > 0 */
            if(hx>hy||((hx==hy)&&(lx>ly))) {    /* x > y, x -= ulp */
                if(lx==0) hx -= 1;
                lx -= 1;
            } else {                            /* x < y, x += ulp */
                lx += 1;
                if(lx==0) hx += 1;
            }
        } else {                                /* x < 0 */
            if(hy>=0||hx>hy||((hx==hy)&&(lx>ly))){/* x < y, x -= ulp */
                if(lx==0) hx -= 1;
                lx -= 1;
            } else {                            /* x > y, x += ulp */
                lx += 1;
                if(lx==0) hx += 1;
            }
        }
        hy = hx&0x7ff00000;
        if(hy>=0x7ff00000) return x+x;  /* overflow  */
        if(hy<0x00100000) {             /* underflow */
            y = x*x;
            if(y!=x) {          /* raise underflow flag */
                __setHILO(y, hx, lx);
                return y;
            }
        }
        __setHILO(x, hx, lx);
        return x;
}