s_nextafter.c

标准c库代码,可以应用于各个系统提供了大量的基本函数

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

/*
FUNCTION
       <<nextafter>>, <<nextafterf>>---get next number

INDEX
	nextafter
INDEX
	nextafterf

ANSI_SYNOPSIS
       #include <math.h>
       double nextafter(double <[val]>, double <[dir]>);
       float nextafterf(float <[val]>, float <[dir]>);

TRAD_SYNOPSIS
       #include <math.h>

       double nextafter(<[val]>, <[dir]>)
              double <[val]>;
              double <[exp]>;

       float nextafter(<[val]>, <[dir]>)
              float <[val]>;
              float <[dir]>;


DESCRIPTION
<<nextafter>> returns the double) precision floating point number
closest to <[val]> in the direction toward <[dir]>.  <<nextafterf>>
performs the same operation in single precision.  For example,
<<nextafter(0.0,1.0)>> returns the smallest positive number which is
representable in double precision.

RETURNS
Returns the next closest number to <[val]> in the direction toward
<[dir]>.

PORTABILITY
	Neither <<nextafter>> nor <<nextafterf>> is required by ANSI C
	or by the System V Interface Definition (Issue 2).
*/

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

#include "fdlibm.h"

#ifdef __STDC__
	double nextafter(double x, double y)
#else
	double nextafter(x,y)
	double x,y;
#endif
{
#ifndef _DOUBLE_IS_32BITS
	__int32_t	hx,hy,ix,iy;
	__uint32_t lx,ly;

	EXTRACT_WORDS(hx,lx,x);
	EXTRACT_WORDS(hy,ly,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 */
	    INSERT_WORDS(x,hy&0x80000000,1);	/* 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 */
	        INSERT_WORDS(y,hx,lx);
		return y;
	    }
	}
	INSERT_WORDS(x,hx,lx);
	return x;
#else /* defined (_DOUBLE_IS_32BITS) */
	return (double) nextafterf ((float) x, (float) y);
#endif /* defined (_DOUBLE_IS_32BITS) */
}