prime/src/Runtime/s_nextafter.c

/* @(#)s_nextafter.c 1.2 95/01/04 */
/*
 * ====================================================
 * 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.
 * ====================================================
 */

/* 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
{
  _INT32 hx, hy, ix, iy;
  _UINT32 lx, ly;

  hx = __HI(x);         /* high word of x */
  lx = __LO(x);         /* low  word of x */
  hy = __HI(y);         /* high word of y */
  ly = __LO(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 */
    __HI(x) = hy & 0x80000000; /* return +-minsubnormal */
    __LO(x) = 1;
    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 */
      __HI(y) = hx;
      __LO(y) = lx;
      return y;
    }
  }
  __HI(x) = hx;
  __LO(x) = lx;
  return x;
}