#include "libm.h"

double nextafter(double x, double y) {
  union {
    double f;
    uint64_t i;
  } ux = {x}, uy = {y};
  uint64_t ax, ay;
  int e;

  if (isnan(x) || isnan(y))
    return x + y;
  if (ux.i == uy.i)
    return y;
  ax = ux.i & -1ULL / 2;
  ay = uy.i & -1ULL / 2;
  if (ax == 0) {
    if (ay == 0)
      return y;
    ux.i = (uy.i & 1ULL << 63) | 1;
  } else if (ax > ay || ((ux.i ^ uy.i) & 1ULL << 63))
    ux.i--;
  else
    ux.i++;
  e = ux.i >> 52 & 0x7ff;
  /* raise overflow if ux.f is infinite and x is finite */
  if (e == 0x7ff)
    FORCE_EVAL(x + x);
  /* raise underflow if ux.f is subnormal or zero */
  if (e == 0)
    FORCE_EVAL(x * x + ux.f * ux.f);
  return ux.f;
}