#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;
}