#include <math.h>

double fmax(double x, double y) {
    if (isnan(x))
        return y;
    if (isnan(y))
        return x;
    /* handle signed zeros, see C99 Annex F.9.9.2 */
    if (signbit(x) != signbit(y))
        return signbit(x) ? y : x;
    return x < y ? y : x;
}