Function test

code/mathematics/fht.test.cpp:13–34

}

void test() {
    rep(it,0,1000) {
        int k = rng() % 10;
        int n = 1<<k;
        vi a(n), b(n);
        rep(i,0,n) {
            a[i] = uniform_int_distribution(-100, 100)(rng);
            b[i] = uniform_int_distribution(-100, 100)(rng);
        }
        vi ans = slow(a,b);

        fht(a);
        fht(b);

        rep(i,0,n) {
            a[i] *= b[i];
        }

        fht(a,true);
        assert(a == ans);
    }
}