// find in which bucket a key is (just basic modulo)
int get_key(const Key &K) {
if(BUFFER) return BUFFER[K.index];
int k = E->extract(K);
k %= N;
if(k < 0) k += N;
return k;
}
int extract(const Key &K) {
int v1 = E1->extract(K);
int v2 = E2->extract(K);
return A1*(v1/d1) + A2*(v2/d2);
}