Rolling hash with double modulus and bases.

Rolling hash

The code below supports:

• Two bases/modulus hashing.
• Gets the hash value of segments from both prefix and suffix perspectives.

Code

const int N = 1e6 + 5;
const ll MOD = 1e9 + 9;
const ll MOD2 = 998244353;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll base = 311;
ll base2 = rng() % 300;
ll pb[N], pb2[N];
struct double_hash {
    int sz;
    vector < ll > pref[2], suff[2];
    double_hash(string s) {
        sz = s.size();
        for (int i = 0; i < 2; ++i) pref[i].resize(sz + 5, 0), suff[i].resize(sz + 5, 0);
        pref[0][0] = pref[1][0] = s[0] - 'a' + 1 LL;
        for (int i = 1; i < sz; ++i) {
            pref[0][i] = (pref[0][i - 1] * base % MOD + (s[i] - 'a' + 1 LL)) % MOD;
            pref[1][i] = (pref[1][i - 1] * base2 % MOD2 + (s[i] - 'a' + 1 LL)) % MOD2;
        }
        reverse(s.begin(), s.end());
        suff[0][0] = suff[1][0] = s[0] - 'a' + 1 LL;
        for (int i = 1; i < sz; ++i) {
            suff[0][i] = (suff[0][i - 1] * base % MOD + (s[i] - 'a' + 1 LL) % MOD) % MOD;
            suff[1][i] = (suff[1][i - 1] * base2 % MOD2 + (s[i] - 'a' + 1 LL) % MOD2) % MOD2;
        }
    }
    pair < ll, ll > get_prefix(int l, int r) {
        ll val = (pref[0][r] - (l ? pref[0][l - 1] : 0 LL) * pb[r - l + 1] + MOD * MOD) % MOD;
        ll val2 = (pref[1][r] - (l ? pref[1][l - 1] : 0 LL) * pb2[r - l + 1] + MOD2 * MOD2) % MOD2;
        return pair < ll, ll > (val, val2);
    }
    pair < ll, ll > get_suffix(int l, int r) {
        l = sz - l - 1, r = sz - r - 1;
        swap(l, r);
        ll val = (suff[0][r] - (l ? suff[0][l - 1] : 0 LL) * pb[r - l + 1] + MOD * MOD) % MOD;
        ll val2 = (suff[1][r] - (l ? suff[1][l - 1] : 0 LL) * pb2[r - l + 1] + MOD2 * MOD2) % MOD2;
        return pair < ll, ll > (val, val2);
    }
};

int main() {
    #
    ifdef kvutxdy
    freopen("C:/Users/khanh/OneDrive/RoadtoPurple/Code/Codeforces/input.in", "r", stdin);#
    endif
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    pb[0] = pb2[0] = 1 LL;
    for (int i = 1; i < N; ++i) {
        pb[i] = pb[i - 1] * base % MOD;
        pb2[i] = pb2[i - 1] * base2 % MOD2;
    }
}