suffix-automaton.cpp

class SuffixAutomaton {
 public:
  struct state {
    int len, link;
    array<int, 26> next;
  };

  vector<state> st;
  int sz = 0, last;

  SuffixAutomaton(const string& s) : st(s.size() << 1) {
    sa_init();
    for (auto v : s) sa_extend((int)(v - 'a'));
  }

  void sa_init() {
    st[0].len = 0;
    st[0].link = -1;
    sz++;
    last = 0;
  }

  void sa_extend(int c) {
    int cur = sz++;
    st[cur].len = st[last].len + 1;
    int p = last;
    while (p != -1 && !st[p].next[c]) {
      st[p].next[c] = cur;
      p = st[p].link;
    }
    if (p == -1)
      st[cur].link = 0;
    else {
      int q = st[p].next[c];
      if (st[p].len + 1 == st[q].len)
        st[cur].link = q;
      else {
        int clone = sz++;
        st[clone].len = st[p].len + 1;
        st[clone].link = st[q].link;
        st[clone].next = st[q].next;
        while (p != -1 && st[p].next[c] == q) {
          st[p].next[c] = clone;
          p = st[p].link;
        }
        st[q].link = st[cur].link = clone;
      }
    }
    last = cur;
  }

  // longest common substring O(N)
  int lcs(const string& T) {
    int v = 0, l = 0, best = 0;
    for (int i = 0; i < T.size(); i++) {
      while (v && !st[v].next[T[i] - 'a']) {
        v = st[v].link;
        l = st[v].len;
      }
      if (st[v].next[T[i] - 'a']) {
        v = st[v].next[T[i] - 'a'];
        l++;
      }
      best = max(best, l);
    }
    return best;
  }
};