substring.cpp

// GistID: 7984760
// KMP substring search && FSM substring search
#include <string>
#include <iostream>
#include <vector>

using namespace std;

typedef vector<vector<int> > FSMType;

const int R = 256;

template<class T>
ostream & operator<<(ostream & os, vector<T>vec) {
    os << "[";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << ",";
    }
    os << "]";
}

FSMType build_fsm(const string &pattern)
{
    FSMType fsm(pattern.size(), vector<int>(R, 0));
    int X = 0;
    for (int i = 0; i < pattern.size(); i++) {
        for (int j = 0; j < R; j++) {
            if (j == pattern[i]) {
                fsm[i][j] = i+1;
            } else {
                fsm[i][j] = fsm[X][j];
            }
        }
        if (i >= 1) {
            X = fsm[X][pattern[i]];
        }
    }
    return fsm;
}

int fsm_search(const string &text, const string &pattern)
{
    FSMType fsm = build_fsm(pattern);
    int state = 0, fin = fsm.size();
    int i = 0;
    while (i < text.length() && state != fin) {
        state = fsm[state][text[i]];
        i++;
    }
    if (state == fin) {
        return i - pattern.size();
    } else {
        return -1;
    }
}

vector<int> build_next(const string &pattern)
{
    vector<int> next(pattern.size(), 0);
    next[0] = 0;
    for (int i = 1; i < pattern.size(); i++) {
        int p = next[i-1];
        while (p > 0 && pattern[p] != pattern[i]) {
            p = next[p-1];
        }
        if (pattern[p] == pattern[i]) {
            next[i] = p+1;
        }
    }
    return next;
}

int kmp_search(const string &text, const string &pattern)
{
    int p = 0, q = 0;

    vector<int> next = build_next(pattern);

    while (q != text.length() && p != pattern.length()) {
        if (pattern[p] == text[q]) {
            p++;
            q++;
        } else {
            if (p == 0) {
                q++;
            } else {
                p = max(next[p-1]+1, 0);
            }
        }
    }


    if (p == pattern.length()) {
        return q-p;
    } else {
        return -1;
    }
}

int search(const string &text, const string &pattern)
{
    //return fsm_search(text, pattern);
    return kmp_search(text, pattern);
}

int main(int argc, const char *argv[])
{
    string text = "ababaabababcaaaab";
    string pattern = "babc";
    int pos = search(text, pattern);
    cout << "pos: " << pos << endl;
    if (pos > 0)
        cout << "found: " << text.substr(pos, text.size() - pos) << endl;
    return 0;
}