Rabin-Karp 字符串匹配算法

public class Main  
{ 
    // d is the number of characters in the input alphabet 
    public final static int d = 256; 
      
    /* pat -> pattern 
        txt -> text 
        q -> A prime number 
    */
    static void search(String pat, String txt, int q) 
    { 
        int M = pat.length(); 
        int N = txt.length(); 
        int i, j; 
        int p = 0; // hash value for pattern 
        int t = 0; // hash value for txt 
        int h = 1; 
      
        //  pow(d, M-1)%q 
        for (i = 0; i < M-1; i++) 
            h = (h*d)%q; 
      
        // Calculate the hash value of pattern and first 
        // window of text 
        for (i = 0; i < M; i++) 
        { 
            p = (d*p + pat.charAt(i))%q; 
            t = (d*t + txt.charAt(i))%q; 
        } 
      
        // Slide the pattern over text one by one 
        for (i = 0; i <= N - M; i++) 
        { 
      
            // Check the hash values of current window of text 
            // and pattern. If the hash values match then only 
            // check for characters on by one 
            if ( p == t ) 
            { 
                /* Check for characters one by one */
                for (j = 0; j < M; j++) 
                { 
                    if (txt.charAt(i+j) != pat.charAt(j)) 
                        break; 
                } 
      
                // if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1] 
                if (j == M) 
                    System.out.println("Pattern found at index " + i); 
            } 
      
            // Calculate hash value for next window of text: Remove 
            // leading digit, add trailing digit 
            if ( i < N-M ) 
            { 
                t = (d*(t - txt.charAt(i)*h) + txt.charAt(i+M))%q; 
      
                // We might get negative value of t, converting it 
                // to positive 
                if (t < 0) 
                t = (t + q); 
            } 
        } 
    } 
      
    /* Driver program to test above function */
    public static void main(String[] args) 
    { 
        String txt = "GEEKS FOR GEEKS"; 
        String pat = "GEEK"; 
        int q = 101; // A prime number 
        search(pat, txt, q); 
    } 
}