KMP算法之dfa与next数组

public class KMP {
private String pat;
private int[][] dfa;

//dfa方法
public KMP(String pat) {
    this.pat = pat;
    int M = pat.length();
    int R = 256;
    dfa = new int[R][M];
    dfa[pat.charAt(0)][0] = 1;
    for (int X = 0, j = 1; j < M; j++) {
        for (int c = 0; c < R; c++) {
            dfa[c][j] = dfa[c][X];
        }
        dfa[pat.charAt(j)][j] = j + 1;
        X = dfa[pat.charAt(j)][X];
    }
}

public int search(String txt) {
    int i, j, N = txt.length(), M = pat.length();
    for (i = 0, j = 0; i < N && j < M; i++) {
        j = dfa[txt.charAt(i)][j];
    }
    if (j == M) {
        return i - M;
    } else {
        return N;
    }
}

public static void main(String[] args) {
//        String pat = args[0];
    String pat = "aacaa";
//        String txt = args[1];
    String txt = "aabrcdereaacaabced";
    KMP kmp = new KMP(pat);
    StdOut.println("text:    " + txt);
    int offset = kmp.search(txt);
    StdOut.print("pattern: ");
    for (int i = 0; i < offset; i++) {
        StdOut.print(" ");
    }
    StdOut.println(pat);
}


//next数组方法
public static int Next(String ts, String ps) {
    char[] t = ts.toCharArray();
    char[] p = ps.toCharArray();
    int i = 0; // 主串的位置
    int j = 0; // 模式串的位置
    int[] next = getNext(ps);
    while (i < t.length && j < p.length) {
        if (j == -1 || t[i] == p[j]) { // 当j为-1时，要移动的是i，当然j也要归0
            i++;
            j++;
        } else {
            // i不需要回溯了
            j = next[j]; // j回到指定位置
        }
    }

    if (j == p.length) {
        return i - j;
    } else {
        return -1;
    }
}


public static int[] getNext(String ps) {
    char[] p = ps.toCharArray();
    int[] next = new int[p.length];
    next[0] = -1;
    int j = 0;
    int k = -1;
    while (j < p.length - 1) {
        if (k == -1 || p[j] == p[k]) {
            if (p[++j] == p[++k]) { // 当两个字符相等时要跳过
                next[j] = next[k];
            } else {
                next[j] = k;
            }
        } else {
            k = next[k];
        }
    }
    return next;
}


}