后缀数组的Java实现

后缀数组简介

后缀数组是一种很有用的数据结构，它经常用在字符串处理的相关问题当中，关于后缀数组的一般介绍，我就不作赘述了，因为网上已经有很多的资料了，我一开始也是不太懂后缀数组，后来看别人写的博客，慢慢的就理解了。下面是一段用Java实现的后缀数组的代码。

import java.util.Arrays;

public class SuffixArray {

    public static final int SIZE = 256;

    public static int[] generateSA(String s) {
        char[] ch = s.toCharArray();
        int len = ch.length;
        int m = SIZE;
        int[] sa = new int[len];
        int[] x = new int[len];
        int[] y = new int[len];
        int[] ysort = new int[len];
        int[] bucket = new int[SIZE];

        Arrays.fill(bucket, 0);
        for (int i = 0; i < len; i++) {
            x[i] = ch[i];
            bucket[ch[i]]++;
        }
        for (int i = 1; i < m; i++)
            bucket[i] += bucket[i-1];
        for (int i = len-1; i >= 0; i--)
            sa[--bucket[ch[i]]] = i;

        for (int step = 1, p = 0; p < len; step *= 2, m = p) {
            p = 0;
            for (int i = len - step; i < len; i++)
                y[p++] = i;
            for (int i = 0; i < len; i++) {
                if (sa[i] >= step)
                    y[p++] = sa[i] - step;
            }

            for (int i = 0; i < len; i++)
                ysort[i] = x[y[i]];

            Arrays.fill(bucket, 0);
            for (int i = 0; i < len; i++)
                bucket[ysort[i]]++;
            for (int i = 1; i < m; i++)
                bucket[i] += bucket[i-1];
            for (int i = len - 1; i >= 0; i--)
                sa[--bucket[ysort[i]]] = y[i];

            Arrays.fill(y, 0);
            System.arraycopy(x, 0, y, 0, len);

            x[sa[0]] = 0;
            p = 1;
            for (int i = 1; i < len; i++)
                x[sa[i]] = compare(y, sa[i], sa[i-1], step) ? p-1 : p++;
        }
        return sa;
    }

    public static int[] generateRank(int[] sa) {
        int len = sa.length;
        int[] rank = new int[len];
        for (int i = 0; i < len; i++)
            rank[sa[i]] = i;
        return rank;
    }

    public static int[] generateH(String s, int[] sa) {
        char[] ch = s.toCharArray();
        int len = sa.length;
        int[] h = new int[len];
        int[] rank = generateRank(sa);

        Arrays.fill(h, 0);
        int index = 0;
        for (int i = 0; i < len; i++) {
            if (rank[i] == 0)
                continue;
            for (int j = sa[rank[i]-1]; inBounds(i+index, j+index, len) && ch[i+index] == ch[j+index];)
                index++;
            h[rank[i]] = index;
            if (index > 0)
                index--;
        }
        return h;
    }

    private static boolean inBounds(int i, int j, int len) {
        return i < len && j < len;
    }

    private static boolean compare(int[] y, int a, int b, int l) {
        int[] tmp = new int[y.length + 1];
        System.arraycopy(y, 0, tmp, 0, y.length);
        return tmp[a] == tmp[b] && tmp[a+l] == tmp[b+l];
    }

    public static void main(String[] args) {
        String s = "world";
        int[] sa = generateSA(s);
        int[] rank = generateRank(sa);
        System.out.println("sa   : " + Arrays.toString(sa));
        System.out.println("rank : " + Arrays.toString(rank));
        for (int i = 0; i < sa.length; i++)
            System.out.print(s.substring(sa[i])+" ");
    }

}

在以后的几篇文章当中，我会写一下后缀数组的具体应用。