KMP substring search algorithm

Kmp.java

public class Kmp {
    public static int indexOf(String t, String p) {
        return compile(p).matcher(t).find();
    }

    public static Pattern compile(String p) {
        return new Pattern(p);
    }

    public static class Pattern {
        protected final String p;
        protected final int[] d;

        public Pattern(String p) {
            this.p = p;

            int m = p.length();
            d = new int[m];

            int i = 1;
            int j = 0;

            while (i < m) {
                if (p.charAt(i) == p.charAt(j)) {
                    d[i] = j + 1;

                    i += 1;
                    j += 1;
                } else {
                    if (j > 0) {
                        j = d[j - 1];
                    } else {
                        i += 1;
                    }
                }
            }
        }

        public Matcher matcher(String t) {
            return new Matcher(t, p, d);
        }
    }

    public static class Matcher {
        protected final String t;
        protected final String p;
        protected final int[] d;
        protected int i;
        protected int j;

        public Matcher(String t, String p, int[] d) {
            this.t = t;
            this.p = p;
            this.d = d;
        }

        public int find() {
            int n = t.length();
            int m = p.length();

            while (i < n) {
                if (t.charAt(i) == p.charAt(j)) {
                    i += 1;
                    j += 1;

                    if (j == m) {
                        int r = i - j;
                        j = 0;
                        return r;
                    }
                } else {
                    if (j > 0) {
                        j = d[j - 1];
                    } else {
                        i += 1;
                    }
                }
            }

            return -1;
        }
    }
}