public class RandomIndex {
	
	private final int n;
	private final int m;
	private int i;
	private final TinyCipher tc;
	
	public RandomIndex(int n, byte[] k) {
		this.n = n;
		int b = BitTools.lb(n - 1);
		this.m = 1 << b;
		this.i = 0;
		this.tc = new TinyCipher(b, k);
	}
	
	public int next() {
		int j;
		while((j = nextFromM()) >= n);
		return j;
	}
	
	private int nextFromM() {
		if (i >= m) throw new IndexOutOfBoundsException();
		int j = tc.encrypt(i);
		++i;
		return j;
	}
	
}