@Override
		public void run() {
			System.out.println("begin: "+ begin+" len: "+len);
			for (int i = begin; i < begin+len; i++) {
				// i: start char index
				// j: password length
				for (int j = 1; j <= maxPassLen; j++) {
					
					byte[] buff = new byte[j];
					buff[0] = (byte) CHARS[i]; // set first character.
					perm(1, buff, j);
				}
			}
			latch.countDown();
		}