static String rand_string(int len) {
		Random gen = new Random();
		StringBuilder ret = new StringBuilder();
		while (len-- > 0) {
			int ch = gen.nextInt(53);
			if (ch == 52) {
				ret.append(' ');
			} else if (ch < 26) {
				ret.append(ch + 'a');
			} else {
				ret.append(ch - 26 + 'A');
			}
		}
		return ret.toString();
	}