Base62

class Base62 {
	public Base62(BigInteger num) {
		base62 = setBase62FromString(num);
	}

	public String setBase62FromString(BigInteger num) {
		StringBuilder builder = new StringBuilder();
		BigInteger modRes;
		BigInteger n62 = new BigInteger("62");
		do {
			modRes = num.mod(n62);
			builder.insert(0, alphabet.charAt(modRes.intValue()));
			num = num.divide(n62);
			if(num.compareTo(n62) == -1) {
				builder.insert(0, alphabet.charAt(num.intValue()));
			}
		} while(num.compareTo(n62) == 1);

		return builder.toString();
	}

	public BigInteger toBigInteger() {
		BigInteger result = new BigInteger("0");
		int b = 0;
		for(int i = base62.length()-1; i != -1; i--) {
			int decValue = alphabet.indexOf(base62.charAt(b));
			BigInteger next = new BigInteger(
					String.valueOf(
							(int)(decValue * Math.pow(62.0, i))
					)
			);
			result = result.add(next);
			b += 1;
		}
		return result;
	}

	public String toString() {
		return base62;
	}

	private String base62;
	private final String alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
}