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static String base62Encode(final byte[] bytes, final int length) {
    final int size = (int) ceil((bytes.length * BYTE_BITS) / DIGIT_BITS);
    final StringBuilder sb = new StringBuilder(size);

    // Allocate an extra zero value byte in the first position, so that the unsigned 32bit UTC 
    // timestamp value is not treated as negative value and thus being encoded as
    // "000000000000000000000000000"
    byte[] b = new byte[bytes.length+1];
    System.arraycopy(bytes, 0, b, 1, bytes.length);

    BigInteger value = new BigInteger(b);
    while (value.compareTo(ZERO) > 0) {
        final BigInteger[] quotientAndRemainder = value.divideAndRemainder(BASE);
        sb.append(BASE_62_CHARACTERS[abs(quotientAndRemainder[1].intValue())]);
        value = quotientAndRemainder[0];
    }

    while (length > 0 && sb.length() < length) {
        sb.append('0');
    }

    return sb.reverse().toString();
}