@classmethod
 def apply(cls, dest, diff):
    """
    Apply the given patch string diff to the given string dest
    :param dest: Destination string
    :param diff: Patch string
    :return:
    """
    if diff is None:
        return
    if not isinstance(dest, MutableString):
        raise ValueError
    if not dest:
        return

    pos = len(dest) - 1
    try:
        for i in range(int(len(diff) / 2)):
            cmd = diff[2 * i]
            param = diff[2 * i + 1]
            par_num = ord(param) - ord('a') + 1
            if cmd == '-':
                pos -= (par_num - 1)
            elif cmd == 'R':
                cls.__check_index(dest, pos)
                dest[pos] = param
            elif cmd == 'D':
                o = pos
                pos -= (par_num - 1)
                cls.__check_index(dest, pos)
                dest[pos:o + 1] = ''
            elif cmd == 'I':
                pos += 1
                cls.__check_offset(dest, pos)
                dest.insert(pos, param)
            pos -= 1
    except IndexError:
        # swallow, same thing happens in original Java version
        pass

@classmethod
def __check_index(cls, s, index):
    if index < 0 or index >= len(s):
        raise IndexError

@classmethod
def __check_offset(cls, s, offset):
    if offset < 0 or offset > len(s):
        raise IndexError