Get a unique 32 bits signed integer for the given string Id.

def get_int_id(self, str_id):
        """Get a unique 32 bits signed integer for the given string Id."""
        if not str_id in self.mapping:
            if self.curr_id == IdRemapper.INT_MAX:
                return None # No more 32 bits signed integers available
            self.mapping[str_id] = self.curr_id
            self.r_mapping[self.curr_id] = str_id
            self.curr_id += 1
        return self.mapping[str_id]