# Copyright (C) 2012 Nippon Telegraph and Telephone Corporation.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
# See the License for the specific language governing permissions and
# limitations under the License.


def carry_around_add(a, b):
    c = a + b
    return (c & 0xffff) + (c >> 16)


def checksum(data):
    s = 0
    for i in range(0, len(data), 2):
        w = data[i] + (data[i + 1] << 8)
        s = carry_around_add(s, w)
    return ~s & 0xffff