1 /*
2  * Copyright (C) 2016 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package com.android.cts.vpnfirewall;
18 
19 public final class Rfc1071 {
checksum(byte[] data, int length)20     static int checksum(byte[] data, int length) {
21         int sum = 0;
22 
23         // High bytes (even indices)
24         for (int i = 0; i < length; i += 2) {
25             sum += (data[i] & 0xFF) << 8;
26             sum = (sum & 0xFFFF) + (sum >> 16);
27         }
28 
29         // Low bytes (odd indices)
30         for (int i = 1; i < length; i += 2) {
31             sum += (data[i] & 0xFF);
32             sum = (sum & 0xFFFF) + (sum >> 16);
33         }
34 
35         // Fix any one's-complement errors- sometimes it is necessary to rotate twice.
36         sum = (sum & 0xFFFF) + (sum >> 16);
37         return sum ^ 0xFFFF;
38     }
39 }
40