Performs an IP checksum (used in IP header and across UDP payload) on the specified portion of a ByteBuffer - Android java.nio

Android examples for java.nio:ByteBuffer

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Description

Performs an IP checksum (used in IP header and across UDP payload) on the specified portion of a ByteBuffer

Demo Code

/*//ww w.  j  a v a  2s .  co m
 * Copyright (C) 2015 The Android Open Source Project
 *
 * 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.
 */

import android.system.OsConstants;
import android.system.OsConstants;
import java.net.Inet6Address;
import java.net.InetAddress;
import java.nio.BufferOverflowException;
import java.nio.BufferUnderflowException;
import java.nio.ByteBuffer;
import java.nio.ShortBuffer;

public class Main{

    /**
     * Performs an IP checksum (used in IP header and across UDP
     * payload) on the specified portion of a ByteBuffer.  The seed
     * allows the checksum to commence with a specified value.
     */
    private static int checksum(ByteBuffer buf, int seed, int start, int end) {
        int sum = seed;
        final int bufPosition = buf.position();

        // set position of original ByteBuffer, so that the ShortBuffer
        // will be correctly initialized
        buf.position(start);
        ShortBuffer shortBuf = buf.asShortBuffer();

        // re-set ByteBuffer position
        buf.position(bufPosition);

        final int numShorts = (end - start) / 2;
        for (int i = 0; i < numShorts; i++) {
            sum += intAbs(shortBuf.get(i));
        }
        start += numShorts * 2;

        // see if a singleton byte remains
        if (end != start) {
            short b = buf.get(start);

            // make it unsigned
            if (b < 0) {
                b += 256;
            }

            sum += b * 256;
        }

        sum = ((sum >> 16) & 0xFFFF) + (sum & 0xFFFF);
        sum = ((sum + ((sum >> 16) & 0xFFFF)) & 0xFFFF);
        int negated = ~sum;
        return intAbs((short) negated);
    }

}

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