Re: [Jnetstream-users] Error in TCP decode

[Mark B. <voy...@ya...>]

Thanks Frederic,
    I'm still out of commission myself.

I will revist the BitBuffer as that is a core routine that has to be 100% 
correct all the time under all circumstances.

Cheers,
mark...

----- Original Message ----- 
From: "Frédéric Dreier" <fre...@dr...>
Cc: <Jne...@li...>
Sent: Wednesday, August 06, 2008 4:31 PM
Subject: Re: [Jnetstream-users] Error in TCP decode


Good morning Mark,

I will be unreachable until next monday so I already send you what I found.

- my implementation of readBitsOneAtATime(). Perhaps it is not so good
but it just does the trick until the original one will be fixed.
- something in FastScanner didn't work after I changed
readBitsOneAtATime(). 32 kept to be added to the ip header length.

Regards,
Frédéric


Fast Scanner ---------------------------------
    /**
     *
     */
    public final boolean scanIp4() {
        addToCache(IP4);
        r1 = readUByte(Ip4.StaticField.PROTOCOL);
        if (DEBUG) {
            logger.debug(String.format("Ip4 p=%d, r1=%d\n", offset, r1));
        }
        //offset += bits.getBits(offset + 4, 4) * 4 * 8 + 32; <<== why +32 ?
        offset += bits.getBits(offset + 4, 4) * 32;
        switch (r1) {
            case 1:
                return scanIcmp();

            case 6:
                return scanTcp();

            case 17:
                return scanUdp();
        }

        return true;
    }

BitBuffer -------------------------------------

    private static final int[] RIGHT_BIT_MASK = new int[33];
    static {
        // initialize mask used by readBitsOneAtATime()
        for (int i = 1; i < RIGHT_BIT_MASK.length; i++) {
            RIGHT_BIT_MASK[i] = (RIGHT_BIT_MASK[i - 1] << 1) | 1;
        }
    }

    private int readBitsOneAtATime(int length) {
        // basic checks to avoid unnecessary computation
        if (length == 0) {
            return 0;
        }
        if (length > 32) {
            throw new IndexOutOfBoundsException("0 >= range <= 32bits");
        }
        // value is stored in long since length+posBits
        // could be greater than 32.
        long value = current;
        int acc = 8 - posBits;
        // concatenate buffer byte per byte
        for (; acc < length; acc += 8) {
            // fetch next byte
            posBits = 8;
            fetch();
            // append new current byte
            value = (value << 8) | (current & 0xFF);
        }
        // right-align the bits and mask high bits
        value = (value >> (acc - length)) & RIGHT_BIT_MASK[length];
        // update posBits value
        posBits = (posBits + length) % 8;
        // return
        return (int) value;
    }


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