/home/jander/temp/etherdune/Checksum.cpp

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// EtherDune IP Checksum module
// Author: Javier Peletier <jm@friendev.com>
// Summary: memory-efficient algorithm to calculate total and partial network checksums
//
// Copyright (c) 2015 All Rights Reserved, http://friendev.com
//
// This source is subject to the GPLv2 license.
// Please see the License.txt file for more information.
// All other rights reserved.
//
// THIS CODE AND INFORMATION ARE PROVIDED "AS IS" WITHOUT WARRANTY OF ANY 
// KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
// PARTICULAR PURPOSE.


#include "Checksum.h"

uint16_t Checksum::add(uint16_t a, uint16_t b)
{
    a += b;

    if (a<b)
        a++;

    return a;
}

uint16_t Checksum::add(uint16_t a, uint16_t b, bool odd)
{
    if (odd)
        b = (b << 8) | (b >> 8);

    return add(a, b);

}


uint16_t Checksum::calc(uint16_t len, const uint8_t *data)
{
    uint16_t sum = 0;
    const uint16_t *dataptr;
    const uint16_t *last;

    dataptr = (uint16_t*)data;
    last = dataptr + (len >> 1);

    while (dataptr < last)
    {
        sum = add(sum, *dataptr);
        dataptr++;
    }

    if (len & 1)
        sum = add(sum, *((uint8_t*)dataptr));

    return sum;

}

uint16_t Checksum::calc(uint16_t checksum, uint16_t len, const uint8_t *data)
{
    return add(checksum, calc(len, data));
}