/*
 * Copyright 2015 Eistec AB
 *
 * This file is subject to the terms and conditions of the GNU Lesser
 * General Public License v2.1. See the file LICENSE in the top level
 * directory for more details.
 */

/**
 * @ingroup     sys_checksum_fletcher16
 * @{
 *
 * @file
 * @brief       Fletcher16 implementation
 *
 * @author      Joakim Nohlgård <joakim.nohlgard@eistec.se>
 *
 * @}
 */

#include "checksum/fletcher16.h"

uint16_t fletcher16(const uint8_t *data, size_t bytes)
{
    uint16_t sum1 = 0xff, sum2 = 0xff;

    while (bytes) {
        size_t tlen = bytes > 20 ? 20 : bytes;
        bytes -= tlen;
        do {
            sum2 += sum1 += *data++;
        } while (--tlen);
        sum1 = (sum1 & 0xff) + (sum1 >> 8);
        sum2 = (sum2 & 0xff) + (sum2 >> 8);
    }
    /* Second reduction step to reduce sums to 8 bits */
    sum1 = (sum1 & 0xff) + (sum1 >> 8);
    sum2 = (sum2 & 0xff) + (sum2 >> 8);
    return (sum2 << 8) | sum1;
}