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@include "internet_checksum.gawk";

{
    printf("Internet Checksum\nInput:\t%s\nValue:\t%d\n", $0 ,internet_checksum($0));
}

#! /usr/bin/gawk
# RFC1071 Computing the Internet Checksum (in gawk)
# written by Sean Summers <seansummers@gmail.com> 2015-09-15 v0.1
#
# use '@include "internet_checksum.gawk"' to include in a script

function _ord_init(low, high, i, t) {
    # creates _ord_ array for ascii lookup
    low = 0; high = 127;
    for (i = low; i <= high; i++) {
        t = sprintf("%c", i);
        _ord_[t] = i;
    }
}

function internet_checksum(str, chunk) {
    # checksum from a string and chunk size (by default 4)
    # with chunk==2, this [should be] the RFC1071 Internet Checksum (haven't tested yet)
    chunk = (chunk == 0) ? 4 : chunk;
    size = length(str);
    split(str, chars, "");
    sum = 0;
    for (i = 1; i <= size; i += chunk) {
        cur = 0;
        val = _ord_[chars[i]];
        for (j = 0; (j < chunk) && ((i + j) <= size); j++) {
            cur += lshift(val, 8 * j);
        }
        sum += cur;
    }
    return compl(sum);
}
BEGIN {_ord_init()}