jcrush:

Assuming you're parsing log files (naturally sorted in order by time), I'd suggest something like this:

my ($prevDateHour,%curHour);
while (<>) {
    my ($dateHour, $usrMac, $apMac) = parsit();
    if ($dateHour ne $prevDateHour) {
        for my $ap (sort keys %curHour) {
            print $prevDateHour, $ap, scalar(keys %{$curHour{$ap}}), "
+\n";
        }
        %curHour=();
        $prevDateHour = $dateHour;
    }
    $curHour{$apMac}{$usrMac} = 1;
}
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(If they're not sorted, or you have multiple files, merge/sort them into a single file first.) This solution will work no matter *how* many hours your days have. ;^D

Update: Tweaked code a little (reset curHour, prevDateHour so reporting could work correctly).

...roboticus

When your only tool is a hammer, all problems look like your thumb.