RogerBW's Blog

The Weekly Challenge 185: Big MAC and Mask 06 October 2022

I’ve been doing the Weekly Challenges. The latest involved various string manipulations. (Note that this is open until 9 October 2022.)

Task 1: MAC address

You are given MAC address in the form i.e. hhhh.hhhh.hhhh.

Write a script to convert the address in the form hh:hh:hh:hh:hh:hh.

I'm not required to validate it. So my basic approach here is to scan the string, accepting only hex digits and ignoring everything else, and inserting colons where appropriate based on the count of valid characters. Rust's match does this very neatly:

fn recomposemac(inp: &str) -> String {
    let mut out = String::new();
    let mut count: u8 = 0;
    for c in inp.chars() {
        match c {
            '0'..='9' | 'a'..='f' => {
                if count == 2 {
                    count = 0;
                count += 1;
            _ => {}

In PostScript, I build the output as an array of characters, then convert it to a string at the end.

/recomposemac {
    6 dict begin
    /zero (0) 0 get def
    /nine (9) 0 get def
    /a (a) 0 get def
    /f (f) 0 get def
    /colon (:) 0 get def
    /ct 0 def
    [ exch
          /c exch def
          c zero ge c nine le and
          c a ge c f le and or {
              ct 2 eq {
                  /ct 0 def
              } if
              /ct ct 1 add def
          } if
      } forall
    ] a2s
} bind def

Task 2: Mask Code

You are given a list of codes in many random format.

Write a script to mask first four characters (a-z,0-9) and keep the rest as it is.

In other words, change the first four alphanumeric characters to "x" characters. This is conceptually very similar to part 1 (with the slight extra complication of having to process a whole list of strings rather than just one), and indeed both of them are quite similar to last week's task 2.


sub recomposemaskcode(@list) {
    my @out;
    for @list -> $ins {
        my $count = 0;
        my $os = '';
        for $ins.comb -> $c {
            if (($c ge '0' && $c le '9') || ($c ge 'a' && $c le 'z')) {
                if ($count < 4) {
                    $os ~= 'x';
                } else {
                    $os ~= $c;
            } else {
                $os ~= $c;
    return @out;

Probably one should avoid the two occurrences of appending $c, for example with a nextafter the $os ~= 'x', and I'd probably do this in production code – but not all languages can do it cleanly.

Full code on github.

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