RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved quinelikes and list processing. (Note that this ends today.)

Task 1: Self Spammer

Write a program which outputs one word of its own script / source code at random. A word is anything between whitespace, including symbols.

Clearly this is an allied trade to the quine (a program that produces its own source). And I can see the value of this kind of game. But it's really not my style, so I didn't play.

Task 2: Order Game

You are given an array of integers, @ints, whose length is a power of 2.

Write a script to play the order game (min and max) and return the last element.

Examples explain that "the order game (min and max)" consists of running a sliding window of 2 along the list, and taking either the lower or the higher value in alternation. This is done repeatedly until there is only one value left.

One could probably do this in place with splice operations, but it seemed more straightforward to have one list for reading and one for writing. For example in Perl (using the slightly clumsy slideatatime from List::MoreUtils):

sub ordergame($a) {

Initialise my working copy of the list.

  my @p = @{$a};

While there are at least two entries left,

  while (scalar @p > 1) {

Make a new (output) list and initialise the flip-flop.

    my @q;
    my $mm = 1;

Iterate over the overlapping pairs of list elements.

    my $dd = slideatatime 1, 2, @p;
    while (my @j = $dd->()) {

Take minimum or maximum and flip the state.

      if (scalar @j == 2) {
        if ($mm) {
          push @q, min(@j);
        } else {
          push @q, max(@j);
        }
        $mm = 1 - $mm;
      }
    }

Finally, copy the output list to the working copy, and ultimately return the sole entry.

    @p = @q;
  }
  return $p[0];
}

I'm rather fond of the PostScript implementation. It uses the same algorithm as above, but instead of having two explicitly-named lists, it uses the stack to build up the output list and then assigns it to the working copy. (My rotor is inspired by Raku's, but doesn't return partial groups.)

/ordergame {
    0 dict begin
    /p exch def
    {
        p length 1 le {
            exit
        } if
        /mm true def
        [
            p 2 -1 rotor {
                aload pop
                mm {
                    min
                } {
                    max
                } ifelse
                /mm mm not def
            } forall
        ] /p exch def
    } loop
    p 0 get
    end
} bind def

Full code on github.