RogerBW's Blog

Perl Weekly Challenge 45: bad codes and quines 21 February 2020

I’ve been doing the Perl Weekly Challenges. The latest was about a bad encoding scheme and producing the source of a program.

The encoder clearly needs to strip spaces, chop the source into sections, and slice transversely. This is all fairly standard stuff.

my $l=length($in)-1;
my @out;
foreach my $c (0..$o{n}-1) {
  my $out;
  for (my $k=$c;$k<=$l;$k+=$o{n}) {
    $out.=substr($in,$k,1);
  }
  push @out,$out;
}

print join(' ',@out),"\n";

It turns out that to decode you don't even need to know the key number (because it's given by the "word" length)). Decoding was not required, but one might as well.

my @in;
while (<>) {
  chomp;
  push @in,split ' ',$_;
}

my @out;
foreach my $n (0..length($in[0])-1) {
  foreach my $w (@in) {
    if (length($w)<$n+1) {
      last;
    }
    push @out,substr($w,$n,1);
  }
}

print join('',@out),"\n";

The Perl6 (Raku) encoder is similar.

As for producing quines, I've never had much interest; like perl golf (producing a program in as few characters as possible), I think it's harmful to the language community when this sort of thing is regarded as a major activity. So here's two fingers up to the computational purist, and a program that simply reads its own source code.

#! /usr/bin/perl

use strict;
use warnings;

open F,$0;
print <F>;

Perl6:

#! /usr/bin/perl6

my $f=open :r,$*PROGRAM-NAME;
for $f.lines {
  say $_;
}

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