RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved string parsing and subnet masks. (Note that this ends today.)

I didn't end up feeling particularly enthused about either of these problems, so I only did them in the core languages (Perl and Raku, Rust because I find it the best language for actually working up an algorithm, and PostScript because I love it).

Task 1: String Lie Detector

You are given a string.

Write a script that parses a self-referential string and determines whether its claims about itself are true. The string will make statements about its own composition, specifically the number of vowels and consonants it contains.

I have a string parser, of sorts.

sub stringliedetector($a) {

Split input into list of words.

  my @words = $a.split(' ').grep({$_ ne ""});

Count vowels and consonants in first word.

  my $vowels = 0;
  my $consonants = 0;
  for @words[0].comb -> $c {
    if ($c ~~ /<[aeiou]>/) {
      $vowels++;
    } else {
      $consonants++;
    }
  }

Build numeric lookup table.

  my %w2n = (
    "zero" => 0,
    "one" => 1,
    "two" => 2,
    "three" => 3,
    "four" => 4,
    "five" => 5,
    "six" => 6,
    "seven" => 7,
    "eight" => 8,
    "nine" => 9,
    "ten" => 10,
      );
  my $valid = True;
  my $mc = 0;

Iterate through descriptive words.

  for 2 .. @words.end -> $wi {
    my $w = @words[$wi];

If it's a number, set the number field.

    if (%w2n{$w}:exists) {
      $mc = %w2n{$w};

If it's "vowel" or "vowels", match number field to vowel count.

    } elsif ($w ~~ /^vowels?$/) {
      if ($mc != $vowels) {
        $valid = False;
      }

If it's "consonant" or "consonants", match number field to consonant count.

    } elsif ($w ~~ /^consonants?$/) {
      if ($mc != $consonants) {
        $valid = False;
      }
    }

And of any of those things has failed, drop out of the loop.

    if (!$valid) {
      last;
    }
  }
  $valid;  
}

This relies on the input being correct: "three vowels consonants" would match on three of each.

Task 2: Subnet Sheriff

You are given an IPv4 address and an IPv4 network (in CIDR format).

Write a script to determine whether both are valid and the address falls within the network. For more information see the Wikipedia article.

I only do some of the validation… Use the library modules!

Parse an address into a 32-bit integer. This will incorrectly pass addresses with a number of fields other than 4. Perl:

sub parseaddr($a) {
  my $out = 0;
  foreach my $os (split /\./, $a) {
    if ($os >= 0 && $os < 256) {
      $out *= 256;
      $out += $os;
    } else {
      return -1;
    }
  }
  $out;
}

sub subnetsheriff($addrs, $nets) {

Parse the address.

  my $addr = parseaddr($addrs);
  if ($addr >= 0) {

Split the network and parse the address part.

    my @netsc = split '/', $nets;
    my $netmask = parseaddr($netsc[0]);
    if ($netmask >= 0) {

Parse the prefix length.

      my $netlen = 0 + $netsc[1];
      if ($netlen >= 0 && $netlen <= 32) {

Build an address mask, and generate the first and last addresses in the range using it.

        my $bits = (1 << (32 - $netlen)) - 1;
        my $netlo = $netmask & !$bits;
        my $nethi = $netmask | $bits;

Check that the address is within the range.

        if ($addr >= $netlo && $addr <= $nethi) {
          return 1;
        }
      }
    }
  }

Or if any of this has failed, report failure.

  0;
}

Full code on codeberg.