RogerBW's Blog

Perl Weekly Challenge 39: text parsing 25 December 2019

I’ve been doing the Perl Weekly Challenges. Last week's was about parsing a specific file format.

This is very straightforward stuff: read the file and generate a list of events from it.

my %ev;
open I,'<','data1';
while (<I>) {
  chomp;
  my @e=$_ =~ /(IN|OUT): (\d+):(\d+)/g;
  while (@e) {
    my $delta=(shift @e eq 'IN')?1:-1;
    my $t=(60*shift @e)+shift @e;
    $ev{$t}+=$delta;
  }
}

Replay the events in time order (we're tracking total occupancy of a building), and calculate for how long that occupancy was greater than zero. The example data were so simple that one could do this by casual inspection anyway, but it seems polite to solve for the general case.

my $ontime=0;
my $occ=0;
my $laston=0;
foreach my $t (sort {$a <=> $b} keys %ev) {
  my $lastocc=$occ;
  $occ+=$ev{$t};
  if ($lastocc==0 && $occ>0) {
    $laston=$t;
  } elsif ($lastocc>0 && $occ==0) {
    $ontime+=($t-$laston);
  }
}
print "$ontime\n";

Perl6 makes this more complicated by returning Match objects but is otherwise very similar.

for $fh.lines {
  my @e=($_ ~~ m:g/(IN|OUT) ':' \s* (\d+) ':' (\d+)/);
  while (@e) {
    my @match=@e.shift.values;
    my $delta=(@match.shift eq 'IN')??1!!-1;
    my $t=(60*@match.shift)+@match.shift;
    %ev{$t}+=$delta;
  }
}

And part 2… was to demonstrate Reverse Polish Notation, which I'd already written as an example for challenge 34 ("demonstrate a dispatch table").

See also:
Perl Weekly Challenge 34

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