RogerBW's Blog

Perl Weekly Challenge 48: survivor and palindromic dates 16 March 2020

I’ve been doing the Perl Weekly Challenges. The latest was about calculating the survivor of a stabbing game, and determining palindromic dates.

For the survivor challenge, the main thought was in working out an appropriate data structure; it turned out that filling a list with ID numbers, then splicing it as they're removed, worked best.

my @list=(1..50);
my $n=0;
while (scalar @list > 1) {
  if ($n>$#list) {
  splice @list,$n,1;

print $list[0],"\n";

Perl6 is the same apart from minor changes in syntax. (With the same off-by-one error.)

For palindromic dates, I started off doing it the obvious way, using actual date formatting functions.

my $format='%m%d%Y';
foreach my $d (10957..376199) {
  my $u=strftime($format,gmtime($d*86400));
  if ((scalar reverse $u) eq $u) {
    print "$u\n";

This lets me trivially change the format being used to search for other types of palindromic date. However, Perl6 doesn't have strftime, so I built nested loops, with a checker to skip over non-existent days at the ends of months.

for 2000..2999 -> $y {
  for 1..12 -> $m {
    for 1..31 -> $d {
      if ($d==31 && ($m==4 || $m==6 || $m==9 || $m==11)) {
      } elsif ($m==2 && $d==30) {
      } elsif ($m==2 && $d==29 && ($y % 4 != 0 || ($y % 100 == 0 && $y % 400 != 0))) {
      my $u=sprintf('%02d%02d%04d',$m,$d,$y);
      if ($u eq $u.flip) {
        say $u;

Which is fine and gets the job done, but where my Perl5 version above runs in about 6.4 seconds, this takes 109 on the same machine. That seems odd. Surely there's nothing particularly complicated going on here? In order to test it, I reimplemented this nested-loop algorithm in Perl5… where it runs in 0.2 seconds (so presumably there's a fair old overhead on the strftime calls in the original).

This is the first time I've seen Perl6 being this dog-slow at something. The sprintf maybe?

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