RogerBW's Blog

Perl Weekly Challenge 49: multiples and LRU cache 22 March 2020

I’ve been doing the Perl Weekly Challenges. The latest was about searching for particular multiples of a number, and implementing a fixed-size cache.

Write a script to accept a positive number as command line argument and print the smallest multiple of the given number consists of digits 0 and 1.

For example:

For given number 55, the smallest multiple is 110 consisting of digits 0 and 1.

The easy way to match multiples seemed to be repeated addition combined with a regexp filter.

foreach my $n (@ARGV) {
  my $t=$n;
  while (1) {
    if ($t =~ /^[01]+$/) {
      print "$t\n";
    } else {

and much the same in Perl6. There might be a clever mathematical way to generate these but one didn't come to mind.

Write a script to demonstrate LRU Cache feature. It should support operations get and set. Accept the capacity of the LRU Cache as command line argument.

Definition of LRU: An access to an item is defined as a get or a set operation of the item. “Least recently used” item is the one with the oldest access time.

For the LRU cache, the basic need is a hash to store the data and a way to determine which entry has been most recently used. There are various possibilities for that combination, but none of them reduces to a pure hash, so I ended up with a plain list, updated by grepping to remove the key of the most-recently-used item and sticking it on the end. Plus lots of the usual Perl5 object sugar packed round it.

One insight: it's easier to add the new item to the cache and then delete the oldest if necessary than to check before adding whether a deletion will be needed.

package Local::LRU;

sub new {
  my $class = shift;
  my $self={};
  $self->{size}=shift || 3;
  bless $self,$class;
  return $self;

sub set {
  my $self=shift;
  my $k=shift;
  my $v=shift;
  if (scalar @{$self->{lru}} > $self->{size}) {
    delete $self->{store}{$self->{lru}[0]};
    shift @{$self->{lru}};

sub get {
  my $self=shift;
  my $k=shift;
  if (exists $self->{store}{$k}) {
    return $self->{store}{$k};
  } else {
    return -1;

sub update_lru {
  my $self=shift;
  my $k=shift;
  my @l=grep {$_ != $k} @{$self->{lru}};
  push @l,$k;

Then a basic test harness:

my $q=Local::LRU->new($ARGV[0] || 3);
print $q->get(2)," = 5\n";
print $q->get(1)," = 3\n";
print $q->get(4)," = -1\n";
print $q->get(3)," = -1\n";

Perl6 has different object sugar, which I haven't used before. It's a bit odd, but on balance quite pleasant to use.

class LRU {
  has Int $.size;
  has %!store;
  has @!lru;

  method set(Int $k, Int $v) {
    if @!lru.elems > $.size {

  method get(Int $k) {
    if %!store{$k}:exists {
      return %!store{$k};
    } else {
      return -1;

  method update_lru(Int $k) {
    my @l=grep {$_ != $k}, @!lru;

my $ => (@*ARGS[0] or 3));
print $q.get(2)," = 5\n";
print $q.get(1)," = 3\n";
print $q.get(4)," = -1\n";
print $q.get(3)," = -1\n";

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