NOTE: I am trying to find the name of the specific LRU algorithm, not

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Asked: May 20, 20262026-05-20T19:42:57+00:00 2026-05-20T19:42:57+00:00

NOTE: I am trying to find the name of the specific LRU algorithm, not

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NOTE: I am trying to find the name of the specific LRU algorithm, not that this is a caching algorithm (I know it is, I wrote it). Telling me this is a caching algorithm is like telling someone looking for the name red-black tree that it is a tree balancing algorithm.

I recently created the following algorithm, but I am fairly certain someone must have done this before me and given it a name. Does this look familiar to anyone?

Purpose: Keep a fixed size pool of strings and the number of times they have been seen. If the pool exceeds the max size, only keep the most recently used items.

Pseudocode:

var cur
var old

func add_key(key)

    if cur not defined
        put a hash in cur

    if key in old
        copy value from old to cur for this key
        delete key from old

    increment cur[key]

    if there are too many keys in cur
        replace old with cur
        empty cur
        copy value from old to cur for this key
        delete key from old

    return cur[key]

A simple implementation in Perl 5 looks like:

#!/usr/bin/perl

use strict;
use warnings;

{ package Fixed::LRU::Counter;

    sub new {
        my ($class, $max) = @_;
        return bless {
            max => $max,
            cur => {},
            old => {},
        }, $class;
    }

    sub add_key {
        my ($self, $k) = @_;

        if ($self->{old}{$k}) {
            $self->{cur}{$k} = $self->{old}{$k};
            delete $self->{old}{$k};
        }

        $self->{cur}{$k}++;

        if (keys %{$self->{cur}} > $self->{max}) {
            $self->{old} = $self->{cur};
            $self->{cur} = { $k => $self->{old}{$k} };
            delete $self->{old}{$k};
        }

        return $self->{cur}{$k};
    }
}

my $c = Fixed::LRU::Counter->new(3);

for my $k (qw/a a b c d e f f g a f/) {
    print "$k: ", $c->add_key($k), "\n";
}