#!/usr/bin/env perl -l

use strict;
use warnings;

use Data::Dump;

my @parent = (
    { Europe => [ qw{ London Moscow } ] },
    { Asia => [ qw{ Tokyo Seoul } ] },
    { Africa => [ qw{ Cairo Algiers } ] },
);

my @current;
reset_current(\@parent, \@current);

print '*** Starting State ***';
print_current_state(\@current);

print '*** Asia Down ***';
failover(Asia => \@current);
print_current_state(\@current);

print '*** Reset State ***';
reset_current(\@parent, \@current);
print_current_state(\@current);

print '*** Europe Down ***';
failover(Europe => \@current);
print_current_state(\@current);

print '*** Asia Down ***';
failover(Asia => \@current);
print_current_state(\@current);

sub failover {
    my ($down, $current) = @_;

    my @cities_to_failover
        = map { (keys %$_)[0] eq $down ? @{$_->{$down}} : () } @$current;

    for (@$current) {
        my ($continent) = keys %$_;
        next unless @{$_->{$continent}};

        if ($continent eq $down) {
            @{$_->{$continent}} = ();
        }
        else {
            push @{$_->{$continent}}, shift @cities_to_failover;
        }
    }
}

sub print_current_state {
    my ($current) = @_;
    dd $current;
}

sub reset_current {
    my ($parent, $current) = @_;

    @$current = ();

    for (@$parent) {
        my ($continent) = keys %$_;
        push @$current, { $continent => [ @{$_->{$continent}} ] };
    }
}

##</code><code>##

*** Starting State ***
[
  { Europe => ["London", "Moscow"] },
  { Asia => ["Tokyo", "Seoul"] },
  { Africa => ["Cairo", "Algiers"] },
]

*** Asia Down ***
[
  { Europe => ["London", "Moscow", "Tokyo"] },
  { Asia => [] },
  { Africa => ["Cairo", "Algiers", "Seoul"] },
]

*** Reset State ***
[
  { Europe => ["London", "Moscow"] },
  { Asia => ["Tokyo", "Seoul"] },
  { Africa => ["Cairo", "Algiers"] },
]

*** Europe Down ***
[
  { Europe => [] },
  { Asia => ["Tokyo", "Seoul", "London"] },
  { Africa => ["Cairo", "Algiers", "Moscow"] },
]

*** Asia Down ***
[
  { Europe => [] },
  { Asia => [] },
  { Africa => ["Cairo", "Algiers", "Moscow", "Tokyo"] },
]