use constant SRC  => 0;
use constant TGT  => 1;
use constant NET  => 2;
use constant SEEN => 3;
use constant STOP => 4;
use constant WORK => 5;
use constant PATH => 6;

sub find_path {
    &init if ! defined $_[STOP];
    return $_[PATH] if $_[PATH];
    &next_hop;
    return $_[PATH] if $_[PATH];
    goto &find_path if @{$_[WORK]};
    return 'path not found';
}

sub next_hop {
    my ($tgt, $stop, $net, @work) = (@_[TGT, STOP, NET], @{$_[WORK]});
    $_[WORK] = [];
    for (@work) {
        my ($node, $path) = @{$_}{qw/key path/};
        next if $_[SEEN]->{$node}++;
        $_[PATH] = $path, return if $node eq $tgt;
        $_[PATH] = "$path=>$tgt", return if $stop->{$node};
        push @{$_[WORK]}, map {key => $_, path => "$path=>$_"}, @{$net->{$node}};
    }
}

sub init {
    my ($src, $tgt, $net) = @_[SRC, TGT, NET];
    %{$_[STOP]} = map {$_ => 1} @{$net->{$tgt}};
    for (@{$net->{$src}}) {
        $_[PATH] = "$src=>$_", return if $_ eq $tgt;
        push @{$_[WORK]}, {key => $_, path => "$src=>$_"};
    }
}

##</code><code>##

sub find_path {
    my ($beg, $end, $net, $seen, $stop, $work) = @_;
    if (! defined $stop) {
        %$stop = map {$_ => 1} @{$net->{$end}};
        for (@{$net->{$beg}}) {
            return "$beg=>$_" if $_ eq $end; 
            push @$work, {key => $_, path => "$beg=>$_"};
        }
    }
    my $next = [];
    for (@$work) {
        my ($node, $path) = @{$_}{qw/key path/};
        next if $seen->{$node}++;
        return $path if $node eq $end;
        return "$path=>$end" if $stop->{$node};
        push @$next, map {key => $_, path => "$path=>$_"}, @{$net->{$node}};
    }
    return find_path($beg, $end, $net, $seen, $stop, $next) if @$next;
    return 'path not found';
}