random_graph: edges empty inside random_edge subroutine (changed behaviour of module)

[kester-habermann]

I have some old code that used to work in 0.96 and no longer works in the current version. Either this got broken or I misundertand how this is supposed to work and it only worked for me by accident in the past.

#!/usr/bin/perl

use Graph;

sub d($$$$) {
    my ($g, $u, $v, $p) = @_;

    print("edges: ", scalar $g->edges(), "\n");
    my $result = 1;

    $result = 0 if $g->out_degree($u) > 0;
    $result = 0 if $g->in_degree($v) > 0;

    return $result;
}

my @verts = qw/A B C D E/;

my $g = Graph->random_graph(vertices => \@verts,
    random_edge => \&d,
    edges => scalar @verts,
    random_seed => 1234);

print "$g\n";

Inside of the code passed to random_edge, the graph has no edges. This breaks the checks that follow. The code is supposed to generate a random graph with each node having exactly one incoming and one outgoing edge.

Result using version: 0.9732:

edges: 0
edges: 0
edges: 0
edges: 0
edges: 0
C-A,C-B,C-D,C-E,E-C

C has four outgoing edges. The last version this worked as expected was version: 0.9716:

edges: 0
edges: 1
edges: 1
edges: 1
edges: 1
edges: 2
edges: 2
edges: 2
edges: 2
edges: 2
edges: 2
edges: 3
edges: 3
edges: 3
edges: 3
edges: 4
edges: 4
edges: 4
edges: 4
edges: 4
A-B,B-A,C-D,D-E,E-C

Each vertex has exactly one outgoing and one incoming edge and we see that edges inside the subroutine is not empty. Version 0.9717 shows the same behaviour as the latest version.