What you want here is called a topological sort. First create a directed dependency graph of tasks and then apply a topological sort to that graph to produce an ordered list of tasks. Happily, there is a module
Sort::Topological that accomplishes this task:
my %children = (
'a' => [ 'b', 'c' ],
'c' => [ 'x' ],
'b' => [ 'x' ],
'x' => [ 'y' ],
'y' => [ 'z' ],
'z' => [ ],
);
sub children { @{$children{$_[0]} || []}; }
my @unsorted = ( 'z', 'a', 'x', 'c', 'b', 'y' );
my @sorted = toposort(\&children, \@unsorted);
%children gives the dependency graph, and the
sub children takes care of the null case.
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