Accessing parent level using Data::Visitor::Callback (Perl) - perl

Question

This is a long-shot, but are there any perl developers out there who know anything about Data::Visitor::Callback?
I have a complex data structure that I am traversing. Each time I find a hash that has a 'total' key, I need to build up a URL. I have some of the data that I need to create these URLs but some of the data comes from higher up in the structure.
I don't think I can access the levels above and that makes it impossible to build my URLs. I only just realised that I needed data from higher up the structure.
If I can't make Data::Visitor::Callback work for me, then it means rolling my own traversal code - which is a pain.
Data I am traversing is converted from the following JSON (the "count" keys are renamed to "total" as part of the conversion process):
[
{
"field": "field_name",
"value": "A",
"count": 647,
"pivot": [
{
"field": "field_name",
"value": "B",
"count": 618,
"pivot": [
{
"field": "field_name",
"value": "C1",
"count": 572
},
{
"field": "field_name",
"value": "C2",
"count": 266
},
{
"field": "field_name",
"value": "C3",
"count": 237
}
]
},
...
Once I get to the deepest level (C), I need both A and B values in order to construct my URLs.
Because Data::Visitor::Callback is acting on each leaf independently, I'm not sure that it 'knows' where in the structure it is.
All help very much appreciated.
Thanks.

Answer 1

Given the JSON you posted is in the variable $json_string, the following code uses recursion to add the parents to all the children in the hash key parent, that way you can access the parents in your code.
use strict;
use warnings;
use JSON;
my $data = decode_json($json_string);
add_parent_to_children($_) for #$data;
sub add_parent_to_children {
my $node = shift;
my $parent = shift;
$node->{parent} = $parent if $parent;
if ($node->{pivot}) {
add_parent_to_children($_, $node) for #{$node->{pivot}};
}
}
Demo:
my $c3 = $data->[0]{pivot}[0]{pivot}[2];
print "$c3->{value}\n"; # prints C3
print "$c3->{parent}{value}\n"; # prints B
print "$c3->{parent}{parent}{value}\n"; # prints A