Convert a flat datastructure into a tree - perl

Question

I have an array of hashes. Each element in the array is a node in a hierarchical tree and has referential data for who the parent is. I will have thousands and hundreds of thousands of nodes in the tree... essentially an unknown set of nodes has to be converted to JSON (shown below) for use with http://bl.ocks.org/robschmuecker/7880033
UPDATE: position_id is a node in the heretical tree. placement_id is the parent's position_id (adjacency referential tree).
UPDATE: Here's the full AoH Data::Dumper result with Nested Set and Adjacency result from a modified version of DBIx::Tree::NestedSet (custom).
$VAR1 = [
{
'lft' => '673',
'id' => '109',
'date_created' => '2015-08-15',
'level' => '7',
'user_id' => '13',
'placement_id' => '11',
'position_id' => '13',
'status' => '1',
'structure_id' => '1',
'rght' => '684'
},
{
'placement_id' => '13',
'position_id' => '22',
'status' => '1',
'structure_id' => '1',
'rght' => '679',
'lft' => '674',
'date_created' => '2015-08-15',
'id' => '116',
'level' => '8',
'user_id' => '22'
},
{
'user_id' => '101',
'level' => '9',
'id' => '200',
'date_created' => '2015-08-15',
'lft' => '675',
'rght' => '676',
'structure_id' => '1',
'status' => '1',
'position_id' => '101',
'placement_id' => '22'
},
{
'date_created' => '2015-08-15',
'id' => '201',
'level' => '9',
'user_id' => '374',
'lft' => '677',
'structure_id' => '1',
'rght' => '678',
'placement_id' => '22',
'position_id' => '374',
'status' => '1'
},
{
'lft' => '680',
'user_id' => '95',
'level' => '8',
'id' => '117',
'date_created' => '2015-08-15',
'status' => '1',
'position_id' => '95',
'placement_id' => '13',
'rght' => '681',
'structure_id' => '1'
}
];
THIS IS THE GOAL, For this example I need to end up with:
{
"name": "13",
"children": [
{
"name": "22",
"children": [
{
"name": "101"
},
{
"name": "374"
}
]
},
{
"name": "95"
}
]
}
You can also see the format I am trying to arrive at here (minus size):
http://bl.ocks.org/robschmuecker/7880033#flare.json
My failed approach(es) included various attempts at looping through the array of hashes to create a recursive Hash of Hashes that can then be used with the JSON Perl module to create the actual JSON I need.

Answer 1

my $data = [
{ position_id => 123, placement_id => undef },
{ position_id => 456, placement_id => 123 },
{ position_id => 789, placement_id => 123 },
# ...
];
my $roots;
{
my %recs_by_name;
my %children_by_parent_name;
for my $row (#$data) {
my $name = $row->{position_id};
my $parent_name = $row->{placement_id};
my $rec = {
name => $name,
};
push #{ $children_by_parent_name{$parent_name // 'root'} }, $rec;
$recs_by_name{$name} = $rec;
}
$roots = delete($children_by_parent_name{root}) || [];
for my $name (keys(%children_by_parent_name)) {
my $children = $children_by_parent_name{$name};
if ( my $rec = $recs_by_name{$name} ) {
$rec->{children} = $children;
} else {
die("Parent $name doesn't exist.\n");
push #$roots, #$children;
}
}
}
print(Dumper($roots));
Tested.
You appear to have the depth of each node available to you (level). Simpler code could be used if your data was sorted by increasing depths.

Answer 2

While it was #ikegami who ultimately answered the question that led to the solution. I believe the following adaptation adds 4 important elements/clarifications I found helpful, and thought others reading this question and answer would also find useful.
1- Clear addition of all key,value pairs from the originating AoH to the resulting HOH. See while loop.
2- A Child node counter.
3- Inclusion and use of the encode_json function from JSON
4- The result is also an Array with a Hash as the first element. Newbies (like me) might find the explicit #{$roots}[0] passed to encode_json as helpful.
At first I had a similar adapted solution posted as an UPDATE within my question, but was admonished that it was bad etiquette and instructed to post an answer.
#ikegami's deserves the credit for the core of the solution.
sub get_jsonTree {
my ($array_of_hashes_ref) = #_;
my $roots;
my %recs_by_name;
my %children_by_parent_name;
my %count;
for my $row (#$array_of_hashes_ref) {
my $name = $row->{position_id};
my $parent_name = $row->{placement_id};
my $rec = {
name => $name,
};
## Added to loop through all key,value pairs and add them to $rec
while ( my ($key, $value) = each(%$row) ) {
$rec->{$key} = $value;
}
##Added To Count Child Nodes
$count{$parent_name} = 0 if (!$count{$parent_name});
$rec->{'child_count'} = $count{$parent_name};
$count{$parent_name}++;
push #{ $children_by_parent_name{$parent_name // 'root'} }, $rec;
$recs_by_name{$name} = $rec;
}
$roots = delete($children_by_parent_name{root}) || [];
for my $name (keys(%children_by_parent_name)) {
my $children = $children_by_parent_name{$name};
if ( my $rec = $recs_by_name{$name} ) {
$rec->{children} = $children;
} else {
$util{'test'} .= "Parent $name doesn't exist.\n<BR>";
push #$roots, #$children;
}
}
use JSON;
my $json_str = encode_json(#{$roots}[0]);
return $json_str;
}
my $array_of_hashes_ref = [
{ position_id => 123, placement_id => undef },
{ position_id => 456, placement_id => 123 },
{ position_id => 789, placement_id => 123 },
# ...
];
my $json_str = &get_jsonTree($array_of_hashes_ref);