#!/usr/bin/perl

use strict;
use warnings;
use Storable 'dclone';
use Data::Dumper;
use Test::More no_plan => 1;

sub orig_tree_data {
  my ( $old_data, $key_name, $next_group, @remaining_keys ) = @_;
  my ( @new_data, $last_key, $build_box );

  foreach ( @{ $old_data } ) {
    if ( $last_key and ( $last_key ne $_->{ $key_name } ) ) {
      push @new_data, {
        $key_name   => $last_key,
        $next_group => ( @remaining_keys )
          ? tree_data( $build_box, @remaining_keys )
          : $build_box,
      };
      undef $build_box;
    }
    $last_key = delete $_->{ $key_name };
    push @{ $build_box }, $_;
  }
  push @new_data, {
    $key_name   => $last_key,
    $next_group => ( @remaining_keys )
      ? tree_data( $build_box, @remaining_keys )
      : $build_box,
  };

  return \@new_data;
}

sub tree_data {
  my ( $old_data, $key_name, $next_group, @remaining_keys ) = @_;
  
  # guard against bad input and serve as recursion end case
  return $old_data unless defined $key_name;
  
  my @new_data;
  my $last_key = '';  # if this is initialized, you dont have to check below
  my $build_box = []; # initalize this explicity (more self documenting)
  
  # encapsulate code into inner sub
  # fortunately for us, all variables 
  # are closed over too :)
  my $push_new_data = sub {
      return unless $last_key;
      push @new_data, {
          $key_name   => $last_key,
          $next_group => tree_data( $build_box, @remaining_keys )
      };
      # re-init this 
      $build_box = [];
  };

  foreach ( @{ $old_data } ) {
    if ( $last_key ne $_->{ $key_name } ) {
      $push_new_data->();
    }
    $last_key = delete $_->{ $key_name };
    push @{ $build_box }, $_;
  }
  $push_new_data->();

  return \@new_data;
}

my $input = [
  {
    'team'       => 'A-Team',
    'employee'   => 'Face',
    'work_day'   => '2006-08-28',
    'other_data' => '123456789',
  },
  {
    'team'       => 'A-Team',
    'employee'   => 'Murdock',
    'work_day'   => '2006-08-28',
    'other_data' => '123456789',
  },
  {
    'team'       => 'Military',
    'employee'   => 'Decker',
    'work_day'   => '2006-08-28',
    'other_data' => '123456789',
  },
];
my $input2 = dclone($input);
my $input3 = dclone($input);
my $input4 = dclone($input);

is_deeply(
    tree_data( $input, 'team', 'employees' ),
    orig_tree_data( $input2, 'team', 'employees' ),
    '... both the same');
    
is_deeply(
    tree_data($input3, 'team', 'employees', 'employee', 'work_days'),
    orig_tree_data($input4, 'team', 'employees', 'employee', 'work_days'),    
    '... both the same again');