Add "descendants" property to collection of nodes in nested set format with a generator

with_descendants_generator.php

<?php
/**
 * Simply returns the titles of the nodes passed to it
 *
 * @param array $nodes Collection of nodes we want to have the titles of
 */
function titles(array $nodes) {
  return array_map(
    function ($node) {
      return $node['title'];
    },
    $nodes
  );
}

/**
 * Generator that adds the descendants property to each node in a collection
 *
 * @param array $nodes pre-order walk of nodes in nested set format
 */
function with_descendants(array $nodes) {
  // This method of determining descendants only works if we have a continuous
  // list of bounds. The first node should be the root node of this subtree,
  // so if it fits "snugly" around the other nodes and we assume the nodes are
  // in pre-order, all should be fine
  assert(
    (($nodes[0]['right'] - $nodes[0]['left'] - 1) / 2) === count($nodes) - 1,
    "Cannot determine descendants when tree is not normalised"
  );
  
  // Just so we only have to process each node once to get their title
  // we cache the titles first
  $titles = titles($nodes);
  
  // Then we run through ordered nodes
  for ($i = 0; $i < count($nodes); $i++) {
    // We sometimes need to access the node
    $node = $nodes[$i];
    
    // We want to access the subtree of this node, so we calculate which
    // node would be the first node in the subtree, and how many nodes
    // would be in that subtree
    $first_node = $i + 1;
    $number_of_nodes = ($node['right'] - $node['left'] - 1) / 2;
      
    // Now we take the relevant slice out of titles
    $node['descendants'] = array_slice($titles, $first_node, $number_of_nodes);
    
    // We have added our property, so we can yield the node
    yield $node;
  }
}