From 4cdf05f6bc62f0718d3b8da36fdd7daaaefdaaaf Mon Sep 17 00:00:00 2001
From: Boone Gorges <boonebgorges@gmail.com>
Date: Tue, 5 May 2015 19:37:53 +0000
Subject: [PATCH] Improve performance of `get_page_children()`.

The new algorithm uses a hash table rather than function recursion, reducing
complexity to O(N). On large numbers of pages, the performance improvement is
several orders of magnitude.

Props santagada, hailin, mihai.
Fixes #10852.
Built from https://develop.svn.wordpress.org/trunk@32355


git-svn-id: http://core.svn.wordpress.org/trunk@32326 1a063a9b-81f0-0310-95a4-ce76da25c4cd
---
 wp-includes/post.php    | 35 ++++++++++++++++++++++++-----------
 wp-includes/version.php |  2 +-
 2 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/wp-includes/post.php b/wp-includes/post.php
index 4d1c61a768..3716cbae72 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -4317,25 +4317,38 @@ function get_page_by_title( $page_title, $output = OBJECT, $post_type = 'page' )
 }
 
 /**
- * Retrieve child pages from list of pages matching page ID.
+ * Identify descendants of a given page ID in a list of page objects.
  *
- * Matches against the pages parameter against the page ID. Also matches all
- * children for the same to retrieve all children of a page. Does not make any
- * SQL queries to get the children.
+ * Descendants are identified from the `$pages` array passed to the function. No database queries are performed.
  *
  * @since 1.5.1
  *
- * @param int   $page_id    Page ID.
- * @param array $pages      List of pages' objects.
+ * @param int   $page_id Page ID.
+ * @param array $pages   List of page objects from which descendants should be identified.
  * @return array List of page children.
  */
 function get_page_children( $page_id, $pages ) {
-	$page_list = array();
+	// Build a hash of ID -> children.
+	$children = array();
 	foreach ( (array) $pages as $page ) {
-		if ( $page->post_parent == $page_id ) {
-			$page_list[] = $page;
-			if ( $children = get_page_children( $page->ID, $pages ) ) {
-				$page_list = array_merge( $page_list, $children );
+		$children[ intval( $page->post_parent ) ][] = $page;
+	}
+
+	$page_list = array();
+
+	// Start the search by looking at immediate children.
+	if ( isset( $children[ $page_id ] ) ) {
+		// Always start at the end of the stack in order to preserve original `$pages` order.
+		$to_look = array_reverse( $children[ $page_id ] );
+
+		while ( $to_look ) {
+			$p = array_pop( $to_look );
+			$page_list[] = $p;
+			if ( isset( $children[ $p->ID ] ) ) {
+				foreach ( array_reverse( $children[ $p->ID ] ) as $child ) {
+					// Append to the `$to_look` stack to descend the tree.
+					$to_look[] = $child;
+				}
 			}
 		}
 	}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 357cb8f207..c8831440cb 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.3-alpha-32354';
+$wp_version = '4.3-alpha-32355';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.