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wp_find_hierarchy_loop_tortoise_hare() WP 3.1.0

Use the "The Tortoise and the Hare" algorithm to detect loops.

For every step of the algorithm, the hare takes two steps and the tortoise one. If the hare ever laps the tortoise, there must be a loop.

No Hooks.

Return

Mixed. Scalar ID of some arbitrary member of the loop, or array of IDs of all members of loop if $_return_loop

Usage

wp_find_hierarchy_loop_tortoise_hare( $callback, $start, $override, $callback_args, $_return_loop );
$callback(callable) (required)
Function that accepts ( ID, callback_arg, ... ) and outputs parent_ID.
$start(int) (required)
The ID to start the loop check at.
$override(array)
An array of ( ID => parent_ID, ... ) to use instead of $callback.
Default: empty array
$callback_args(array)
Additional arguments to send to $callback.
Default: empty array
$_return_loop(true/false)
Return loop members or just detect presence of loop? Only set to true if you already know the given $start is part of a loop (otherwise the returned array might include branches).
Default: false

Code of wp find hierarchy loop tortoise hare: wp-includes/functions.php WP 5.2.1

<?php
function wp_find_hierarchy_loop_tortoise_hare( $callback, $start, $override = array(), $callback_args = array(), $_return_loop = false ) {
	$tortoise = $hare = $evanescent_hare = $start;
	$return   = array();

	// Set evanescent_hare to one past hare
	// Increment hare two steps
	while (
		$tortoise
	&&
		( $evanescent_hare = isset( $override[ $hare ] ) ? $override[ $hare ] : call_user_func_array( $callback, array_merge( array( $hare ), $callback_args ) ) )
	&&
		( $hare = isset( $override[ $evanescent_hare ] ) ? $override[ $evanescent_hare ] : call_user_func_array( $callback, array_merge( array( $evanescent_hare ), $callback_args ) ) )
	) {
		if ( $_return_loop ) {
			$return[ $tortoise ] = $return[ $evanescent_hare ] = $return[ $hare ] = true;
		}

		// tortoise got lapped - must be a loop
		if ( $tortoise == $evanescent_hare || $tortoise == $hare ) {
			return $_return_loop ? $return : $tortoise;
		}

		// Increment tortoise by one step
		$tortoise = isset( $override[ $tortoise ] ) ? $override[ $tortoise ] : call_user_func_array( $callback, array_merge( array( $tortoise ), $callback_args ) );
	}

	return false;
}