VersionPress\Synchronizers\SynchronizerFactory::resolveSynchronizationSequence PHP Method

    private function resolveSynchronizationSequence()
    {
        $unresolved = $this->dbSchema->getAllReferences();
        $sequence = [];
        $triedRemovingSelfReferences = false;
        while (count($unresolved) > 0) {
            $resolvedInThisStep = [];
            // 1st step - move all entities with resolved dependencies to $sequence
            foreach ($unresolved as $entity => $deps) {
                if (count($deps) === 0) {
                    unset($unresolved[$entity]);
                    $sequence[] = $entity;
                    $resolvedInThisStep[] = $entity;
                }
            }
            // 2nd step - update unresolved dependencies of remaining entities
            foreach ($resolvedInThisStep as $resolvedEntity) {
                foreach ($unresolved as $unresolvedEntity => $deps) {
                    $unresolved[$unresolvedEntity] = array_diff($deps, [$resolvedEntity]);
                }
            }
            // Nothing changed - circular dependency
            if (count($resolvedInThisStep) === 0) {
                if (!$triedRemovingSelfReferences) {
                    // At first try to remove all self-references as they have to run in 2-pass sync anyway
                    $triedRemovingSelfReferences = true;
                    foreach ($unresolved as $unresolvedEntity => $deps) {
                        $unresolved[$unresolvedEntity] = array_diff($deps, [$unresolvedEntity]);
                    }
                } else {
                    // Simply eliminate dependencies one by one until it is resolvable
                    reset($unresolved);
                    $firstEntity = key($unresolved);
                    array_pop($unresolved[$firstEntity]);
                }
            }
        }
        return $sequence;
    }