/**
* @return string|null
*/
public function hash()
{
if ($this->hash) {
return $this->hash;
}
if (is_null($this->first) && is_null($this->second)) {
return null;
}
$first = $this->first;
$second = $this->second;
if ($this->first instanceof ITreeNode) {
$first = $this->first->hash();
$second = $this->second->hash();
if ($first === null || $second === null) {
return null;
}
}
$hash = call_user_func($this->hasher, $first . $second);
if (!is_string($hash)) {
throw new UnexpectedValueException('Hash callback must return a string.');
}
$this->hash = $hash;
$this->first = false;
$this->second = false;
return $this->hash;
}
/**
* @return null
*/
private function resolveHashes()
{
$res = $this->treeRoot->hash();
if (is_null($res)) {
return;
}
if ($this->finished) {
call_user_func($this->finished, $res);
}
}