public static function buildTree($s)
{
$t = new Tree();
$seenSymbols = array();
$pq = new PriorityQueue();
// Add leaf nodes for each symbol to the priority queue.
for ($i = 0; $i < strlen($s); ++$i) {
$c = $s[$i];
if (array_key_exists($c, $seenSymbols)) {
continue;
}
$occurrences = 0;
for ($j = $i; $j < strlen($s); ++$j) {
if ($s[$j] != $c) {
continue;
}
$occurrences++;
}
$node = new Node($c, $occurrences);
$pq->enqueue($node);
$t->leaves[$c] = $node;
$seenSymbols[$c] = true;
}
if ($pq->size() == 0) {
return $t;
}
// While there is more than one node left in the priority queue:
while ($pq->size() > 1) {
// Remove the two nodes of highest priority (lowest probability).
$node1 = $pq->dequeue();
$node2 = $pq->dequeue();
// Create a new internal node with the two nodes as children with
// p_total = p_1 + p_2
$newNode = new Node(null, $node1->weight + $node2->weight);
$newNode->left = $node1;
$newNode->right = $node2;
$node1->parent = $newNode;
$node2->parent = $newNode;
// Add the new node to the queue.
$pq->enqueue($newNode);
}
// The final node in the priority queue is the root.
$t->root = $pq->dequeue();
return $t;
}
