<?php

/**
 * A Directed Acyclic Graph - used for doing topological sorts on dependencies, such as the before/after conditions
 * in config yaml fragments
 */
class SS_DAG {
	/** @var array|null - The nodes/vertices in the graph. Should be a numeric sequence of items (no string keys, no gaps). */
	protected $data;

	/** @var array - The edges in the graph, in $to_idx => [$from_idx1, $from_idx2, ...] format */
	protected $dag;

	function __construct($data = null) {
		$data = $data ? array_values($data) : array();

		$this->data = $data;
		$this->dag = array_fill_keys(array_keys($data), array());
	}

	/**
	 * Add another node/vertex
	 * @param $item anything - The item to add to the graph
	 */
	function additem($item) {
		$this->data[] = $item;
		$this->dag[] = array();
	}

	/**
	 * Add an edge from one vertex to another
	 * @param $from integer|any - The index in $data of the node/vertex, or the node/vertex itself, that the edge goes from
	 * @param $to integer|any - The index in $data of the node/vertex, or the node/vertex itself, that the edge goes to
	 *
	 * When passing actual nodes (as opposed to indexes), uses array_search with strict = true to find
	 */
	function addedge($from, $to) {
		$i = is_numeric($from) ? $from : array_search($from, $this->data, true);
		$j = is_numeric($to) ? $to : array_search($to, $this->data, true);

		if ($i === false) throw new Exception("Couldnt find 'from' item in data when adding edge to DAG");
		if ($j === false) throw new Exception("Couldnt find 'to' item in data when adding edge to DAG");

		if (!isset($this->dag[$j])) $this->dag[$j] = array();
		$this->dag[$j][] = $i;
	}

	/**
	 * Sort graph so that each node (a) comes before any nodes (b) where an edge exists from a to b
	 * @return array - The nodes
	 * @throws Exception - If the graph is cyclic (and so can't be sorted)
	 */
	function sort() {
		$data = $this->data; $dag = $this->dag; $sorted = array();

		while (true) {
			$withedges = array_filter($dag, 'count');
			$starts = array_diff_key($dag, $withedges);

			if (!count($starts)) break;

			foreach ($starts as $i => $foo) $sorted[] = $data[$i];

			foreach ($withedges as $j => $deps) {
				$withedges[$j] = array_diff($withedges[$j], array_keys($starts));
			}

			$dag = $withedges;
		}

		if ($dag) throw new Exception("DAG has cyclic requirements");
		return $sorted;
	}
}