Add super rudimentary dependency graph solver

app/Util/DependencyGraph.php

@@ -0,0 +1,123 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Util;
+
+class DependencyGraph
+{
+	/** @var DependencyGraphNode[] */
+	protected $nodes;
+
+	public function __construct()
+	{
+		$this->nodes = [];
+	}
+
+	public function add(string $name, array $depends_on = []): void
+	{
+		if (!isset($this->nodes[$name]))
+		{
+			$node = new DependencyGraphNode($name);
+			$this->nodes[$name] = $node;
+			$node->DependsOn = $depends_on;
+		}
+		else
+		{
+			// Probably unnecessary -- if the same node is added multiple times
+			// with different dependencies something's probably way messed up,
+			// but... we'll try our best?!
+			$this->nodes[$name] = array_unique(array_merge($this->nodes[$name]->DependsOn, $depends_on));
+		}
+	}
+
+	public function link(): void
+	{
+		foreach ($this->nodes as $node)
+		{
+			foreach ($node->DependsOn as $depends_on)
+			{
+				if (!isset($this->nodes[$depends_on]))
+				{
+					throw new \Exception('Unmet dependency on node: '.$depends_on);
+				}
+
+				if (!in_array($node->Name, $this->nodes[$depends_on]->DependedOnBy))
+				{
+					$this->nodes[$depends_on]->DependedOnBy[] = $node->Name;
+				}
+			}
+		}
+	}
+
+	public function flatten(): array
+	{
+		$state = [];
+
+		// Find leaf nodes, add each one to our state array.
+		foreach ($this->nodes as $node)
+		{
+			if (sizeof($node->DependedOnBy) == 0)
+			{
+				$state[] = $node;
+			}
+		}
+
+		// Then walk through the array and insert each node's dependencies before
+		// it recursively.
+		$iter = 0;
+		for ($i = 0; $i < sizeof($state); $i++)
+		{
+			if ($state[$i]->Processed)
+			{
+				continue;
+			}
+			$state[$i]->Processed = true;
+			$depends_on = [];
+			foreach ($state[$i]->DependsOn as $depends_on_name)
+			{
+				$depends_on[] = $this->nodes[$depends_on_name];
+			}
+			array_splice($state, $i, 0, $depends_on);
+			--$i;
+		}
+
+		// Remove all but the first appearance of a node.
+		// array_unique guarantees that the _first_ element will be retained,
+		//   so this satisfies what we're trying to do.
+		return array_unique(array_map(static function($node) { return $node->Name; }, $state));
+	}
+}
+
+class DependencyGraphNode
+{
+	/** @var string */
+	public $Name;
+
+	/** @var string */
+	public $DependsOn;
+
+	/** @var string */
+	public $DependedOnBy;
+
+	/** @var bool */
+	public $Processed;
+
+	public function __construct(string $name)
+	{
+		$this->Name = $name;
+		$this->DependsOn = [];
+		$this->DependedOnBy = [];
+		$this->Processed = false;
+	}
+
+	public function dependsOn(string $node): void
+	{
+		$this->DependsOn[] = $node;
+	}
+
+	public function dependedOnBy(string $node): void
+	{
+		$this->DependedOnBy[] = $node;
+	}
+}