fix: validate battlefield reachability inputs

src/Domain/Battlefield.php

@@ -9,29 +9,46 @@ use SplQueue;
 
 final readonly class Battlefield
 {
+    /** @var array<string, Terrain> */
+    private array $terrain;
+
     /**
-     * @param array<string, Terrain> $terrain
+     * @param array<array-key, mixed> $terrain
      */
     public function __construct(
         public int $width,
         public int $height,
-        private array $terrain = [],
+        array $terrain = [],
     ) {
         if ($width < 8 || $width > 16 || $height < 8 || $height > 16) {
             throw new InvalidArgumentException('Battlefield dimensions must each be between 8 and 16.');
         }
 
-        foreach (array_keys($terrain) as $key) {
-            if (preg_match('/^(\d+):(\d+)$/', $key, $matches) !== 1) {
+        $validatedTerrain = [];
+
+        foreach ($terrain as $key => $terrainType) {
+            if (!$terrainType instanceof Terrain) {
+                throw new InvalidArgumentException("Invalid terrain value at coordinate {$key}.");
+            }
+
+            if (!is_string($key) || preg_match('/^(\d+):(\d+)$/', $key, $matches) !== 1) {
                 throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
             }
 
             $position = new Position((int) $matches[1], (int) $matches[2]);
 
+            if ($key !== $position->key()) {
+                throw new InvalidArgumentException("Invalid terrain coordinate: {$key}.");
+            }
+
             if (!$this->contains($position)) {
                 throw new InvalidArgumentException("Terrain coordinate {$key} is outside the battlefield.");
             }
+
+            $validatedTerrain[$key] = $terrainType;
         }
+
+        $this->terrain = $validatedTerrain;
     }
 
     public function contains(Position $position): bool
@@ -53,9 +70,21 @@ final readonly class Battlefield
      */
     public function reachable(Position $start, int $budget, array $occupied): array
     {
+        if (!$this->contains($start)) {
+            throw new InvalidArgumentException('Reachability start position is outside the battlefield.');
+        }
+
+        if ($budget < 0) {
+            throw new InvalidArgumentException('Reachability budget cannot be negative.');
+        }
+
         $occupiedKeys = [];
 
         foreach ($occupied as $position) {
+            if (!$this->contains($position)) {
+                throw new InvalidArgumentException('Occupied position is outside the battlefield.');
+            }
+
             $occupiedKeys[$position->key()] = true;
         }