feat: validate and resolve unit movement

src/Domain/CombatEngine.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace BattleForge\Domain;
+
+final class CombatEngine
+{
+    public function move(MatchState $match, string $unitId, Position $destination): MatchState
+    {
+        $this->assertMatchActive($match);
+
+        $unit = $match->unit($unitId);
+        $this->assertCanAct($match, $unit);
+
+        $occupied = [];
+
+        foreach ($match->units as $otherUnit) {
+            if ($otherUnit->id !== $unit->id && !$otherUnit->isDefeated()) {
+                $occupied[] = $otherUnit->position;
+            }
+        }
+
+        $reachable = $match->battlefield->reachable($unit->position, $unit->speed, $occupied);
+
+        if ($destination->key() === $unit->position->key() || !isset($reachable[$destination->key()])) {
+            throw new CombatException('Destination is not reachable.');
+        }
+
+        $movedUnit = $unit->moveTo($destination)->spendAction();
+        $next = $match->withUnit($movedUnit);
+        $actionLog = [...$match->actionLog, "{$unit->id} moved to {$destination->key()}"];
+
+        return $next->copy(actionLog: $actionLog);
+    }
+
+    private function assertMatchActive(MatchState $match): void
+    {
+        if ($match->winnerTeamId !== null) {
+            throw new CombatException('The match is already complete.');
+        }
+    }
+
+    private function assertCanAct(MatchState $match, UnitState $unit): void
+    {
+        if ($unit->teamId !== $match->activeTeamId) {
+            throw new CombatException("It is not this unit's turn.");
+        }
+
+        if ($unit->isDefeated()) {
+            throw new CombatException('Defeated units cannot act.');
+        }
+
+        if ($unit->actionsRemaining === 0) {
+            throw new CombatException('Unit has no actions remaining.');
+        }
+    }
+}

src/Domain/CombatException.php

@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace BattleForge\Domain;
+
+use DomainException;
+
+final class CombatException extends DomainException
+{
+}