279 lines
11 KiB
C#
279 lines
11 KiB
C#
// Assets/_Project/Scripts/Gameplay/PathfindingService.cs
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using System.Collections.Generic;
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using UnityEngine;
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using TD.Core;
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using TD.Levels;
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namespace TD.Gameplay
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{
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/// <summary>
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/// Scene singleton that computes shortest-path routes from any tile to the
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/// nearest goal tile using A* on the runtime walkability grid.
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/// </summary>
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/// <remarks>
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/// <b>Algorithm:</b> A* with Manhattan-distance heuristic. Grid cost is uniform
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/// (1 per step, 4-connected, no diagonals), so A* is optimal and significantly
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/// faster than plain BFS on large grids thanks to the heuristic.
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///
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/// <b>Who calls this:</b>
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/// <list type="bullet">
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/// <item><see cref="EnemyMovement"/> calls <see cref="ComputePath"/> once on
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/// spawn and again whenever <see cref="OnPathsInvalidated"/> fires.</item>
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/// </list>
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///
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/// <b>Invalidation:</b> Subscribes to <see cref="LevelLoader.OnWalkabilityChanged"/>.
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/// When a tower is placed or sold, <c>LevelLoader.SetWalkable</c> fires that event
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/// and <see cref="OnPathsInvalidated"/> is relayed to all active enemies, which
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/// each recompute their own path from their current tile.
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///
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/// <b>Goal tile set:</b> Built once on <c>Start</c> from
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/// <c>LevelLoader.LevelData.Goals[].TileArea</c>. Goal tiles never change at
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/// runtime (they are baked into the level), so there is no need to rebuild the set.
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///
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/// <b>No caching:</b> Paths are computed on demand per enemy. On typical TD grid
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/// sizes (50×50 or smaller) a single A* run takes <1 ms. If profiling shows
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/// otherwise, add a per-startTile cache here.
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/// </remarks>
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public class PathfindingService : MonoBehaviour
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{
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// ----- Singleton --------------------------------------------------
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public static PathfindingService Instance { get; private set; }
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// ----- Events -----------------------------------------------------
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/// <summary>
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/// Fired on every peer when the walkability grid changes (tower placed/sold).
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/// <see cref="EnemyMovement"/> subscribes per-instance to recompute its path.
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/// </summary>
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public event System.Action OnPathsInvalidated;
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// ----- State ------------------------------------------------------
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// Built once on Start from LevelData.Goals[].TileArea.
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private HashSet<Vector2Int> goalTiles;
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// A* scratch collections — allocated once and cleared per run to avoid GC.
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// PathfindingService is a singleton, so single-instance scratch is safe.
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private readonly Dictionary<Vector2Int, Vector2Int> cameFrom = new Dictionary<Vector2Int, Vector2Int>();
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private readonly Dictionary<Vector2Int, int> gScore = new Dictionary<Vector2Int, int>();
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private readonly SimplePriorityQueue openSet = new SimplePriorityQueue();
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// ----- Lifecycle --------------------------------------------------
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private void Awake()
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{
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if (Instance != null && Instance != this)
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{
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Debug.LogError("[PathfindingService] Duplicate instance detected. " +
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"Only one PathfindingService should exist per scene.");
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Destroy(gameObject);
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return;
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}
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Instance = this;
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}
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private void Start()
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{
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BuildGoalTileSet();
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var loader = LevelLoader.Instance;
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if (loader != null)
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loader.OnWalkabilityChanged += HandleWalkabilityChanged;
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else
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Debug.LogWarning("[PathfindingService] LevelLoader not found in Start. " +
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"Path invalidation will not work.");
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}
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private void OnDestroy()
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{
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if (Instance == this) Instance = null;
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var loader = LevelLoader.Instance;
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if (loader != null)
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loader.OnWalkabilityChanged -= HandleWalkabilityChanged;
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}
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// ----- Public API -------------------------------------------------
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/// <summary>
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/// Computes the shortest walkable path from <paramref name="startTile"/> to
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/// the nearest goal tile. Returns an ordered list of tiles to visit, starting
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/// with the first step AFTER <paramref name="startTile"/> and ending with the
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/// reached goal tile. Returns an empty list if no path exists or LevelLoader
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/// is unavailable (should not occur — TowerPlacementManager guarantees a path
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/// always exists after any placement).
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/// </summary>
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public List<Vector2Int> ComputePath(Vector2Int startTile)
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{
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var loader = LevelLoader.Instance;
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if (loader == null || !loader.IsLoaded || goalTiles == null || goalTiles.Count == 0)
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{
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Debug.LogWarning("[PathfindingService] Cannot compute path: " +
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"LevelLoader unavailable or no goal tiles baked.");
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return new List<Vector2Int>();
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}
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return RunAStar(startTile, loader);
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}
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// ----- A* implementation ------------------------------------------
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private List<Vector2Int> RunAStar(Vector2Int start, LevelLoader loader)
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{
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cameFrom.Clear();
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gScore.Clear();
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openSet.Clear();
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gScore[start] = 0;
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openSet.Enqueue(start, Heuristic(start));
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while (openSet.Count > 0)
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{
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Vector2Int current = openSet.Dequeue();
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if (goalTiles.Contains(current))
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return ReconstructPath(start, current);
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int currentG = gScore.TryGetValue(current, out int g) ? g : int.MaxValue;
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foreach (var neighbor in GridCoordinates.GetNeighbors(current))
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{
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if (!loader.IsWalkable(neighbor)) continue;
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int tentativeG = currentG + 1;
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if (gScore.TryGetValue(neighbor, out int existingG)
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&& tentativeG >= existingG) continue;
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cameFrom[neighbor] = current;
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gScore[neighbor] = tentativeG;
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int f = tentativeG + Heuristic(neighbor);
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// Re-enqueue with updated priority. SimplePriorityQueue handles
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// duplicate entries by ignoring higher-cost duplicates on dequeue.
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openSet.Enqueue(neighbor, f);
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}
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}
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// No path found — TowerPlacementManager should have prevented this.
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Debug.LogWarning($"[PathfindingService] A* found no path from {start}. " +
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"Check that TowerPlacementManager BFS is validating correctly.");
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return new List<Vector2Int>();
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}
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// Manhattan distance to the nearest goal tile. Admissible heuristic for
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// a 4-connected uniform-cost grid.
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private int Heuristic(Vector2Int tile)
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{
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int best = int.MaxValue;
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foreach (var goal in goalTiles)
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{
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int d = GridCoordinates.ManhattanDistance(tile, goal);
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if (d < best) best = d;
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}
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return best;
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}
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private List<Vector2Int> ReconstructPath(Vector2Int start, Vector2Int goal)
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{
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var path = new List<Vector2Int>();
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Vector2Int current = goal;
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while (current != start)
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{
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path.Add(current);
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if (!cameFrom.TryGetValue(current, out current))
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break; // shouldn't happen
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}
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path.Reverse();
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return path;
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}
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// ----- Helpers ----------------------------------------------------
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private void BuildGoalTileSet()
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{
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goalTiles = new HashSet<Vector2Int>();
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var loader = LevelLoader.Instance;
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if (loader == null || loader.LevelData == null || loader.LevelData.Goals == null)
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{
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Debug.LogWarning("[PathfindingService] No LevelData or Goals found. " +
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"Enemies will have no destination.");
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return;
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}
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foreach (var goal in loader.LevelData.Goals)
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{
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if (goal.TileArea == null) continue;
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foreach (var tile in goal.TileArea)
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goalTiles.Add(tile);
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}
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Debug.Log($"[PathfindingService] Goal tile set built: {goalTiles.Count} tiles.");
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}
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private void HandleWalkabilityChanged()
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{
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OnPathsInvalidated?.Invoke();
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}
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}
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// -------------------------------------------------------------------------
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// Minimal priority queue for A*. Uses a sorted list of (priority, tile) pairs.
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// Suitable for the grid sizes in this project (typically < 10,000 tiles).
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// Replace with a binary heap if profiling shows this as a bottleneck.
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// -------------------------------------------------------------------------
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internal sealed class SimplePriorityQueue
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{
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private readonly List<(int priority, Vector2Int tile)> heap
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= new List<(int, Vector2Int)>();
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public int Count => heap.Count;
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public void Clear() => heap.Clear();
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public void Enqueue(Vector2Int tile, int priority)
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{
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heap.Add((priority, tile));
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SiftUp(heap.Count - 1);
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}
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public Vector2Int Dequeue()
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{
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Vector2Int result = heap[0].tile;
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int last = heap.Count - 1;
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heap[0] = heap[last];
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heap.RemoveAt(last);
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if (heap.Count > 0) SiftDown(0);
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return result;
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}
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private void SiftUp(int i)
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{
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while (i > 0)
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{
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int parent = (i - 1) / 2;
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if (heap[parent].priority <= heap[i].priority) break;
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(heap[i], heap[parent]) = (heap[parent], heap[i]);
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i = parent;
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}
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}
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private void SiftDown(int i)
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{
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int count = heap.Count;
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while (true)
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{
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int smallest = i;
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int left = 2 * i + 1;
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int right = 2 * i + 2;
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if (left < count && heap[left].priority < heap[smallest].priority) smallest = left;
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if (right < count && heap[right].priority < heap[smallest].priority) smallest = right;
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if (smallest == i) break;
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(heap[i], heap[smallest]) = (heap[smallest], heap[i]);
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i = smallest;
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}
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}
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}
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}
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