using UnityEngine;
using System.Collections.Generic;

/// <summary>
/// Simple AI entity that navigates the maze using fog of war
/// Only knows about areas it has explored or can currently see
/// </summary>
[RequireComponent(typeof(Rigidbody2D))]
public class MazeAIEntity : MonoBehaviour
{
    [Header("AI Settings")]
    [SerializeField] private MazeController mazeController;
    [SerializeField] private MazeFogOfWar fogOfWar;
    [SerializeField] private float visionRange = 5f;
    [SerializeField] private float movementSpeed = 2f;
    [SerializeField] private float pathUpdateInterval = 1f;

    [Header("Pathfinding")]
    [SerializeField] private bool showPath = true;
    [SerializeField] private LineRenderer pathRenderer;

    private MazeData maze;
    private MazePathfinder pathfinder;
    private List<Vector2Int> currentPath = new();
    private int currentPathIndex = 0;
    private Vector2Int currentTarget;
    private float lastPathUpdate = 0f;

    void Start()
    {
        maze = mazeController.GetCurrentMaze();
        pathfinder = new MazePathfinder(maze);

        // Start at a random start point
        if (maze.StartPoints.Count > 0)
        {
            int startIndex = Random.Range(0, maze.StartPoints.Count);
            Vector2Int startPos = maze.StartPoints[startIndex];
            transform.position = new Vector3(startPos.x + 0.5f, startPos.y + 0.5f, 0);

            // Mark starting area as explored
            ExploreCurrentArea();
        }

        if (pathRenderer == null)
        {
            pathRenderer = gameObject.AddComponent<LineRenderer>();
            pathRenderer.startWidth = 0.1f;
            pathRenderer.endWidth = 0.1f;
            pathRenderer.material = new Material(Shader.Find("Sprites/Default"));
            pathRenderer.startColor = Color.yellow;
            pathRenderer.endColor = Color.yellow;
        }
    }

    void Update()
    {
        if (maze == null) return;

        // Update vision and exploration
        UpdateVision();

        // Update pathfinding
        if (Time.time - lastPathUpdate > pathUpdateInterval)
        {
            UpdatePathfinding();
            lastPathUpdate = Time.time;
        }

        // Move along path
        FollowPath();
    }

    /// <summary>
    /// Updates the entity's vision and explores new areas
    /// </summary>
    private void UpdateVision()
    {
        Vector2Int currentTile = WorldToTile(transform.position);

        if (fogOfWar != null)
        {
            fogOfWar.UpdateEntityVision(gameObject, currentTile, visionRange);
        }
        else
        {
            // If no fog of war, explore current area
            ExploreCurrentArea();
        }
    }

    /// <summary>
    /// Explores the area around the current position
    /// </summary>
    private void ExploreCurrentArea()
    {
        Vector2Int center = WorldToTile(transform.position);
        HashSet<Vector2Int> explored = new();

        int range = Mathf.CeilToInt(visionRange);
        for (int x = center.x - range; x <= center.x + range; x++)
        {
            for (int y = center.y - range; y <= center.y + range; y++)
            {
                Vector2Int tile = new Vector2Int(x, y);
                if (Vector2Int.Distance(center, tile) <= visionRange && maze.IsInBounds(x, y))
                {
                    explored.Add(tile);
                }
            }
        }

        if (fogOfWar != null)
        {
            fogOfWar.ExploreTiles(explored);
        }
    }

    /// <summary>
    /// Updates pathfinding towards an exit
    /// </summary>
    private void UpdatePathfinding()
    {
        if (maze.ExitPoints.Count == 0) return;

        Vector2Int currentPos = WorldToTile(transform.position);

        // Find closest unexplored exit
        Vector2Int targetExit = FindBestExit(currentPos);

        if (targetExit != currentPos)
        {
            // Only pathfind to areas we know about
            HashSet<Vector2Int> knownTiles = fogOfWar != null ?
                fogOfWar.GetExploredTiles() : GetAllTiles();

            // Find path within known areas
            currentPath = FindPathInKnownArea(currentPos, targetExit, knownTiles);

            if (currentPath.Count > 0)
            {
                currentPathIndex = 0;
                UpdatePathVisualization();
            }
        }
    }

    /// <summary>
    /// Finds the best exit to path towards
    /// </summary>
    private Vector2Int FindBestExit(Vector2Int from)
    {
        Vector2Int bestExit = from;
        float bestDistance = float.MaxValue;

        foreach (var exit in maze.ExitPoints)
        {
            // Only consider exits we know about
            if (fogOfWar == null || fogOfWar.IsTileExplored(exit))
            {
                float distance = Vector2Int.Distance(from, exit);
                if (distance < bestDistance)
                {
                    bestDistance = distance;
                    bestExit = exit;
                }
            }
        }

        return bestExit;
    }

    /// <summary>
    /// Finds a path within explored areas
    /// </summary>
    private List<Vector2Int> FindPathInKnownArea(Vector2Int start, Vector2Int goal, HashSet<Vector2Int> knownTiles)
    {
        // A* within known tiles using min-heap open set (O(n log n) vs O(n²))
        var openSet = new MinHeap<Vector2Int>();
        var cameFrom = new Dictionary<Vector2Int, Vector2Int>();
        var gScore = new Dictionary<Vector2Int, float> { [start] = 0f };

        float h0 = Mathf.Abs(start.x - goal.x) + Mathf.Abs(start.y - goal.y);
        openSet.Enqueue(start, h0);

        while (openSet.Count > 0)
        {
            Vector2Int current = openSet.Dequeue();

            if (current == goal)
                return ReconstructPath(cameFrom, current);

            foreach (var neighbor in maze.GetAdjacentWalkable(current.x, current.y))
            {
                if (!knownTiles.Contains(neighbor)) continue;

                float tentativeG = gScore[current] + 1f;
                if (!gScore.TryGetValue(neighbor, out float existingG) || tentativeG < existingG)
                {
                    cameFrom[neighbor] = current;
                    gScore[neighbor] = tentativeG;
                    float f = tentativeG + Mathf.Abs(neighbor.x - goal.x) + Mathf.Abs(neighbor.y - goal.y);
                    if (openSet.Contains(neighbor, out _)) openSet.UpdatePriority(neighbor, f);
                    else openSet.Enqueue(neighbor, f);
                }
            }
        }

        return new List<Vector2Int>();
    }

    /// <summary>
    /// Reconstructs the path from A* results using Add+Reverse (O(n)) instead of Insert(0,...) (O(n²))
    /// </summary>
    private static List<Vector2Int> ReconstructPath(Dictionary<Vector2Int, Vector2Int> cameFrom, Vector2Int current)
    {
        var path = new List<Vector2Int>();
        while (cameFrom.TryGetValue(current, out var prev))
        {
            path.Add(current);
            current = prev;
        }
        path.Add(current);
        path.Reverse();
        return path;
    }

    /// <summary>
    /// Gets all tiles (fallback when no fog of war)
    /// </summary>
    private HashSet<Vector2Int> GetAllTiles()
    {
        HashSet<Vector2Int> allTiles = new();
        for (int x = 0; x < maze.Width; x++)
        {
            for (int y = 0; y < maze.Height; y++)
            {
                allTiles.Add(new Vector2Int(x, y));
            }
        }
        return allTiles;
    }

    /// <summary>
    /// Moves along the current path
    /// </summary>
    private void FollowPath()
    {
        if (currentPath.Count == 0 || currentPathIndex >= currentPath.Count) return;

        Vector2Int targetTile = currentPath[currentPathIndex];
        Vector3 targetPos = new Vector3(targetTile.x + 0.5f, targetTile.y + 0.5f, 0);
        Vector3 currentPos = transform.position;

        // Move towards target
        Vector3 direction = (targetPos - currentPos).normalized;
        transform.position += direction * movementSpeed * Time.deltaTime;

        // Check if reached target
        if (Vector3.Distance(currentPos, targetPos) < 0.1f)
        {
            currentPathIndex++;

            // If reached end of path, find new path
            if (currentPathIndex >= currentPath.Count)
            {
                currentPath.Clear();
                UpdatePathfinding();
            }
        }
    }

    /// <summary>
    /// Updates the path visualization
    /// </summary>
    private void UpdatePathVisualization()
    {
        if (!showPath || pathRenderer == null) return;

        pathRenderer.positionCount = currentPath.Count;
        for (int i = 0; i < currentPath.Count; i++)
        {
            Vector2Int tile = currentPath[i];
            pathRenderer.SetPosition(i, new Vector3(tile.x + 0.5f, tile.y + 0.5f, -0.1f));
        }
    }

    /// <summary>
    /// Converts world position to tile coordinates
    /// </summary>
    private Vector2Int WorldToTile(Vector3 worldPos)
    {
        return new Vector2Int(Mathf.FloorToInt(worldPos.x), Mathf.FloorToInt(worldPos.y));
    }

    void OnDestroy()
    {
        if (fogOfWar != null)
        {
            fogOfWar.RemoveEntity(gameObject);
        }
    }
}