using System.Collections.Generic;
using System.Linq;
using UnityEngine;

public class GearFollower : MonoBehaviour
{
    [Header("参数")]
    public SpriteRenderer targetSpriteRenderer;
    public float snapDistance = 0.3f;
    public float detachDistance = 1.0f;
    public float tractionSpeed = 2f;

    private List<Vector2> edgePoints;
    private int currentEdgeIndex = 0;
    private int tractionDirection = 1;

    private enum State { Follow, Traction }
    private State currentState = State.Follow;

    private Camera mainCamera;

    void Start()
    {
        mainCamera = Camera.main;
        LoadEdgePointsFromSprite();
    }

    void Update()
    {
        Vector2 mouseWorldPos = mainCamera.ScreenToWorldPoint(Input.mousePosition);

        if (Input.GetMouseButton(0))
        {
            transform.Rotate(Vector3.forward, -360f * Time.deltaTime);
        }

        switch (currentState)
        {
            case State.Follow:
                UpdateFollowState(mouseWorldPos);
                break;
            case State.Traction:
                UpdateTractionState(mouseWorldPos);
                break;
        }
    }

    private void LoadEdgePointsFromSprite()
    {
        edgePoints = new List<Vector2>();
        if (targetSpriteRenderer == null)
        {
            Debug.LogError("targetSpriteRenderer 为空");
            return;
        }

        List<Vector2> shapePoints = new List<Vector2>();
        targetSpriteRenderer.sprite.GetPhysicsShape(0, shapePoints);

        if (shapePoints.Count == 0)
        {
            Debug.LogError("目标Sprite没有物理形状点");
            return;
        }

        edgePoints = shapePoints
            .Select(p => (Vector2)targetSpriteRenderer.transform.TransformPoint(p))
            .ToList();

        if (edgePoints[0] != edgePoints[edgePoints.Count - 1])
        {
            edgePoints.Add(edgePoints[0]);
        }
    }

    private void UpdateFollowState(Vector2 mousePos)
    {
        transform.position = mousePos;

        int nearestIndex = FindNearestEdgePointIndex(mousePos, out float dist);
        if (dist < snapDistance)
        {
            currentEdgeIndex = nearestIndex;

            // 判断应该向前还是向后吸附
            Vector2 prev = edgePoints[WrapIndex(currentEdgeIndex - 1)];
            Vector2 next = edgePoints[WrapIndex(currentEdgeIndex + 1)];

            float distToPrev = Vector2.Distance(mousePos, prev);
            float distToNext = Vector2.Distance(mousePos, next);

            tractionDirection = (distToNext < distToPrev) ? 1 : -1;

            transform.position = edgePoints[currentEdgeIndex];
            currentState = State.Traction;
        }
    }

    private float directionSwitchCooldown = 0.2f;
    private float directionSwitchTimer = 0f;

    private void UpdateTractionState(Vector2 mousePos)
    {
        Vector2 gearPos = transform.position;

        if (Vector2.Distance(mousePos, gearPos) > detachDistance)
        {
            currentState = State.Follow;
            return;
        }

        if (!Input.GetMouseButton(0))
        {
            // 不按左键不移动
            return;
        }

        int nextIndex = WrapIndex(currentEdgeIndex + tractionDirection);
        Vector2 from = edgePoints[currentEdgeIndex];
        Vector2 to = edgePoints[nextIndex];

        // 方向判断优化：使用角度判断是否反方向
        Vector2 forward = (to - from).normalized;
        Vector2 toMouse = (mousePos - from).normalized;
        float angle = Vector2.SignedAngle(forward, toMouse); // -180 ~ 180

        // 当角度大于120度，说明鼠标方向明显是反的
        if (Mathf.Abs(angle) > 120f)
        {
            directionSwitchTimer += Time.deltaTime;
            if (directionSwitchTimer > directionSwitchCooldown)
            {
                tractionDirection *= -1;
                directionSwitchTimer = 0f;

                // 更新前进段
                nextIndex = WrapIndex(currentEdgeIndex + tractionDirection);
                from = edgePoints[currentEdgeIndex];
                to = edgePoints[nextIndex];
                forward = (to - from).normalized;
            }
        }
        else
        {
            directionSwitchTimer = 0f;
        }

        Vector2 targetPoint = GetClosestPointOnSegment(from, to, mousePos);
        Vector2 moveDir = (targetPoint - gearPos);
        float distance = moveDir.magnitude;

        float maxStep = tractionSpeed * Time.deltaTime;

        if (distance <= maxStep)
        {
            transform.position = targetPoint;

            // 如果到达段末，前进一段
            if (Vector2.Distance(targetPoint, to) < 0.01f)
            {
                currentEdgeIndex = nextIndex;
            }
        }
        else
        {
            transform.position += (Vector3)(moveDir.normalized * maxStep);
        }
    }


    private int FindNearestEdgePointIndex(Vector2 pos, out float minDist)
    {
        int nearestIndex = 0;
        minDist = float.MaxValue;

        for (int i = 0; i < edgePoints.Count; i++)
        {
            float dist = Vector2.Distance(pos, edgePoints[i]);
            if (dist < minDist)
            {
                minDist = dist;
                nearestIndex = i;
            }
        }
        return nearestIndex;
    }

    private Vector2 GetClosestPointOnSegment(Vector2 a, Vector2 b, Vector2 p)
    {
        Vector2 ab = b - a;
        float t = Vector2.Dot(p - a, ab) / ab.sqrMagnitude;
        t = Mathf.Clamp01(t);
        return a + ab * t;
    }

    private int WrapIndex(int index)
    {
        if (index < 0) return edgePoints.Count - 2;
        if (index >= edgePoints.Count - 1) return 0;
        return index;
    }
}