using System;
using NBF;
using UnityEngine;

[RequireComponent(typeof(LineRenderer))]
public class Rope : MonoBehaviour
{
    [Header("Anchors")] [SerializeField] public Rigidbody startAnchor;
    [SerializeField] public Rigidbody endAnchor;

    [Header("Physics (Dynamic Nodes, Fixed Segment Len)")] [SerializeField, Min(0.01f), Tooltip("物理每段固定长度（越小越细致越耗）")]
    private float physicsSegmentLen = 0.15f;

    [SerializeField, Range(2, 200)] private int minPhysicsNodes = 12;

    [SerializeField, Range(2, 400), Tooltip("物理节点上限（仅用于性能保护；与“最大长度不限制”不是一回事）")]
    private int maxPhysicsNodes = 120;

    [SerializeField] private float gravityStrength = 2.0f;
    [SerializeField, Range(0f, 1f)] private float velocityDampen = 0.95f;

    [SerializeField, Range(0.0f, 1.0f), Tooltip("约束修正强度，越大越硬。0.6~0.9 常用")]
    private float stiffness = 0.8f;

    [SerializeField, Range(1, 80), Tooltip("迭代次数。鱼线 10~30 通常够用")]
    private int iterations = 20;

    [Header("Length Control (No Min/Max Clamp)")]
    [Tooltip("初始总长度（米）。如果为 0，则用 physicsSegmentLen*(minPhysicsNodes-1) 作为初始长度")]
    [SerializeField, Min(0f)]
    private float initialLength = 0f;

    [Tooltip("长度变化平滑时间（越小越跟手，越大越稳）")] [SerializeField, Min(0.0001f)]
    private float lengthSmoothTime = 0.15f;

    [Tooltip("当长度在变化时，额外把速度压掉一些（防抖）。0=不额外处理，1=变化时几乎清速度（建议只在收线生效）")] [SerializeField, Range(0f, 1f)]
    private float lengthChangeVelocityKill = 0.6f;

    [Tooltip("允许的最小松弛余量（避免目标长度刚好等于锚点距离时抖动）")] [SerializeField, Min(0f)]
    private float minSlack = 0.002f;

    [Header("Head Segment Clamp")] [Tooltip("第一段（起点->第1节点）允许的最小长度，避免收线时第一段被压到0导致数值炸")] [SerializeField, Min(0.0001f)]
    private float headMinLen = 0.01f;

    [Header("Node Count Stability")] [SerializeField, Tooltip("节点数切换迟滞（米）。避免长度在临界点抖动导致节点数来回跳 -> 卡顿")]
    private float nodeHysteresis = 0.05f;

    [Header("Simple Ground/Water Constraint (Cheap)")] [SerializeField]
    private bool constrainToGround = true;

    [SerializeField] private LayerMask groundMask = ~0;
    [SerializeField, Min(0f)] private float groundRadius = 0.01f;
    [SerializeField, Min(0f)] private float groundCastHeight = 1.0f;
    [SerializeField, Min(0.01f)] private float groundCastDistance = 2.5f;

    [Header("Render (High Resolution)")] [SerializeField, Min(1), Tooltip("静止时每段物理线段插值加密数量（越大越顺，越耗）")]
    private int renderSubdivisionsIdle = 6;

    [SerializeField, Min(1), Tooltip("甩动时每段物理线段插值加密数量（动态降LOD以防卡顿）")]
    private int renderSubdivisionsMoving = 2;

    [SerializeField, Min(0f), Tooltip("平均速度超过该阈值认为在甩动（用于动态降 subdiv）")]
    private float movingSpeedThreshold = 2.0f;

    [SerializeField, Tooltip("是否使用 Catmull-Rom 平滑（开启更顺，但更耗）")]
    private bool smooth = true;

    [SerializeField, Min(0.0001f)] private float lineWidth = 0.002f;

    [Header("Air Drag (Stable)")] [SerializeField, Range(0f, 5f), Tooltip("空气阻力（Y向），指数衰减，越大越不飘")]
    private float airDrag = 0.9f;

    [SerializeField, Range(0f, 2f), Tooltip("横向额外阻力（XZ），指数衰减，越大越不左右飘")]
    private float airDragXZ = 0.6f;

    private LineRenderer _lineRenderer;

    // physics (注意：数组固定为 maxPhysicsNodes，physicsNodes 表示有效长度)
    private int _physicsNodes;
    private Vector3[] _pCurr;
    private Vector3[] _pPrev;

    // render
    private Vector3[] _rPoints;
    private int _rCountCached = -1;

    private Vector3 _gravity;

    // length control runtime
    private float _targetLength;
    private float _currentLength;
    private float _lengthSmoothVel;

    // rest length head
    private float _headRestLen;

    // node stability
    private int _lastDesiredNodes = 0;

    private FRod _rod;
    public void Init(FRod rod) => _rod = rod;

    private void Awake()
    {
        _lineRenderer = GetComponent<LineRenderer>();
        _gravity = new Vector3(0f, -gravityStrength, 0f);

        InitLengthSystem();
        AllocateAndInitNodes();
        RebuildRenderBufferIfNeeded(renderSubdivisionsIdle);
    }

    private void OnValidate()
    {
        renderSubdivisionsIdle = Mathf.Max(renderSubdivisionsIdle, 1);
        renderSubdivisionsMoving = Mathf.Max(renderSubdivisionsMoving, 1);
        iterations = Mathf.Clamp(iterations, 1, 80);
        groundCastDistance = Mathf.Max(groundCastDistance, 0.01f);
        groundCastHeight = Mathf.Max(groundCastHeight, 0f);
        lineWidth = Mathf.Max(lineWidth, 0.0001f);

        lengthSmoothTime = Mathf.Max(lengthSmoothTime, 0.0001f);

        physicsSegmentLen = Mathf.Max(physicsSegmentLen, 0.01f);
        minPhysicsNodes = Mathf.Max(minPhysicsNodes, 2);
        maxPhysicsNodes = Mathf.Max(maxPhysicsNodes, minPhysicsNodes);

        headMinLen = Mathf.Max(headMinLen, 0.0001f);
        nodeHysteresis = Mathf.Max(0f, nodeHysteresis);
        
    }

    private void InitLengthSystem()
    {
        float defaultLen = physicsSegmentLen * (Mathf.Max(minPhysicsNodes, 2) - 1);
        _currentLength = (initialLength > 0f) ? initialLength : defaultLen;
        _targetLength = _currentLength;
    }

    private void AllocateAndInitNodes()
    {
        _physicsNodes = Mathf.Clamp(ComputeDesiredNodesStable(_currentLength), 2, maxPhysicsNodes);

        // ✅ 永久分配到最大，运行时不再 new，避免GC卡顿
        _pCurr = new Vector3[maxPhysicsNodes];
        _pPrev = new Vector3[maxPhysicsNodes];

        Vector3 start = startAnchor ? startAnchor.position : transform.position;
        Vector3 dir = Vector3.down;

        for (int i = 0; i < _physicsNodes; i++)
        {
            Vector3 pos = start + dir * (physicsSegmentLen * i);
            _pCurr[i] = pos;
            _pPrev[i] = pos;
        }

        UpdateHeadRestLenFromCurrentLength();

        if (startAnchor && endAnchor)
            LockAnchorsHard();
    }

    private int ComputeDesiredNodes(float lengthMeters)
    {
        int desired = Mathf.FloorToInt(Mathf.Max(0f, lengthMeters) / physicsSegmentLen) + 1;
        desired = Mathf.Clamp(desired, minPhysicsNodes, maxPhysicsNodes);
        return desired;
    }

    private int ComputeDesiredNodesStable(float lengthMeters)
    {
        int desired = ComputeDesiredNodes(lengthMeters);

        if (_lastDesiredNodes == 0)
        {
            _lastDesiredNodes = desired;
            return desired;
        }

        if (desired == _lastDesiredNodes)
            return desired;

        // 边界（lastDesiredNodes 对应的长度临界）
        float boundary = (_lastDesiredNodes - 1) * physicsSegmentLen;

        // 在临界附近就不切换，避免来回跳
        if (Mathf.Abs(lengthMeters - boundary) < nodeHysteresis)
            return _lastDesiredNodes;

        _lastDesiredNodes = desired;
        return desired;
    }

    public void SetTargetLength(float lengthMeters)
    {
        _targetLength = Mathf.Max(0f, lengthMeters);
    }

    public float GetCurrentLength() => _currentLength;
    public float GetTargetLength() => _targetLength;

    private void FixedUpdate()
    {
        if (!startAnchor || !endAnchor) return;

        _gravity.y = -gravityStrength;

        UpdateLengthSmooth();
        UpdateNodesFromLength(); // ✅ 一次性增减，无GC
        UpdateHeadRestLenFromCurrentLength();

        // simulate
        Simulate();

        // 确保端点正确后再迭代
        LockAnchorsHard();

        // constraints
        for (int it = 0; it < iterations; it++)
            SolveDistanceConstraints_HeadOnly();

        // 迭代后锁一次足够
        LockAnchorsHard();

        if (constrainToGround)
            ConstrainToGround();

        // 约束后再锁一次（保险）
        LockAnchorsHard();
    }

    private void LateUpdate()
    {
        if (!startAnchor || !endAnchor || _pCurr == null || _physicsNodes < 2) return;

        // 端点“跟手”：渲染前强行同步 transform，消除慢一拍
        int last = _physicsNodes - 1;
        Vector3 s = startAnchor.transform.position;
        Vector3 e = endAnchor.transform.position;

        _pCurr[0] = s;
        _pPrev[0] = s;
        _pCurr[last] = e;
        _pPrev[last] = e;

        DrawHighResLine();
    }

    private void UpdateLengthSmooth()
    {
        float minFeasible = 0.01f;

        float desired = Mathf.Max(_targetLength, minFeasible);

        _currentLength = Mathf.SmoothDamp(
            _currentLength,
            desired,
            ref _lengthSmoothVel,
            lengthSmoothTime,
            Mathf.Infinity,
            Time.fixedDeltaTime
        );
    }

    private void UpdateNodesFromLength()
    {
        int desired = ComputeDesiredNodesStable(_currentLength);
        desired = Mathf.Clamp(desired, 2, maxPhysicsNodes);
        if (desired == _physicsNodes) return;

        if (desired > _physicsNodes)
        {
            AddNodesAtStart(desired - _physicsNodes);
        }
        else
        {
            RemoveNodesAtStart(_physicsNodes - desired);
        }

        _physicsNodes = desired;

        // 渲染缓存按最大 subdiv 预留即可
        RebuildRenderBufferIfNeeded(renderSubdivisionsIdle);
    }


    private void AddNodesAtStart(int addCount)
    {
        if (addCount <= 0) return;

        int oldCount = _physicsNodes;
        int newCount = Mathf.Min(oldCount + addCount, maxPhysicsNodes);
        addCount = newCount - oldCount;
        if (addCount <= 0) return;

        // 把 [1..oldCount-1] 整体往后挪 addCount，给 [1..addCount] 腾位置
        // oldCount-1 个元素（包含尾端锚点位，尾端后面会被锁）
        Array.Copy(_pCurr, 1, _pCurr, 1 + addCount, oldCount - 1);
        Array.Copy(_pPrev, 1, _pPrev, 1 + addCount, oldCount - 1);

        Vector3 s = startAnchor.position;

        // 方向：用“挪完后的第一个动态点”指向来估计
        Vector3 dir = Vector3.down;
        int firstOld = 1 + addCount;
        if (firstOld < 1 + addCount + (oldCount - 2) && oldCount >= 2)
        {
            Vector3 toOld1 = (_pCurr[firstOld] - s);
            if (toOld1.sqrMagnitude > 1e-6f) dir = toOld1.normalized;
        }

        // 继承速度
        float dt = Time.fixedDeltaTime;
        Vector3 inheritVel = Vector3.zero;
        if (oldCount >= 2 && firstOld < maxPhysicsNodes)
        {
            inheritVel = (_pCurr[firstOld] - _pPrev[firstOld]) / Mathf.Max(dt, 1e-6f);
        }

        for (int k = 1; k <= addCount; k++)
        {
            Vector3 pos = s + dir * (physicsSegmentLen * k);
            _pCurr[k] = pos;
            _pPrev[k] = pos - inheritVel * dt;
        }

        LockAnchorsHard();
    }

    private void RemoveNodesAtStart(int removeCount)
    {
        if (removeCount <= 0) return;

        int oldCount = _physicsNodes;
        int newCount = Mathf.Max(oldCount - removeCount, 2);
        removeCount = oldCount - newCount;
        if (removeCount <= 0) return;

        // 把 [1+removeCount .. 1+removeCount+(newCount-2)-1] 挪到 [1..newCount-2]
        // 中间动态节点数量 = newCount-2
        Array.Copy(_pCurr, 1 + removeCount, _pCurr, 1, newCount - 2);
        Array.Copy(_pPrev, 1 + removeCount, _pPrev, 1, newCount - 2);

        LockAnchorsHard();
    }

    private void UpdateHeadRestLenFromCurrentLength()
    {
        int fixedSegCount = Mathf.Max(0, _physicsNodes - 2);
        float baseLen = fixedSegCount * physicsSegmentLen;

        _headRestLen = _currentLength - baseLen;
        _headRestLen = Mathf.Clamp(_headRestLen, headMinLen, physicsSegmentLen * 1.5f);
    }

    private void Simulate()
    {
        float dt = Time.fixedDeltaTime;
        float invDt = 1f / Mathf.Max(dt, 1e-6f);

        // 指数衰减：更稳定好调
        float kY = Mathf.Exp(-airDrag * dt);
        float kXZ = Mathf.Exp(-airDragXZ * dt);

        for (int i = 1; i < _physicsNodes - 1; i++)
        {
            Vector3 vel = (_pCurr[i] - _pPrev[i]) * invDt;

            vel.x *= kXZ;
            vel.z *= kXZ;
            vel.y *= kY;

            vel *= velocityDampen;

            Vector3 next = _pCurr[i] + vel * dt;

            _pPrev[i] = _pCurr[i];
            _pCurr[i] = next + _gravity * (dt * dt);
        }
    }

    private void LockAnchorsHard()
    {
        if (!startAnchor || !endAnchor || _pCurr == null || _pPrev == null || _physicsNodes < 2) return;

        float dt = Time.fixedDeltaTime;
        Vector3 s = startAnchor.position;
        Vector3 e = endAnchor.position;

        _pCurr[0] = s;
        _pPrev[0] = s - startAnchor.linearVelocity * dt;

        int last = _physicsNodes - 1;
        _pCurr[last] = e;
        _pPrev[last] = e - endAnchor.linearVelocity * dt;
    }

    private void SolveDistanceConstraints_HeadOnly()
    {
        for (int i = 0; i < _physicsNodes - 1; i++)
        {
            float rest = (i == 0) ? _headRestLen : physicsSegmentLen;

            Vector3 a = _pCurr[i];
            Vector3 b = _pCurr[i + 1];

            Vector3 delta = b - a;
            float dist = delta.magnitude;
            if (dist < 1e-6f) continue;

            float diff = (dist - rest) / dist;
            Vector3 corr = delta * diff * stiffness;

            if (i != 0)
                _pCurr[i] += corr * 0.5f;

            if (i + 1 != _physicsNodes - 1)
                _pCurr[i + 1] -= corr * 0.5f;
        }
    }

    private void ConstrainToGround()
    {
        for (int i = 1; i < _physicsNodes - 1; i++)
        {
            Vector3 p = _pCurr[i];

            if (constrainToGround && groundMask != 0)
            {
                Vector3 origin = p + Vector3.up * groundCastHeight;
                if (Physics.Raycast(origin, Vector3.down, out RaycastHit hit, groundCastDistance, groundMask,
                        QueryTriggerInteraction.Ignore))
                {
                    float minY = hit.point.y + groundRadius;
                    if (p.y < minY) p.y = minY;
                }
            }

            _pCurr[i] = p;
        }
    }

    private void DrawHighResLine()
    {
        if (_pCurr == null || _physicsNodes < 2) return;

        _lineRenderer.startWidth = lineWidth;
        _lineRenderer.endWidth = lineWidth;

        if (!smooth)
        {
            _lineRenderer.positionCount = _physicsNodes;
            // 只传有效段
            // LineRenderer.SetPositions 会读数组的前 positionCount 个
            _lineRenderer.SetPositions(_pCurr);
            return;
        }

        // 动态降 subdiv：甩动时降低点数，减少CPU开销
        int subdiv = PickRenderSubdivisions();
        RebuildRenderBufferIfNeeded(subdiv);

        int idx = 0;

        for (int seg = 0; seg < _physicsNodes - 1; seg++)
        {
            Vector3 p0 = _pCurr[Mathf.Max(seg - 1, 0)];
            Vector3 p1 = _pCurr[seg];
            Vector3 p2 = _pCurr[seg + 1];
            Vector3 p3 = _pCurr[Mathf.Min(seg + 2, _physicsNodes - 1)];

            for (int s = 0; s < subdiv; s++)
            {
                float t = s / (float)subdiv;
                Vector3 pt = CatmullRom_XZ_LinearY(p0, p1, p2, p3, t);
                _rPoints[idx++] = pt;
            }
        }

        _rPoints[idx++] = _pCurr[_physicsNodes - 1];

        _lineRenderer.positionCount = idx;
        _lineRenderer.SetPositions(_rPoints);
    }

    private int PickRenderSubdivisions()
    {
        int idle = Mathf.Max(1, renderSubdivisionsIdle);
        int moving = Mathf.Max(1, renderSubdivisionsMoving);

        // 计算平均速度（用 Verlet 差分）
        float dt = Time.fixedDeltaTime;
        float invDt = 1f / Mathf.Max(dt, 1e-6f);

        float sum = 0f;
        int count = Mathf.Max(1, _physicsNodes - 2);
        for (int i = 1; i < _physicsNodes - 1; i++)
            sum += (_pCurr[i] - _pPrev[i]).magnitude * invDt;

        float avgSpeed = sum / count;

        return (avgSpeed > movingSpeedThreshold) ? moving : idle;
    }

    private void RebuildRenderBufferIfNeeded(int subdiv)
    {
        int targetCount = (_physicsNodes - 1) * subdiv + 1;
        if (_rPoints == null || _rCountCached != targetCount)
        {
            _rPoints = new Vector3[targetCount];
            _rCountCached = targetCount;
        }
    }

    private static Vector3 CatmullRom(Vector3 p0, Vector3 p1, Vector3 p2, Vector3 p3, float t)
    {
        float t2 = t * t;
        float t3 = t2 * t;

        return 0.5f * (
            (2f * p1) +
            (-p0 + p2) * t +
            (2f * p0 - 5f * p1 + 4f * p2 - p3) * t2 +
            (-p0 + 3f * p1 - 3f * p2 + p3) * t3
        );
    }

    private static Vector3 CatmullRom_XZ_LinearY(Vector3 p0, Vector3 p1, Vector3 p2, Vector3 p3, float t)
    {
        Vector3 cr = CatmullRom(p0, p1, p2, p3, t);
        cr.y = Mathf.Lerp(p1.y, p2.y, t);
        return cr;
    }

    private void OnDrawGizmosSelected()
    {
        if (_pCurr == null) return;
        Gizmos.color = Color.yellow;
        for (int i = 0; i < _physicsNodes; i++)
            Gizmos.DrawSphere(_pCurr[i], 0.01f);
    }
}