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

public interface IWaterProvider
{
    /// <summary>返回该世界坐标下的水面高度</summary>
    float GetWaterHeight(Vector3 worldPos);

    /// <summary>可选：水面法线（没有就返回 Vector3.up）</summary>
    Vector3 GetWaterNormal(Vector3 worldPos);

    /// <summary>可选：水流速度（没有就返回 Vector3.zero）</summary>
    Vector3 GetWaterVelocity(Vector3 worldPos);
}

/// <summary>
/// 多点采样浮力：考虑形状(采样点分布)、重心(Rigidbody.centerOfMass)、扭矩(在点上施力)。
/// 适合小物体（0.01m级），避免“任何形状都一样直直往下/往上顶飞”
/// </summary>
[DisallowMultipleComponent]
[RequireComponent(typeof(Rigidbody))]
public class BuoyancyBody : MonoBehaviour
{
    [Header("Water")]
    [Tooltip("如果不填则使用简单水面：y = WaterLevel")]
    public MonoBehaviour waterProviderBehaviour;
    private IWaterProvider waterProvider;

    [Tooltip("简单水面模式：水面高度（y）")]
    public float waterLevel = 0f;

    [Header("Buoyancy (Physical)")]
    [Tooltip("水密度 kg/m^3。淡水约1000，海水约1025")]
    public float waterDensity = 1000f;

    [Tooltip("物体密度 kg/m^3。决定浮沉：< waterDensity 更容易浮，> waterDensity 更容易沉")]
    public float objectDensity = 300f;

    [Tooltip("如果 > 0 则强制使用该体积(m^3)，否则按 mass/objectDensity 推体积")]
    public float overrideVolume = 0f;

    [Tooltip("浮力强度缩放（调手感用），1为物理值")]
    public float buoyancyScale = 1f;

    [Header("Stabilization / Damping")]
    [Tooltip("水中线性阻尼（越大越不弹、越“粘水”）")]
    public float linearDamping = 2.5f;

    [Tooltip("水中角阻尼（越大越稳定立漂）")]
    public float angularDamping = 2.0f;

    [Tooltip("入水/出水过渡平滑厚度（m）。越大越不弹，但边界更“软”】【0.01~0.05常用】")]
    public float surfaceSmoothing = 0.02f;

    [Tooltip("限制单个采样点最大浮力加速度（m/s^2），防止轻小物体从高处落下冲天")]
    public float maxBuoyantAccelPerPoint = 50f;

    [Header("Center of Mass")]
    [Tooltip("本地空间重心偏移（m）。可用来模拟胶囊体配重，让浮漂能站起来")]
    public Vector3 centerOfMassOffset = Vector3.zero;

    [Header("Sampling (Shape matters!)")]
    [Tooltip("每个Collider生成的采样点数量（越大越精确，越耗性能）。浮漂建议 12~40")]
    [Range(4, 200)] public int pointsPerCollider = 24;

    [Tooltip("采样点是否只保留在Collider内部（对非凸MeshCollider不可靠；浮漂建议用Capsule/Box/Sphere）")]
    public bool keepPointsInsideCollider = true;

    [Tooltip("运行时显示采样点(编辑器Gizmos)")]
    public bool drawGizmos = true;

    [Tooltip("只对这些Collider算浮力（为空则自动抓取子物体所有Collider）")]
    public Collider[] colliders;

    private Rigidbody rb;
    private readonly List<SamplePoint> samplePoints = new();

    private struct SamplePoint
    {
        public Collider col;
        public Vector3 localPos;   // 相对 collider.transform 的本地
    }

    void Awake()
    {
        rb = GetComponent<Rigidbody>();
        ApplyCOM();

        waterProvider = waterProviderBehaviour as IWaterProvider;
        BuildSamplePoints();
    }

    void OnValidate()
    {
        if (!rb) rb = GetComponent<Rigidbody>();
        ApplyCOM();

        // 在编辑器里改参数时重建采样点
        if (Application.isPlaying == false)
        {
            // 避免在Prefab编辑等情况下报错
        }
    }

    void Reset()
    {
        rb = GetComponent<Rigidbody>();
        rb.useGravity = true;
        rb.interpolation = RigidbodyInterpolation.Interpolate;
        rb.collisionDetectionMode = CollisionDetectionMode.Continuous;

        if (colliders == null || colliders.Length == 0)
            colliders = GetComponentsInChildren<Collider>();
    }

    private void ApplyCOM()
    {
        if (!rb) return;
        rb.centerOfMass = centerOfMassOffset;
    }

    /// <summary>手动调用：当你运行时增减Collider或缩放后，建议调用一次</summary>
    public void Rebuild()
    {
        waterProvider = waterProviderBehaviour as IWaterProvider;
        ApplyCOM();
        BuildSamplePoints();
    }

    private void BuildSamplePoints()
    {
        samplePoints.Clear();

        Collider[] cols = colliders;
        if (cols == null || cols.Length == 0)
            cols = GetComponentsInChildren<Collider>();

        foreach (var col in cols)
        {
            if (!col || !col.enabled) continue;

            // 用 bounds 做快速采样，再可选筛内部点
            var b = col.bounds;

            int n = Mathf.Max(4, pointsPerCollider);
            // 使用“分层网格+抖动”的方式，保证不同形状/尺寸采样分布不同
            int dim = Mathf.CeilToInt(Mathf.Pow(n, 1f / 3f));
            dim = Mathf.Max(2, dim);

            Vector3 size = b.size;
            Vector3 step = new Vector3(
                size.x / (dim - 1),
                size.y / (dim - 1),
                size.z / (dim - 1)
            );

            // 为了让小物体也稳定：如果某轴极小，step可能很小，没关系
            int added = 0;
            for (int ix = 0; ix < dim && added < n; ix++)
            for (int iy = 0; iy < dim && added < n; iy++)
            for (int iz = 0; iz < dim && added < n; iz++)
            {
                // 0..1
                float fx = (dim == 1) ? 0.5f : (float)ix / (dim - 1);
                float fy = (dim == 1) ? 0.5f : (float)iy / (dim - 1);
                float fz = (dim == 1) ? 0.5f : (float)iz / (dim - 1);

                // 抖动（避免规则网格导致“锁姿态”）
                Vector3 jitter = new Vector3(
                    (UnityEngine.Random.value - 0.5f) * step.x * 0.25f,
                    (UnityEngine.Random.value - 0.5f) * step.y * 0.25f,
                    (UnityEngine.Random.value - 0.5f) * step.z * 0.25f
                );

                Vector3 world = new Vector3(
                    b.min.x + size.x * fx,
                    b.min.y + size.y * fy,
                    b.min.z + size.z * fz
                ) + jitter;

                if (keepPointsInsideCollider)
                {
                    // 对凸Collider通常可靠：如果点在内部，ClosestPoint会返回点本身
                    Vector3 cp = col.ClosestPoint(world);
                    if ((cp - world).sqrMagnitude > 1e-8f)
                        continue;
                }

                // 转到 collider.transform 本地存储
                Vector3 local = col.transform.InverseTransformPoint(world);
                samplePoints.Add(new SamplePoint { col = col, localPos = local });
                added++;
            }

            // 如果内部点太少，至少补一点：用 collider.transform.position 周围
            if (added < 4)
            {
                for (int i = added; i < 4; i++)
                {
                    Vector3 world = col.bounds.center + UnityEngine.Random.insideUnitSphere * (col.bounds.extents.magnitude * 0.25f);
                    Vector3 local = col.transform.InverseTransformPoint(world);
                    samplePoints.Add(new SamplePoint { col = col, localPos = local });
                }
            }
        }
    }

    void FixedUpdate()
    {
        if (samplePoints.Count == 0) BuildSamplePoints();

        // 体积估计：V = m / rho_object
        float volume = overrideVolume > 0f ? overrideVolume : (rb.mass / Mathf.Max(1e-6f, objectDensity));
        volume = Mathf.Max(1e-9f, volume);

        int count = samplePoints.Count;
        float volPerPoint = volume / count;

        Vector3 gravity = Physics.gravity; // 通常 (0,-9.81,0)
        float gMag = gravity.magnitude;

        // 如果重力为0就不算了
        if (gMag < 1e-6f) return;

        // 逐点施加：浮力 + 阻尼
        for (int i = 0; i < count; i++)
        {
            var sp = samplePoints[i];
            if (!sp.col || !sp.col.enabled) continue;

            Vector3 worldPos = sp.col.transform.TransformPoint(sp.localPos);

            float waterH = waterProvider?.GetWaterHeight(worldPos) ?? waterLevel;
            float depth = waterH - worldPos.y; // >0 表示点在水下

            if (depth <= 0f) continue;

            // 入水过渡：在水面附近用平滑厚度做0..1
            float submergence01 = (surfaceSmoothing <= 1e-6f)
                ? 1f
                : Mathf.Clamp01(depth / surfaceSmoothing);

            // 该点产生的“排水质量”：m_displaced = rho_water * V_point * submergence
            float displacedMass = waterDensity * volPerPoint * submergence01;

            // 浮力 = -g * m_displaced
            Vector3 buoyancy = -gravity * displacedMass * buoyancyScale;

            // 限制单点最大向上加速度，防止小物体落水被“顶飞”
            // a = F/m -> Fmax = amax * m
            float Fmax = maxBuoyantAccelPerPoint * rb.mass;
            if (buoyancy.sqrMagnitude > Fmax * Fmax)
                buoyancy = buoyancy.normalized * Fmax;

            // 水中阻尼（相对水流速度）
            Vector3 waterVel = waterProvider?.GetWaterVelocity(worldPos) ?? Vector3.zero;
            Vector3 pointVel = rb.GetPointVelocity(worldPos);
            Vector3 relVel = pointVel - waterVel;

            // 阻尼随入水程度增强：越深越“粘”
            Vector3 dampingForce = -relVel * (linearDamping * displacedMass);

            // 把力施加在采样点上 -> 自然产生扭矩（形状/重心不同效果不同）
            rb.AddForceAtPosition(buoyancy + dampingForce, worldPos, ForceMode.Force);

            // 额外角阻尼（整体），只在“确实有一部分在水里”时施加更合理，
            // 这里用 submergence01 做比例：越入水越强
            rb.AddTorque(-rb.angularVelocity * (angularDamping * displacedMass), ForceMode.Force);
        }
    }

#if UNITY_EDITOR
    void OnDrawGizmosSelected()
    {
        if (!drawGizmos) return;

        Gizmos.matrix = Matrix4x4.identity;

        // 水面
        Gizmos.color = new Color(0f, 0.6f, 1f, 0.25f);
        Vector3 p = transform.position;
        float y = (waterProvider != null) ? waterProvider.GetWaterHeight(p) : waterLevel;
        Gizmos.DrawCube(new Vector3(p.x, y, p.z), new Vector3(0.5f, 0.001f, 0.5f));

        // 采样点
        if (samplePoints == null) return;
        Gizmos.color = Color.yellow;
        foreach (var sp in samplePoints)
        {
            if (!sp.col) continue;
            Vector3 w = sp.col.transform.TransformPoint(sp.localPos);
            Gizmos.DrawSphere(w, 0.005f);
        }

        // COM
        if (TryGetComponent<Rigidbody>(out var r))
        {
            Gizmos.color = Color.red;
            Gizmos.DrawSphere(transform.TransformPoint(r.centerOfMass), 0.01f);
        }
    }
#endif
}