using System.Collections.Generic;
using UnityEngine;

/*网址
  https://blog.csdn.net/linxinfa/article/details/116808549  
 */

/// <summary>
/// 贝塞尔曲线工具
/// </summary>
public class BezierCurve : MonoBehaviour
{
    /// <summary>
    /// 控制点（包括起始点和终止点）
    /// </summary>
    [SerializeField]
    private Transform[] points;

    /// <summary>
    /// 精确度
    /// </summary>
    [SerializeField]
    private int accuracy = 20;

    private void Update()
    {
        // 绘制贝塞尔曲线
        Vector3 prev_pos = points[0].position;
        for (int i = 0; i <= accuracy; ++i)
        {
            Vector3 to = CalculateBezierPoint(i / (float)accuracy, points);
            Debug.DrawLine(prev_pos, to, Color.green);
            prev_pos = to;
        }
    }

    private void OnDrawGizmos()
    {
        Gizmos.color = Color.white;
        // 绘制控制点（包括起始点和终止点）
        for (int i = 0; i < points.Length; ++i)
        {
            if (i < points.Length - 1)
            {
                if (4 == points.Length && i == 1)
                {
                    continue;
                }
                Vector3 current = points[i].position;
                Vector3 next = points[i + 1].position;

                Gizmos.DrawLine(current, next);
            }
        }
    }

    /// <summary>
    /// 返回曲线路径
    /// </summary>
    /// <returns></returns>
    public List<Vector3> GeneratePaths()
    {
        List<Vector3> paths = new List<Vector3>();
        for (int i = 0; i <= accuracy; ++i)
        {
            Vector3 to = CalculateBezierPoint(i / (float)accuracy, points);
            paths.Add(to);
        }
        return paths;
    }

    /// <summary>
    /// 计算贝塞尔点
    /// </summary>
    private Vector3 CalculateBezierPoint(float t, Transform[] controlPoints)
    {
        Vector3[] points = new Vector3[controlPoints.Length];
        for (int i = 0; i < controlPoints.Length; i++)
        {
            points[i] = controlPoints[i].position;
        }
        return CalculateBezierPoint(t, points);
    }

    /// <summary>
    /// 计算贝塞尔点
    /// </summary>
    private Vector3 CalculateBezierPoint(float t, Vector3[] points)
    {
        int n = points.Length - 1;
        Vector3 result = Vector3.zero;

        for (int i = 0; i <= n; i++)
        {
            result += BinomialCoefficient(n, i) * Mathf.Pow(1 - t, n - i) * Mathf.Pow(t, i) * points[i];
        }
        return result;
    }

    /// <summary>
    /// 二项式系数
    /// </summary>
    private int BinomialCoefficient(int n, int k)
    {
        if (k == 0 || k == n)
        {
            return 1;
        }
        return BinomialCoefficient(n - 1, k - 1) + BinomialCoefficient(n - 1, k);
    }
}