RoApp/Assets/Editor/UI/ShapeEditor/SplineUtility.cs

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

public class SplineUtility
{
    public static void CalculateTangents(Vector3 point, Vector3 prevPoint, Vector3 nextPoint, Vector3 forward, float scale, out Vector3 rightTangent, out Vector3 leftTangent)
    {
        Vector3 v1 = (prevPoint - point).normalized;
        Vector3 v2 = (nextPoint - point).normalized;
        Vector3 v3 = v1 + v2;
        Vector3 cross = forward;

        if (prevPoint != nextPoint)
        {
            bool colinear = Mathf.Abs(v1.x * v2.y - v1.y * v2.x + v1.x * v2.z - v1.z * v2.x + v1.y * v2.z - v1.z * v2.y) < 0.01f;

            if (colinear)
            {
                rightTangent = v2 * scale;
                leftTangent = v1 * scale;
                return;
            }

            cross = Vector3.Cross(v1, v2);
        }

        rightTangent = Vector3.Cross(cross, v3).normalized * scale;
        leftTangent = -rightTangent;
    }

    public static int NextIndex(int index, int pointCount)
    {
        return Mod(index + 1, pointCount);
    }

    public static int PreviousIndex(int index, int pointCount)
    {
        return Mod(index - 1, pointCount);
    }

    private static int Mod(int x, int m)
    {
        int r = x % m;
        return r < 0 ? r + m : r;
    }
}