using UnityEngine;
using System.Collections.Generic;

public class MoellonCutter
{
    public struct CutResult
    {
        public Mesh topMesh;
        public Mesh bottomMesh;
    }

    public CutResult Cut(Mesh originalMesh, Vector3 planePoint, Vector3 planeNormal)
    {
        List<Vector3> topVertices = new List<Vector3>();
        List<int> topTriangles = new List<int>();
        List<Vector3> bottomVertices = new List<Vector3>();
        List<int> bottomTriangles = new List<int>();

        Vector3[] vertices = originalMesh.vertices;
        int[] triangles = originalMesh.triangles;
        Plane cuttingPlane = new Plane(planeNormal.normalized, planePoint);

        for (int i = 0; i < triangles.Length; i += 3)
        {
            int i0 = triangles[i];
            int i1 = triangles[i + 1];
            int i2 = triangles[i + 2];

            Vector3 v0 = vertices[i0];
            Vector3 v1 = vertices[i1];
            Vector3 v2 = vertices[i2];

            bool side0 = cuttingPlane.GetSide(v0);
            bool side1 = cuttingPlane.GetSide(v1);
            bool side2 = cuttingPlane.GetSide(v2);

            if (side0 && side1 && side2)
            {
                AddTriangle(topVertices, topTriangles, v0, v1, v2);
            }
            else if (!side0 && !side1 && !side2)
            {
                AddTriangle(bottomVertices, bottomTriangles, v0, v1, v2);
            }
        }

        return new CutResult
        {
            topMesh = BuildMesh(topVertices, topTriangles),
            bottomMesh = BuildMesh(bottomVertices, bottomTriangles)
        };
    }

    private void AddTriangle(List<Vector3> verts, List<int> tris, Vector3 v0, Vector3 v1, Vector3 v2)
    {
        int baseIndex = verts.Count;
        verts.Add(v0);
        verts.Add(v1);
        verts.Add(v2);
        tris.Add(baseIndex);
        tris.Add(baseIndex + 1);
        tris.Add(baseIndex + 2);
    }

    private Mesh BuildMesh(List<Vector3> vertices, List<int> triangles)
    {
        Mesh mesh = new Mesh();
        mesh.SetVertices(vertices);
        mesh.SetTriangles(triangles, 0);
        mesh.RecalculateNormals();
        mesh.RecalculateBounds();
        return mesh;
    }
}