using UnityEngine;
using UnityEditor;
using System.IO;
using System.Text;

public class MeshAssetObjExporter
{
    [MenuItem("Assets/Export Mesh (.asset) to OBJ", true)]
    static bool ValidateExport()
    {
        return Selection.activeObject is Mesh;
    }

    [MenuItem("Assets/Export Mesh (.asset) to OBJ")]
    static void ExportMeshAssetToObj()
    {
        Mesh mesh = Selection.activeObject as Mesh;
        if (mesh == null)
        {
            Debug.LogWarning("Please select a Mesh .asset file in the Project window.");
            return;
        }

        string path = EditorUtility.SaveFilePanel("Export OBJ", "", mesh.name + ".obj", "obj");
        if (string.IsNullOrEmpty(path)) return;

        ExportMeshToObj(mesh, path);
        Debug.Log("Mesh exported to: " + path);
    }

    static Vector3 ConvertToBlender(Vector3 v)
    {
        // Unity坐标 (X, Y, Z) → Blender坐标 (X, Z, -Y)
        return new Vector3(v.x, v.z, -v.y);
    }

    static void ExportMeshToObj(Mesh mesh, string path)
    {
        StringBuilder sb = new StringBuilder();
        sb.AppendLine("# Exported from Unity MeshAssetObjExporter");
        sb.AppendLine("g " + mesh.name);

        // 顶点
        foreach (Vector3 v in mesh.vertices)
        {
            Vector3 vBlender = ConvertToBlender(v);
            sb.AppendFormat("v {0} {1} {2}\n", vBlender.x, vBlender.y, vBlender.z);
        }

        // 法线
        foreach (Vector3 n in mesh.normals)
        {
            Vector3 nBlender = ConvertToBlender(n);
            sb.AppendFormat("vn {0} {1} {2}\n", nBlender.x, nBlender.y, nBlender.z);
        }

        // UV
        foreach (Vector2 uv in mesh.uv)
        {
            sb.AppendFormat("vt {0} {1}\n", uv.x, uv.y);
        }

        // 面
        for (int i = 0; i < mesh.subMeshCount; i++)
        {
            int[] tris = mesh.GetTriangles(i);
            for (int j = 0; j < tris.Length; j += 3)
            {
                int a = tris[j] + 1;
                int b = tris[j + 1] + 1;
                int c = tris[j + 2] + 1;
                sb.AppendFormat("f {0}/{0}/{0} {1}/{1}/{1} {2}/{2}/{2}\n", a, b, c);
            }
        }

        File.WriteAllText(path, sb.ToString());
    }
}