thief-mission-viewer/project/code/TMV/MeshSurfaceData.cs

using System;
using System.Collections.Generic;
using Godot;
using GArray = Godot.Collections.Array;

namespace KeepersCompound.TMV;

public class MeshSurfaceData
{
    public bool Empty { get; private set; }

    private readonly List<Vector3> _vertices = new();
    private readonly List<Vector3> _normals = new();
    private readonly List<int> _indices = new();
    private readonly List<Vector2> _uv1s = new();
    private readonly List<Vector2> _uv2s = new();

    public MeshSurfaceData()
    {
        Empty = true;
    }

    public void TransformUv2s(int start, int end, Func<Vector2, Vector2> f)
    {
        for (var i = start; i < end; i++)
        {
            _uv2s[i] = f(_uv2s[i]);
        }
    }

    // TODO: Guard against empty polygons being added
    public (int, int) AddPolygon(
        List<Vector3> vertices,
        Vector3 normal,
        List<Vector2> uv1s,
        List<Vector2> uv2s)
    {
        Empty = false;

        var vertexCount = vertices.Count;
        var indexOffset = _vertices.Count;
        _vertices.AddRange(vertices);

        for (var i = 0; i < vertexCount; i++)
        {
            _normals.Add(normal);
        }

        // Simple triangulation. Polys are always convex so we can just do a fan
        for (int j = 1; j < vertexCount - 1; j++)
        {
            _indices.Add(indexOffset);
            _indices.Add(indexOffset + j);
            _indices.Add(indexOffset + j + 1);
        }

        _uv1s.AddRange(uv1s);
        _uv2s.AddRange(uv2s);

        var end = _uv2s.Count;
        var start = end - uv2s.Count;
        return (start, end);
    }

    public GArray BuildSurfaceArray()
    {
        var array = new GArray();
        array.Resize((int)Mesh.ArrayType.Max);
        array[(int)Mesh.ArrayType.Vertex] = _vertices.ToArray();
        array[(int)Mesh.ArrayType.Normal] = _normals.ToArray();
        array[(int)Mesh.ArrayType.Index] = _indices.ToArray();
        array[(int)Mesh.ArrayType.TexUV] = _uv1s.ToArray();
        array[(int)Mesh.ArrayType.TexUV2] = _uv2s.ToArray();

        return array;
    }
}