## Converts raw M2Loader data into a Godot Node3D scene tree.
## Usage:
##   var data = M2Loader.new().load_m2(abs_path)
##   var node = M2Builder.build(data, "res://extracted")
##   add_child(node)
class_name M2Builder

static var _texture_cache: Dictionary = {}

static func build(data: Dictionary, extracted_dir: String = "") -> Node3D:
	var root := Node3D.new()
	root.name = "M2"

	var verts:   PackedVector3Array = data.get("vertices", PackedVector3Array())
	var normals: PackedVector3Array = data.get("normals",  PackedVector3Array())
	var uvs:     PackedVector2Array = data.get("uvs",      PackedVector2Array())
	var indices: PackedInt32Array   = data.get("indices",  PackedInt32Array())
	var batches: Array              = data.get("batches",  [])

	if verts.is_empty() or indices.is_empty() or batches.is_empty():
		return root

	var textures:      PackedStringArray = data.get("textures",       PackedStringArray())
	var materials:     Array             = data.get("materials",       [])
	var tex_combos:    PackedInt32Array  = data.get("texture_combos",  PackedInt32Array())

	var mesh := ArrayMesh.new()

	for batch_variant in batches:
		if not (batch_variant is Dictionary):
			continue
		var batch: Dictionary  = batch_variant
		var idx_start: int     = int(batch.get("index_start",        0))
		var idx_count: int     = int(batch.get("index_count",         0))
		var mat_id: int        = int(batch.get("material_id",        -1))
		var tc_idx: int        = int(batch.get("texture_combo_index", -1))

		if idx_count <= 0 or idx_start + idx_count > indices.size():
			continue

		var batch_indices := indices.slice(idx_start, idx_start + idx_count)
		if batch_indices.is_empty():
			continue

		var arrays := []
		arrays.resize(Mesh.ARRAY_MAX)
		arrays[Mesh.ARRAY_VERTEX] = verts
		if normals.size() == verts.size():
			arrays[Mesh.ARRAY_NORMAL] = normals
		if uvs.size() == verts.size():
			arrays[Mesh.ARRAY_TEX_UV] = uvs
		arrays[Mesh.ARRAY_INDEX] = batch_indices

		mesh.add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, arrays)

		var surf_idx := mesh.get_surface_count() - 1
		var mat_def: Dictionary = materials[mat_id] if mat_id >= 0 and mat_id < materials.size() else {}
		mesh.surface_set_material(surf_idx, _build_material(mat_def, tc_idx, textures, tex_combos, extracted_dir))

	if mesh.get_surface_count() == 0:
		return root

	var mi := MeshInstance3D.new()
	mi.name = "Mesh"
	mi.mesh = mesh
	root.add_child(mi)
	return root


static func _build_material(
		mat_def: Dictionary,
		texture_combo_index: int,
		textures: PackedStringArray,
		tex_combos: PackedInt32Array,
		extracted_dir: String) -> StandardMaterial3D:

	var mat := StandardMaterial3D.new()
	mat.cull_mode          = BaseMaterial3D.CULL_DISABLED
	mat.roughness          = 0.85
	mat.metallic           = 0.0
	mat.shading_mode       = BaseMaterial3D.SHADING_MODE_UNSHADED
	mat.specular_mode      = BaseMaterial3D.SPECULAR_DISABLED
	mat.vertex_color_use_as_albedo = false

	var blend_mode: int = mat_def.get("blend_mode", 0)
	match blend_mode:
		0: mat.transparency = BaseMaterial3D.TRANSPARENCY_DISABLED
		1: mat.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA_SCISSOR
		_: mat.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA

	if texture_combo_index >= 0 and texture_combo_index < tex_combos.size():
		var tex_idx: int = tex_combos[texture_combo_index]
		if tex_idx >= 0 and tex_idx < textures.size():
			var tex_path: String = str(textures[tex_idx]).replace("\\", "/")
			if not tex_path.is_empty():
				var tex := _load_texture(tex_path, extracted_dir)
				if tex:
					mat.albedo_texture = tex
					mat.texture_filter = BaseMaterial3D.TEXTURE_FILTER_LINEAR_WITH_MIPMAPS_ANISOTROPIC

	return mat


static func _load_texture(rel_path: String, extracted_dir: String) -> Texture2D:
	if rel_path.is_empty() or extracted_dir.is_empty():
		return null

	var abs_path := ProjectSettings.globalize_path(extracted_dir.path_join(rel_path))
	if _texture_cache.has(abs_path):
		return _texture_cache[abs_path]

	if not ClassDB.class_exists("BLPLoader"):
		return null

	var loader = ClassDB.instantiate("BLPLoader")
	if loader == null:
		return null

	var img: Image = loader.call("load_image", abs_path)
	if img == null or img.is_empty():
		_texture_cache[abs_path] = null
		return null

	img.generate_mipmaps()
	var tex := ImageTexture.create_from_image(img)
	_texture_cache[abs_path] = tex
	return tex