extends RefCounted
## Converts Skeleton3D-skinned meshes into NON-skinned mesh pieces parented to
## BoneAttachment3D nodes, so rendering never uses GPU vertex-skinning (transform
## feedback). Required on ANGLE/Vulkan/Mali mobile GPUs (e.g. Mali-G720 in mobile
## Chrome/Brave), where Godot's Compatibility skinning path renders skinned meshes
## invisible while non-skinned meshes draw fine.
##
## Each triangle is assigned wholesale to its dominant bone (highest summed weight),
## so segments stay watertight; joints become rigid (no smooth bending), which suits
## the low-poly look. Call RigidSkin.convert_tree(character_root) once after the scene
## is instanced (bones don't need to be posed yet — pieces are baked in bind space).

static func convert_tree(root: Node) -> int:
	var converted := 0
	for skel: Skeleton3D in _find(root, "Skeleton3D", []):
		for mi: Node in skel.get_children():
			if mi is MeshInstance3D and (mi as MeshInstance3D).skin != null \
					and (mi as MeshInstance3D).mesh is ArrayMesh:
				if _convert(skel, mi as MeshInstance3D):
					converted += 1
	return converted


static func _find(n: Node, klass: String, acc: Array) -> Array:
	if n.is_class(klass):
		acc.append(n)
	for c: Node in n.get_children():
		_find(c, klass, acc)
	return acc


static func _convert(skel: Skeleton3D, mi: MeshInstance3D) -> bool:
	var mesh := mi.mesh as ArrayMesh
	var skin := mi.skin

	# bind index -> (bone index, bind pose = mesh-space -> bone-local)
	var bind_bone: PackedInt32Array = []
	var bind_pose: Array[Transform3D] = []
	for b: int in skin.get_bind_count():
		var bone := skin.get_bind_bone(b)
		if bone < 0:
			bone = skel.find_bone(skin.get_bind_name(b))
		bind_bone.append(bone)
		bind_pose.append(skin.get_bind_pose(b))

	# Accumulate SurfaceTool geometry per (destination bone, source surface). Keying on the
	# surface too — not just the bone — is what preserves materials: a head bone that carries
	# both the skin and hair surfaces must stay two pieces with two materials, otherwise every
	# surface funnelled to a bone collapses onto one material (bald matadors, wrong uniforms).
	var groups: Dictionary = {}   # "bone:surface" -> {"st": SurfaceTool, "mat": Material, "bone": int}

	for s: int in mesh.get_surface_count():
		var arr := mesh.surface_get_arrays(s)
		var verts: PackedVector3Array = arr[Mesh.ARRAY_VERTEX]
		var norms: PackedVector3Array = arr[Mesh.ARRAY_NORMAL]
		var uvs: PackedVector2Array = arr[Mesh.ARRAY_TEX_UV] if arr[Mesh.ARRAY_TEX_UV] != null else PackedVector2Array()
		var cols: PackedColorArray = arr[Mesh.ARRAY_COLOR] if arr[Mesh.ARRAY_COLOR] != null else PackedColorArray()
		var bones: PackedInt32Array = arr[Mesh.ARRAY_BONES]
		var weights: PackedFloat32Array = arr[Mesh.ARRAY_WEIGHTS]
		var idx: PackedInt32Array = arr[Mesh.ARRAY_INDEX]
		# Honour a per-instance override (whole mesh) or per-surface override the scene set,
		# falling back to the material baked into the mesh surface.
		var mat: Material = mi.material_override
		if mat == null:
			mat = mi.get_surface_override_material(s)
		if mat == null:
			mat = mesh.surface_get_material(s)
		var infl := 8 if (mesh.surface_get_format(s) & Mesh.ARRAY_FLAG_USE_8_BONE_WEIGHTS) else 4

		var tri := PackedInt32Array()
		if idx != null and idx.size() > 0:
			tri = idx
		else:
			tri.resize(verts.size())
			for i: int in verts.size():
				tri[i] = i

		for t: int in range(0, tri.size(), 3):
			var a := tri[t]
			var b := tri[t + 1]
			var c := tri[t + 2]
			var bind := _dominant_bind([a, b, c], bones, weights, infl)
			var bone := bind_bone[bind]
			var pose := bind_pose[bind]
			var key := "%d:%d" % [bone, s]
			var group: Dictionary = groups.get(key, {})
			if group.is_empty():
				var new_st := SurfaceTool.new()
				new_st.begin(Mesh.PRIMITIVE_TRIANGLES)
				group = {"st": new_st, "mat": mat, "bone": bone}
				groups[key] = group
			var st: SurfaceTool = group["st"]
			for v: int in [a, b, c]:
				if cols.size() > v:
					st.set_color(cols[v])
				if uvs.size() > v:
					st.set_uv(uvs[v])
				if norms.size() > v:
					st.set_normal((pose.basis * norms[v]).normalized())
				st.add_vertex(pose * verts[v])

	if groups.is_empty():
		return false

	# Combine all of a bone's surface-groups into ONE mesh under ONE BoneAttachment (each group
	# stays its own surface + material). Same draw calls as before, but roughly half the nodes to
	# transform/cull every frame — which is what a phone feels while ragdolls drive the skeleton.
	var by_bone: Dictionary = {}   # bone:int -> Array[Dictionary]
	for key: String in groups:
		var group: Dictionary = groups[key]
		var bone: int = group["bone"]
		if not by_bone.has(bone):
			by_bone[bone] = []
		(by_bone[bone] as Array).append(group)

	for bone: int in by_bone:
		var att := BoneAttachment3D.new()
		att.bone_name = skel.get_bone_name(bone)
		skel.add_child(att)
		var piece := MeshInstance3D.new()
		var combined := ArrayMesh.new()
		for group: Dictionary in by_bone[bone]:
			var st: SurfaceTool = group["st"]
			st.commit(combined)
			if group["mat"] != null:
				combined.surface_set_material(combined.get_surface_count() - 1, group["mat"])
		piece.mesh = combined
		att.add_child(piece)

	# Hide (don't free) the original: keeps the node for any gameplay code that references
	# it, and a hidden skinned mesh isn't drawn so it never hits the transform-feedback path.
	mi.visible = false
	return true


static func _dominant_bind(vs: Array, bones: PackedInt32Array, weights: PackedFloat32Array, infl: int) -> int:
	var acc: Dictionary = {}
	for v: int in vs:
		for k: int in infl:
			var w := weights[v * infl + k]
			if w <= 0.0:
				continue
			var bind := bones[v * infl + k]
			acc[bind] = float(acc.get(bind, 0.0)) + w
	var best := 0
	var best_w := -1.0
	for bind: int in acc:
		if acc[bind] > best_w:
			best_w = acc[bind]
			best = bind
	return best