extends Node3D
## Terrain generator — M0 placeholder.
##
## Builds ONE heightmap chunk from deterministic noise, so the world already
## looks like rolling ground rather than a flat plane. Single chunk, generated at
## _ready(). Chunk streaming, biomes and server-side determinism (see
## docs/ARCHITECTURE.md §4) come in later milestones.
##
## Collision uses MeshInstance3D.create_trimesh_collision() (mesh-derived), which
## registers reliably in the physics space. A hand-built ConcavePolygonShape3D
## via set_faces() did NOT register (player/rays passed through) — avoid it.
##
## Exposes `sample_height(x, z)` so other systems (player spawn, prop placement)
## can sit exactly on the surface.

signal ready_built   ## emitted once the mesh + collision exist

@export var size: int = 128          # chunk is size x size metres
@export var resolution: int = 96     # vertices per side (higher = smoother/heavier)
@export var height_scale: float = 10.0
@export var noise_seed: int = 1337

var _noise: FastNoiseLite
var built: bool = false

func _ready() -> void:
	_build()

func _ensure_noise() -> void:
	if _noise != null:
		return
	_noise = FastNoiseLite.new()
	_noise.seed = noise_seed
	_noise.noise_type = FastNoiseLite.TYPE_PERLIN
	_noise.frequency = 0.012
	_noise.fractal_octaves = 4

func _build() -> void:
	_ensure_noise()

	var st := SurfaceTool.new()
	st.begin(Mesh.PRIMITIVE_TRIANGLES)

	var step := float(size) / float(resolution)
	var half := size * 0.5
	var uv_scale := 1.0 / float(size)

	for z in range(resolution):
		for x in range(resolution):
			var x0 := x * step - half
			var z0 := z * step - half
			var x1 := (x + 1) * step - half
			var z1 := (z + 1) * step - half

			var p00 := Vector3(x0, sample_height(x0, z0), z0)
			var p10 := Vector3(x1, sample_height(x1, z0), z0)
			var p01 := Vector3(x0, sample_height(x0, z1), z1)
			var p11 := Vector3(x1, sample_height(x1, z1), z1)

			_add_tri(st, p00, p01, p11, half, uv_scale)
			_add_tri(st, p00, p11, p10, half, uv_scale)

	st.generate_normals()

	var mesh := st.commit()
	var mi := MeshInstance3D.new()
	mi.name = "TerrainMesh"
	mi.mesh = mesh
	var mat := StandardMaterial3D.new()
	mat.albedo_color = Color(0.33, 0.45, 0.22)   # mossy green
	mat.roughness = 0.95
	mi.material_override = mat
	add_child(mi)

	# Collision: a HeightMapShape3D, NOT a concave trimesh.
	#
	# Root cause of the fall-through (confirmed by instrumented tests): a concave
	# polygon shape — whether hand-built via set_faces() or mesh-derived via
	# create_trimesh_collision() — is ONE-SIDED. It only collides on the front of
	# each triangle, so with this mesh's winding the player dropped straight
	# through from above. A box on the same StaticBody collided fine, which pinned
	# the cause to the concave shape, not layers/timing/registration.
	#
	# HeightMapShape3D is the correct primitive for heightmap terrain: a grid of
	# height samples, solid (two-sided), and far cheaper than a trimesh.
	var hm := HeightMapShape3D.new()
	hm.map_width = resolution + 1
	hm.map_depth = resolution + 1
	var data := PackedFloat32Array()
	data.resize((resolution + 1) * (resolution + 1))
	var hi := 0
	for hz in range(resolution + 1):
		for hx in range(resolution + 1):
			var wx := hx * step - half
			var wz := hz * step - half
			data[hi] = sample_height(wx, wz)
			hi += 1
	hm.map_data = data

	var body := StaticBody3D.new()
	body.name = "TerrainBody"
	add_child(body)
	var col := CollisionShape3D.new()
	col.name = "CollisionShape3D"
	col.shape = hm
	body.add_child(col)
	# HeightMapShape3D samples are spaced 1 unit apart; scale the body so the
	# collision grid covers the same `size` metres as the visual mesh.
	body.scale = Vector3(step, 1.0, step)

	built = true
	ready_built.emit()

## World-space ground height at (x, z).
func sample_height(x: float, z: float) -> float:
	_ensure_noise()
	return _noise.get_noise_2d(x, z) * height_scale

func _add_tri(st: SurfaceTool, a: Vector3, b: Vector3, c: Vector3, half: float, uv_scale: float) -> void:
	st.set_uv(Vector2((a.x + half) * uv_scale, (a.z + half) * uv_scale))
	st.add_vertex(a)
	st.set_uv(Vector2((b.x + half) * uv_scale, (b.z + half) * uv_scale))
	st.add_vertex(b)
	st.set_uv(Vector2((c.x + half) * uv_scale, (c.z + half) * uv_scale))
	st.add_vertex(c)