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. This is intentionally
## simple: a single chunk, generated at _ready(). Chunk streaming, biomes and
## server-side determinism (see docs/ARCHITECTURE.md §4) come in later milestones.
##
## The mesh is generated in code (no asset needed) so the project runs the moment
## Godot opens it, with nothing to import.

@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

func _ready() -> void:
	_build()

func _build() -> void:
	var noise := FastNoiseLite.new()
	noise.seed = noise_seed
	noise.noise_type = FastNoiseLite.TYPE_PERLIN
	noise.frequency = 0.012
	noise.fractal_octaves = 4

	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)

	# Build a grid of quads, two triangles each. Each vertex carries a planar UV
	# (world-XZ mapped to 0..1) so the surface is ready for texturing later.
	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, _height(noise, x0, z0), z0)
			var p10 := Vector3(x1, _height(noise, x1, z0), z0)
			var p01 := Vector3(x0, _height(noise, x0, z1), z1)
			var p11 := Vector3(x1, _height(noise, x1, z1), z1)

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

	# Normals give the lighting its shape. Tangents are only needed for normal
	# maps, which this low-poly material doesn't use — so we skip them (and avoid
	# the "UVs required to generate tangents" error).
	st.generate_normals()

	var mesh := st.commit()
	var mi := MeshInstance3D.new()
	mi.mesh = mesh

	# Simple ground material. Low-poly look = flat-ish colour + lighting does the rest.
	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 so the player can walk on it.
	var body := StaticBody3D.new()
	var shape := CollisionShape3D.new()
	var col := mesh.create_trimesh_shape()
	shape.shape = col
	body.add_child(shape)
	add_child(body)

func _height(noise: FastNoiseLite, x: float, z: float) -> float:
	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)