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.
##
## The mesh is generated in code (no asset needed) so the project runs the moment
## Godot opens it, with nothing to import.
##
## Exposes `sample_height(x, z)` so other systems (player spawn, prop placement)
## can sit exactly on the surface instead of guessing a Y and falling through.

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 _build() -> void:
	_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, 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)

	# 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.
	st.generate_normals()

	var mesh := st.commit()
	var mi := MeshInstance3D.new()
	mi.name = "TerrainMesh"
	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. Build the body BEFORE marking built
	# so anything spawning on ready_built already has ground under it.
	var body := StaticBody3D.new()
	body.name = "TerrainBody"
	var shape := CollisionShape3D.new()
	shape.shape = mesh.create_trimesh_shape()
	body.add_child(shape)
	add_child(body)

	built = true
	ready_built.emit()

## World-space ground height at (x, z). Safe to call before or after _build()
## because it lazily creates the noise object.
func sample_height(x: float, z: float) -> float:
	if _noise == null:
		_noise = FastNoiseLite.new()
		_noise.seed = noise_seed
		_noise.noise_type = FastNoiseLite.TYPE_PERLIN
		_noise.frequency = 0.012
		_noise.fractal_octaves = 4
	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)