class_name TileShape
extends RefCounted
## Neighbourhoods on the tile grid, so that a style can draw each tile to fit
## the tiles around it, and a stable number per tile for small variations.

## Bits of a neighbour mask, clockwise from north.
const N := 1
const NE := 2
const E := 4
const SE := 8
const S := 16
const SW := 32
const W := 64
const NW := 128
## Where the neighbour of each bit lies, in the order of the bits.
const OFFSETS: Array[Vector2i] = [
	Vector2i(0, -1), Vector2i(1, -1), Vector2i(1, 0), Vector2i(1, 1),
	Vector2i(0, 1), Vector2i(-1, 1), Vector2i(-1, 0), Vector2i(-1, -1),
]


## One byte per tile of `codes`: 1 if `members` holds its code, else 0.
static func flags(codes: PackedByteArray, members: Array[int]) -> PackedByteArray:
	var result := PackedByteArray()
	result.resize(codes.size())
	for index in codes.size():
		result[index] = 1 if members.has(codes[index]) else 0
	return result


## The neighbours of `tile` whose flag is 1, as a mask of the bits above, on
## a map of `width` by `height` tiles. Beyond the edge of the map, every
## neighbour counts as set if `outside_set` is true.
static func mask(tile_flags: PackedByteArray, width: int, height: int, tile: Vector2i, outside_set: bool) -> int:
	var result := 0
	for bit in 8:
		var at := tile + OFFSETS[bit]
		var inside := at.x >= 0 and at.y >= 0 and at.x < width and at.y < height
		if (inside and tile_flags[at.y * width + at.x] == 1) or (not inside and outside_set):
			result |= 1 << bit
	return result


## A number from 0 to 2147483647 that is the same for `tile` every time and
## looks random from one tile to the next.
static func variant(tile: Vector2i) -> int:
	var h := (tile.x * 374761393 + tile.y * 668265263) & 0x7fffffff
	h = ((h ^ (h >> 13)) * 1274126177) & 0x7fffffff
	return h ^ (h >> 16)