make representation of empty tiles consistent.

previously, world generation would represent empty tiles as the number
0, while other code would expect them to be an object with its `type`
property set to `"empty"`. the object representation is now used
consistently.

js/graphics.mjs

@@ -37,8 +37,7 @@ export function draw_world (box, start_x = 0, start_y = 0, scale = BASE_SCALE) {
 
 	draw_floor(start_x, start_y, scale, !checkerboard(start_x, start_y));
 
-	iter_2d(range(0, world.BOX_SIZE - 1), (x, y) => {
+	world.for_each_tile(box, (x, y, tile) => {
-		const tile = world.get_tile(box, x, y);
 		draw_tile(tile, x * scale + start_x, y * scale + start_y, scale);
 	});
 

js/player.mjs

@@ -33,9 +33,7 @@ on_press(" ", _ => {
 });
 
 function clear_tile (x, y) {
-	world.set_tile(player.box, x, y, {
+	world.set_tile(player.box, x, y, world.empty_tile());
-		type: "empty",
-	});
 }
 
 on_press("Backspace", _ => {

js/world.mjs

@@ -7,12 +7,18 @@ export const CENTER = Math.floor(BOX_SIZE / 2);
 
 function create_world (size) {
 	return {
-		tiles: Array(size).fill(0).map(_ => Array(size).fill(0)),
+		tiles: Array(size).fill(0).map(_ =>
+			Array(size).fill(0).map(_ => empty_tile())
+		),
 		depth: 0,
 	};
 }
 const world = create_world(BOX_SIZE);
 
+export function empty_tile () {
+	return { type: "empty" };
+}
+
 export function create_new () {
 	return create_world(BOX_SIZE);
 }
@@ -73,7 +79,10 @@ function reconstruct_parent (world, parent) {
 }
 
 export function from_json (json) {
-	const result = JSON.parse(json);
+	const result = JSON.parse(json, (key, value) => {
+		if (value === undefined || value === 0 || value === null) return empty_tile();
+		return value;
+	});
 
 	for_each_tile(result, (x, y, tile) => {
 		if (tile.type === "box") {