Add distance transform algorithm

[jciskey]

Adds a basic implementation of the distance transform algorithm.

[khoover]

Suggested change

	for x in 0..ROOM_SIZE {
	for y in 0..ROOM_SIZE {
	let position = unsafe { RoomXY::unchecked_new(x as u8, y as u8) };
	let val = match &room_data.terrain.get_xy(position) {
	screeps::constants::Terrain::Wall => 0,
	_ => 255,
	};
	initial_cm.set(position, val);
	}
	for (xy, cm_val) in initial_cm.iter_mut() {
	*cm_val = match room_data.terrain.get_xy(xy) {
	screeps::constants::Terrain::Wall => 0,
	_ => 255
	};
	}

[khoover]

Suggested change

	// all of its neighbors. However, since we're going RTL:TTB, we can ignore tiles we
	// all of its neighbors. However, since we're going LTR:TTB, we can ignore tiles we

[khoover]

We can also ignore TopRight.

[khoover]

Suggested change

	let mut neighbor_values: [u8; 5] = [255; 5];
	if let Some(neighbor_position) = current_position.checked_add_direction(Direction::Top) {
	let value = cm.get(neighbor_position);
	neighbor_values[0] = value;
	}
	if let Some(neighbor_position) = current_position.checked_add_direction(Direction::Left) {
	let value = cm.get(neighbor_position);
	neighbor_values[1] = value;
	}
	if let Some(neighbor_position) = current_position.checked_add_direction(Direction::TopLeft) {
	let value = cm.get(neighbor_position);
	neighbor_values[2] = value;
	}
	if let Some(neighbor_position) = current_position.checked_add_direction(Direction::TopRight) {
	let value = cm.get(neighbor_position);
	neighbor_values[3] = value;
	}
	if let Some(neighbor_position) = current_position.checked_add_direction(Direction::BottomLeft) {
	let value = cm.get(neighbor_position);
	neighbor_values[4] = value;
	}
	let current_value = cm.get(current_position);
	let neighbors_minimum = match neighbor_values.iter().min() {
	Some(value) => match value {
	255 => 255,
	_ => value + 1
	},
	None => 255,
	};
	let min_value = cmp::min(current_value, neighbors_minimum);
	let min_value = [Direction::Top, Direction::TopLeft, Direction::Left, Direction::BottomLeft].into_iter()
	.filter_map(|dir| current_position.checked_add_direction(dir))
	.map(|position| cm.get(position))
	.min()
	.map(|x| x.saturating_add(1))
	.map(|x| x.min(cm.get(current_position)))
	.unwrap_or_else(|| cm.get(current_position));

Can do basically the same thing for the reverse direction too.