verbose = False

def load_disk_map(filename: str = "input") -> str:
	with open(filename, "r") as fp:
		data = fp.read().rstrip("\n")
	return data

def disk_map_to_blocks(disk_map: str) -> list:
	blocks = []
	for i, s in enumerate(disk_map):
		# file
		if i % 2 == 0:
			ID = i // 2
			blocks += int(s) * [str(ID)]

		# free space
		else:
			blocks += int(s) * ["."]

	return blocks

def move_ltr_to_empty_space(blocks: list) -> str:
	left = 0
	right = len(blocks) - 1

	while left < right:
		if blocks[left] != ".":
			left += 1
		elif blocks[right] == ".":
			right -= 1
		else:
			tmp = blocks[left]
			blocks[left] = blocks[right]
			blocks[right] = tmp
		
		if verbose:
			print(blocks)

	return blocks

def checksum(blocks: str) -> int:
	return sum([position * int(ID) for position, ID in enumerate(blocks) if ID != "."])

if __name__ == "__main__":
	disk_map = load_disk_map("input")
	if verbose:
		print("Disk map:")
		print(disk_map)
		print()

	blocks = disk_map_to_blocks(disk_map)
	if verbose:
		print("Blocks:")
		print(blocks)

	result = move_ltr_to_empty_space(blocks)

	if verbose:
		print()
		print("Checksum:")
	print(checksum(result))