AoC 2024 Day 9 part 2

with open("day9input.txt") as f:
    input_nums = [int(x) for x in f.read()]

len_input = len(input_nums)

file_sizes = input_nums[0:len_input:2]
empty_spaces = input_nums[1:len_input:2]


# construct the disk representation of the input
# store all the files / empty space ranges for later
disk_representation = []
file_ranges = []
empty_ranges = []
for idx in range(len(file_sizes)):
    left = len(disk_representation)
    disk_representation += [idx] * file_sizes[idx]
    right = len(disk_representation)-1
    file_ranges.append([left, right])
    if idx < len(empty_spaces):
        left = len(disk_representation)
        disk_representation += ['.'] * empty_spaces[idx]
        right = len(disk_representation)-1
        if empty_spaces[idx] > 0:
            empty_ranges.append([left, right])

curr_file_id = len(file_sizes) - 1
# once for each file, start looking for a large enough empty space from the beginning of the disk
while curr_file_id > 0:
    curr_file_size = file_ranges[curr_file_id][1] - file_ranges[curr_file_id][0] + 1
    for empty in empty_ranges:
        if empty[0] > file_ranges[curr_file_id][0]:
            # break out of the loop if the empty space isn't to the left of the file
            break
        if curr_file_size <= empty[1] - empty[0] + 1:
            # space found. right_idx is the rightmost index of the file in the disk representation
            # left_idx is the leftmost index of the empty space
            right_idx = file_ranges[curr_file_id][1]
            left_idx = empty[0]
            for _ in range(curr_file_size):
                # looks a little wonky, but this just swaps the values at left_idx and right_idx
                (disk_representation[left_idx],
                 disk_representation[right_idx]) = (disk_representation[right_idx],
                                                    disk_representation[left_idx])
                right_idx -= 1
                left_idx += 1
            empty[0] += curr_file_size
            break
    curr_file_id -= 1


# count up the output, ignoring empty space
output = 0
for idx in range(len(disk_representation)):
    if disk_representation[idx] != '.':
        output += disk_representation[idx] * idx
print(output)