const ALLOW_SEPARATION: bool = false;

pub fn day9() {
    let mut regions: Vec<(Option<usize>, usize)> = crate::input(9)
        .chars()
        .filter_map(|c| c.to_string().parse::<usize>().ok())
        .enumerate()
        .flat_map(|(i, size)| {
            let file_id = (i % 2 == 0).then_some(i / 2);
            if ALLOW_SEPARATION {
                vec![(file_id, 1); size]
            } else {
                vec![(file_id, size)]
            }
        })
        .collect();

    for i in (0..regions.len()).rev() {
        let (Some(_), file_size) = regions[i] else {
            continue;
        };
        let Some(slot_index) = regions[..i]
            .iter()
            .position(|&(slot, slot_size)| slot.is_none() && slot_size >= file_size)
        else {
            continue;
        };
        let slot_size = regions[slot_index].1;

        regions[slot_index] = regions[i];
        regions[i] = (None, file_size);
        if slot_size > file_size {
            regions.insert(slot_index + 1, (None, slot_size - file_size));
        }
    }

    let disk: Vec<Option<usize>> = regions
        .into_iter()
        .flat_map(|(file_id, size)| vec![file_id; size])
        .collect();

    let checksum: usize = disk
        .into_iter()
        .enumerate()
        .filter_map(|(i, f)| Some(i * f? as usize))
        .sum();

    println!("Checksum: {}", checksum);
}