use std::error::Error;
use std::fs::File;
use std::io::{self, BufRead};
use std::vec::Vec;

fn upper_half(half: (u32, u32)) -> (u32, u32) {
    let (low, high) = half;

    if low == high {
        (low, high)
    } else if high - low == 1 {
        (high, high)
    } else {
        (low + (high - low) / 2 + high % 2, high)
    }
}

fn lower_half(half: (u32, u32)) -> (u32, u32) {
    let (low, high) = half;

    if low == high {
        (low, high)
    } else if high - low == 1 {
        (low, low)
    } else {
        (low, high - (high - low) / 2 - high % 2)
    }
}

fn partition(rows: (u32, u32), cols: (u32, u32), location: &str) -> ((u32, u32), (u32, u32)) {
    if location == "" {
        return (rows, cols);
    }

    let head = location.chars().nth(0).unwrap();
    let tail = &location[1..];

    match head {
        'F' => partition(lower_half(rows), cols, tail),
        'B' => partition(upper_half(rows), cols, tail),
        'L' => partition(rows, lower_half(cols), tail),
        'R' => partition(rows, upper_half(cols), tail),
        _ => panic!("Unkown location part: {}", head),
    }
}

fn main() -> Result<(), Box<dyn Error>> {
    let file = File::open("inputs/day5.txt")?;
    let lines = io::BufReader::new(file).lines().map(|l| l.unwrap());

    let mut seat_ids: Vec<u32> = lines
        .map(|loc| partition((0, 127), (0, 7), &loc))
        .map(|((r, _), (c, _))| r * 8 + c)
        .collect();

    seat_ids.sort();
    let max = seat_ids.iter().last().unwrap();

    println!("MaxID: {}", max);

    for pair in seat_ids.windows(2) {
        if pair[1] != pair[0] + 1 {
            println!("Missing ID: {}", pair[0] + 1);
            break;
        }
    }

    Ok(())
}