aoc/src/day5.rs

use aoc_runner_derive::aoc;

use std::fmt;

#[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd)]
pub struct SeatId(u16);
impl fmt::Display for SeatId {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        <Self as fmt::Debug>::fmt(self, f)
    }
}

impl SeatId {
    fn parse(input: &[u8]) -> Option<SeatId> {
        if input.len() != 10 {
            return None;
        }
        let mut seatid = 0u16;

        for b in &input[..7] {
            let value = match b {
                b'F' => 0,
                b'B' => 1,
                _ => return None,
            };
            seatid = (seatid << 1) | value;
        }

        for b in &input[7..10] {
            let value = match b {
                b'L' => 0,
                b'R' => 1,
                _ => return None,
            };
            seatid = (seatid << 1) | value;
        }
        Some(SeatId(seatid))
    }
}

#[aoc(day5, part1)]
pub fn solve_d5_p1(input: &[u8]) -> SeatId {
    input
        .split(|&b| b == b'\n')
        .map(|i| SeatId::parse(i).unwrap())
        .max()
        .unwrap()
}

#[aoc(day5, part2)]
pub fn solve_d5_p2(input: &[u8]) -> SeatId {
    #[derive(Debug)]
    struct MinMaxSum {
        min: u16,
        max: u16,
        sum: u32,
    }
    let MinMaxSum { min, max, sum } = input
        .split(|&b| b == b'\n')
        .map(|i| SeatId::parse(i).unwrap())
        .fold(
            MinMaxSum {
                min: u16::MAX,
                max: 0,
                sum: 0,
            },
            |MinMaxSum { min, max, sum }, seatid| MinMaxSum {
                min: std::cmp::min(min, seatid.0),
                max: std::cmp::max(max, seatid.0),
                sum: sum + seatid.0 as u32,
            },
        );
    let max = max as u32;
    let min = min as u32;
    let num_seats = max - min + 1;
    let expected_sum = (max + min) * num_seats / 2;
    SeatId((expected_sum - sum) as u16)
}