day02.rs

//! # Dive!
//!
//! Both part 1 and part 2 rely on the [`fold`] method. This method comes in useful for a lot
//! of Advent of Code problems so is handy to know about. The input is parsed into a tuple enum
//! [`Sub`] for convenience.
//!
//! [`fold`]: Iterator::fold
use crate::util::iter::*;
use crate::util::parse::*;

#[derive(Clone, Copy)]
pub enum Sub {
    Up(i32),
    Down(i32),
    Forward(i32),
}

pub fn parse(input: &str) -> Vec<Sub> {
    let helper = |[a, b]: [&str; 2]| {
        let amount = b.signed();
        match a {
            "up" => Sub::Up(amount),
            "down" => Sub::Down(amount),
            "forward" => Sub::Forward(amount),
            _ => unreachable!(),
        }
    };
    input.split_ascii_whitespace().chunk::<2>().map(helper).collect()
}

pub fn part1(input: &[Sub]) -> i32 {
    let helper = |(position, depth), next| match next {
        Sub::Up(n) => (position, depth - n),
        Sub::Down(n) => (position, depth + n),
        Sub::Forward(n) => (position + n, depth),
    };
    let (position, depth) = input.iter().copied().fold((0, 0), helper);
    position * depth
}

pub fn part2(input: &[Sub]) -> i32 {
    let helper = |(position, depth, aim), next| match next {
        Sub::Up(n) => (position, depth, aim - n),
        Sub::Down(n) => (position, depth, aim + n),
        Sub::Forward(n) => (position + n, depth + aim * n, aim),
    };
    let (position, depth, _) = input.iter().copied().fold((0, 0, 0), helper);
    position * depth
}