advent/2021/src/day18.rs

use color_eyre::eyre;


#[derive(Debug, Clone)]
enum Node {
    Leaf(u8),
    Branch {
        left: Box<Node>,
        right: Box<Node>,
    }
}
use Node::*;

impl Node {
    fn unwrap_branch(&self) -> (&Node, &Node) {
        match self {
            Node::Leaf(_) => panic!("Attempted to unwrap branches from a leaf node"),
            Node::Branch {left, right} => (left, right),
        }
    }

    fn leftmost_value_mut(&mut self) -> &mut u8 {
        match self {
            Node::Leaf(val) => val,
            Node::Branch {left, ..} => left.leftmost_value_mut(),
        }
    }

    fn rightmost_value_mut(&mut self) -> &mut u8 {
        match self {
            Node::Leaf(val) => val,
            Node::Branch {right, ..} => right.rightmost_value_mut(),
        }
    }

    fn magnitude(&self) -> usize {
        match self {
            Node::Leaf(v) => *v as usize,
            Node::Branch {left, right} => 3 * left.magnitude() + 2 * right.magnitude(),
        }
    }
}


struct Parser<'a> {
    chars: std::str::Chars<'a>,
}

impl<'a> Parser<'a> {
    fn from(src: &'a str) -> Parser<'a> {
        let mut chars = src.chars();
        chars.next(); // throw away the opening bracket to avoid nesting too deeply
        Parser {chars}
    }

    fn parse(&mut self) -> Node {
        let mut left = None;
        loop {
            let node = match self.chars.next() {
                Some('[') => self.parse(),
                Some(','|']') => {continue;},
                Some(c) => Node::Leaf(c.to_digit(10).unwrap() as u8),
                None => {dbg!(left); panic!("Ran out of characters while parsing");},
            };

            if left.is_none() {
                left = Some(node);
            }
            else {
                return Node::Branch {
                    left: Box::new(left.unwrap()),
                    right: Box::new(node),
                };
            }
        }
    }
}

fn load(data: &str) -> Vec<Node> {
    let mut nodes = Vec::new();
    for line in data.lines() {
        let mut parser = Parser::from(line);
        nodes.push(parser.parse());
    }
    nodes
}


fn explode(node: &mut Node, depth: usize) -> Option<(u8, u8)> {
    // only branch nodes can explode
    if let Leaf(_) = node {return None;}

    if depth >= 4 {
        let (left, right) = node.unwrap_branch();
        match (left, right) {
            (Leaf(v1), Leaf(v2)) => {
                let result = (*v1, *v2);
                *node = Leaf(0);
                return Some(result);
            },
            _ => (),
        }
    }

    if let Branch {left, right} = node {
        if let Some((v1, v2)) = explode(left, depth + 1) {
            if v2 > 0 {
                *right.leftmost_value_mut() += v2;
                return Some((v1, 0));
            }
            return Some((v1, v2));
        }

        if let Some((v1, v2)) = explode(right, depth + 1) {
            if v1 > 0 {
                *left.rightmost_value_mut() += v1;
                return Some((0, v2));
            }
            return Some((v1, v2));
        }
    }

    return None;
}


fn split(node: &mut Node) -> bool {
    match node {
        Leaf(v) if *v > 9 => {
            *node = Branch {
                left: Box::new(Leaf(*v / 2)),
                right: Box::new(Leaf((*v + 1) / 2)),
            };
            return true;
        },
        Leaf(_) => return false,
        Branch {left, right} => {
            if split(left) {
                return true;
            }
            else {
                return split(right);
            }
        }
    }
}


fn reduce(node: &mut Node) {
    loop {
        if let Some(..) = explode(node, 0) {
            continue;
        }
        if split(node) {
            continue;
        }
        break;
    }
}


fn add(left: Node, right: Node) -> Node {
    let mut sum = Node::Branch {
        left: Box::new(left),
        right: Box::new(right),
    };
    reduce(&mut sum);
    sum
}


pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
    let nodes = load(data);
    let one = nodes.into_iter().reduce(|sum, next| add(sum, next)).unwrap();

    // I'm sure there's a more efficient way to do this, but
    let mut max = 0;
    let nodes2 = load(data);
    for i in 0..nodes2.len() {
        for j in 0..nodes2.len() {
            if i == j {continue;}
            let a = nodes2[i].clone();
            let b = nodes2[j].clone();
            let magnitude = add(a, b).magnitude();
            if magnitude > max {
                max = magnitude;
            }
        }
    }

    Ok((one.magnitude(), max))
}