advent-of-code/2017/day/9/src/main.rs

use std::fs::File;
use std::io::{Read, BufReader};
use std::char;

#[derive(Debug, Copy, Clone)]
enum Flags {
    InGarbage,
    CancelNext,
    CancelNextInGarbage,
    Normal
}

#[derive(Debug)]
pub struct State {
    pub score: usize,
    pub garbage_count: usize,
    group_depth: usize,
    flags: Flags,
}

fn main() {
    let file = File::open("input").expect("unable to open input file");
    let reader = BufReader::new(file);

    let (score, garbage_count) = stream(reader);
    println!("Part 1 (score): {}", score);
    println!("Part 2 (garbage count): {}", garbage_count);
}

fn stream<R>(reader: R) -> (usize, usize) where R: Read {
    let state = reader.bytes()
        .map(|byte| byte.ok().and_then(|b| char::from_u32(b as u32)))
        .fold(State { group_depth: 0, score: 0, garbage_count: 0, flags: Flags::Normal }, next_state);

    (state.score, state.garbage_count)
}

fn next_state(state: State, chr: Option<char>) -> State {
    use Flags::*;

    match (state.flags, chr) {
        (CancelNext, _)          => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth, flags: Normal },
        (CancelNextInGarbage, _) => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth, flags: InGarbage },
        (Normal, Some('{'))      => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth + 1, flags: Normal },
        (Normal, Some('}'))      => State { score: state.score + state.group_depth, garbage_count: state.garbage_count, group_depth: state.group_depth - 1, flags: Normal },
        (Normal, Some('<'))      => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth, flags: InGarbage },
        (InGarbage, Some('>'))   => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth, flags: Normal },
        (InGarbage, Some('!'))   => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth, flags: CancelNextInGarbage },
        (InGarbage, Some(_))     => State { score: state.score, garbage_count: state.garbage_count + 1, group_depth: state.group_depth, flags: InGarbage },
        (Normal, Some('!'))      => State { score: state.score, garbage_count: state.garbage_count, group_depth: state.group_depth, flags: CancelNext },
        (_, Some(_))             => state,
        (_, None)                => panic!("error reading/converting byte")
    }
}

// {}, score of 1.
#[test]
fn test_empty() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 1);
}

// {{{}}}, score of 1 + 2 + 3 = 6.
#[test]
fn test_empty_nested() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{{{}}}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 6);
}

// {{},{}}, score of 1 + 2 + 2 = 5.
#[test]
fn test_empty_empty() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{{},{}}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 5);
}

// {{{},{},{{}}}}, score of 1 + 2 + 3 + 3 + 3 + 4 = 16.
#[test]
fn test_empty_empty_nested_empty() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{{{},{},{{}}}}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 16);
}

// {<a>,<a>,<a>,<a>}, score of 1.
#[test]
fn test_four_garbage() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<a>,<a>,<a>,<a>}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 1);
}

// {{<ab>},{<ab>},{<ab>},{<ab>}}, score of 1 + 2 + 2 + 2 + 2 = 9.
#[test]
fn test_groups_with_garbage() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{{<ab>},{<ab>},{<ab>},{<ab>}}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 9);
}

// {{<!!>},{<!!>},{<!!>},{<!!>}}, score of 1 + 2 + 2 + 2 + 2 = 9.
#[test]
fn test_groups_with_cancellations() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{{<!!>},{<!!>},{<!!>},{<!!>}}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 9);
}

// {{<a!>},{<a!>},{<a!>},{<ab>}}, score of 1 + 2 = 3.
#[test]
fn test_groups_with_garbage_exclamation() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{{<a!>},{<a!>},{<a!>},{<ab>}}");
    let (score, _) = stream(cursor);
    assert_eq!(score, 3);
}

// Part Two

// <>, 0 characters.
#[test]
fn test_garbage_empty() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 0);
}

// <random characters>, 17 characters.
#[test]
fn test_garbage_count_random_characters() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<random characters>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 17);
}

// <<<<>, 3 characters.
#[test]
fn test_garbage_repeated_start() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<<<<>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 3);
}

// <{!>}>, 2 characters.
#[test]
fn test_garbage_cancel_one() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<{!>}>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 2);
}

// <!!>, 0 characters.
#[test]
fn test_garbage_cancel_cancel() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<!!>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 0);
}

// <!!!>>, 0 characters.
#[test]
fn test_garbage_cancel_cancel_end() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<!!!>>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 0);
}

// <{o"i!a,<{i<a>, 10 characters.
#[test]
fn test_garbage_gibberish() {
    use std::io::Cursor;

    let cursor = Cursor::new(b"{<{o\"i!a,<{i<a>}");
    let (_, garbage_count) = stream(cursor);
    assert_eq!(garbage_count, 10);
}