advent-of-code/aoc2020/src/day08.rs

use std::collections::HashSet;
use std::fmt::Write;

use anyhow::{anyhow, bail, Context, Result};

const INPUT: &str = include_str!("../input/day08.txt");

pub fn run() -> Result<String> {
    let mut res = String::with_capacity(128);

    writeln!(res, "part 1: {}", part1(INPUT)?)?;
    writeln!(res, "part 2: {}", part2(INPUT)?)?;

    Ok(res)
}

fn part1(input: &str) -> Result<i64> {
    let instructions = input
        .lines()
        .map(|line| line.parse())
        .collect::<Result<Vec<Instruction>>>()?;

    let mut interpreter = Interpreter::new(instructions);

    Ok(match interpreter.run() {
        ExitStatus::InfiniteLoop(value) => value,
        ExitStatus::End(_) => bail!("interpreter doesn't have an infinite loop"),
    })
}

fn part2(input: &str) -> Result<i64> {
    let instructions = input
        .lines()
        .map(|line| line.parse())
        .collect::<Result<Vec<Instruction>>>()?;

    for idx in 0..instructions.len() {
        let mut instructions = instructions.clone();

        match instructions[idx] {
            Instruction::Acc(_) => continue,
            Instruction::Jmp(offset) => instructions[idx] = Instruction::Nop(offset),
            Instruction::Nop(offset) => instructions[idx] = Instruction::Jmp(offset),
        };

        let mut interpreter = Interpreter::new(instructions);

        match interpreter.run() {
            ExitStatus::InfiniteLoop(_) => continue,
            ExitStatus::End(value) => return Ok(value),
        }
    }

    Err(anyhow!(
        "interpreter always had an infinite loop, no solution found"
    ))
}

struct Interpreter {
    idx: usize,
    accumulator: i64,
    memory: Vec<Instruction>,
}

enum ExitStatus {
    InfiniteLoop(i64),
    End(i64),
}

impl Interpreter {
    fn new(instructions: Vec<Instruction>) -> Self {
        Self {
            idx: 0,
            accumulator: 0,
            memory: instructions,
        }
    }

    fn step(&mut self) {
        match self.memory[self.idx] {
            Instruction::Acc(arg) => {
                self.accumulator += arg;
                self.idx += 1;
            }
            Instruction::Jmp(offset) => self.idx = self.idx.wrapping_add(offset as usize),
            Instruction::Nop(_) => self.idx += 1,
        }
    }

    fn run(&mut self) -> ExitStatus {
        let mut set = HashSet::new();

        while self.idx < self.memory.len() {
            if !set.insert(self.idx) {
                return ExitStatus::InfiniteLoop(self.accumulator);
            }

            self.step();
        }

        ExitStatus::End(self.accumulator)
    }
}

#[derive(Clone)]
enum Instruction {
    Acc(i64),
    Jmp(i64),
    Nop(i64),
}

impl std::str::FromStr for Instruction {
    type Err = anyhow::Error;

    fn from_str(s: &str) -> Result<Self> {
        let space = s.find(' ').context("couldn't split on space")?;

        let inst = &s[..space];
        let arg = s[(space + 1)..]
            .parse()
            .context("couldn't parse argument for instruction")?;

        Ok(match inst {
            "acc" => Self::Acc(arg),
            "jmp" => Self::Jmp(arg),
            "nop" => Self::Nop(arg),
            _ => bail!("unrecognized instruction `{}`", inst),
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const PROVIDED: &str = include_str!("../input/day08_provided.txt");

    #[test]
    fn part1_provided() {
        assert_eq!(part1(PROVIDED).unwrap(), 5);
    }

    #[test]
    fn part1_real() {
        assert_eq!(part1(INPUT).unwrap(), 1675);
    }

    #[test]
    fn part2_provided() {
        assert_eq!(part2(PROVIDED).unwrap(), 8);
    }

    #[test]
    fn part2_real() {
        assert_eq!(part2(INPUT).unwrap(), 1532);
    }
}