Discussion on: Advent of Code 2020 Solution Megathread - Day 8: Handheld Halting

[Christopher Kruse]

Rust solution.

I spent longer than I care to admit trying to figure out the best way to access/use the data from the initial input in a mutable way: I started with returning the Program from the input parser, but nothing could be mutable that way; I tried boxes, I tried Rc, (both of which I haven't really played around with before); finally settled on making them Clone-able and just duplicating it for the run.

There's some control flow stuff in here that I'm sure there are better ways of solving (if you know or have a suggestion, please drop it in!).

Also gave me nightmares of last year's opcode parser... 😱

As always, on Github.

use aoc_runner_derive::{aoc, aoc_generator};
use regex::Regex;

struct Program {
    instructions: Vec<Instruction>,
    accumulator: isize,
    fp: isize,
    clean_exit: bool,
}

impl Program {
    fn new(instructions: Vec<Instruction>) -> Program {
        Program {
            instructions: instructions,
            accumulator: 0,
            fp: 0,
            clean_exit: true,
        }
    }
    fn run_until_cycle(&mut self) {
        loop {
            let mut inst = self.instructions.get_mut(self.fp as usize);
            match inst {
                Some(inst) => {
                    inst.visit_count += 1;
                    if inst.visit_count == 2 {
                        self.clean_exit = false;
                        return;
                    }
                    match &inst.opcode[..] {
                        "acc" => {
                            self.accumulator += inst.position;
                            self.fp += 1;
                        }
                        "jmp" => self.fp += inst.position,
                        "nop" => self.fp += 1,
                        _ => panic!("Unexpected instruction!"),
                    }
                }
                None => return,
            }
        }
    }
}

#[derive(Clone)]
struct Instruction {
    opcode: String,
    position: isize,
    visit_count: u8,
}

#[aoc_generator(day8)]
fn parse_input_day8(input: &str) -> Vec<Instruction> {
    let instruction_re =
        Regex::new("^(?P<opcode>\\w{3}) (?P<pos>[+-]\\d+)").expect("Couldn't create regex!");
    input
        .lines()
        .map(|line| {
            let captures = instruction_re.captures(line).expect("Didn't match regex!");
            Instruction {
                opcode: String::from(captures.name("opcode").unwrap().as_str()),
                position: str::parse(captures.name("pos").unwrap().as_str()).unwrap(),
                visit_count: 0,
            }
        })
        .collect()
}

#[aoc(day8, part1)]
fn read_accumulator_at_cycle(input: &Vec<Instruction>) -> isize {
    let insts = input.clone();
    let mut pg = Program::new(insts);
    pg.run_until_cycle();
    pg.accumulator
}

#[aoc(day8, part2)]
fn correct_broken_op(input: &Vec<Instruction>) -> isize {
    let mut pg: Program;
    let mut result: isize = 0;
    for (pos, inst) in input.iter().enumerate() {
        if &inst.opcode[..] == "nop" {
            let mut insts = input.clone();
            insts.get_mut(pos).unwrap().opcode = String::from("jmp");
            pg = Program::new(insts);
            pg.run_until_cycle();
            if pg.clean_exit {
                result = pg.accumulator;
                break;
            }
        }
        if &inst.opcode[..] == "jmp" {
            let mut insts = input.clone();
            insts.get_mut(pos).unwrap().opcode = String::from("nop");
            pg = Program::new(insts);
            pg.run_until_cycle();
            if pg.clean_exit {
                result = pg.accumulator;
                break;
            }
        }
    }

    result
}

[Neil Gall]

I think the insight you need is to separate the mutable from the immutable state. The program is immutable (except for the modifications in part 2) but the machine state is mutable. The instructions are immutable but the record of visiting them is mutable. Cloning the whole program and changing one instruction is the right approach however. Good work!