day23.rs

//! [Day 23: Safe Cracking](https://adventofcode.com/2016/day/23)

use assembunny::{BunnyVM, REG_A};

fn compute_until_safe(a: i32, program: &str) -> i32 {
    let mut program = BunnyVM::new(program);
    program.registers[REG_A] = a;

    loop {
        program.step();
        if program.is_terminated() {
            break program.registers[REG_A];
        }
    }
}

fn factorial(n: i32) -> i32 {
    (1..=n).product()
}

pub fn main() {
    let args = aoc::parse_args();
    args.run(solve);
}

/// # Panics
#[must_use]
pub fn solve(program: &str) -> (i32, i32) {
    let part1 = compute_until_safe(7, program);

    // as the program calculates n! + constant,
    // we advantageously can reuse the answer from part 1
    // nota: constant=n1*n2 where n1,n2 are immediate in the program:
    //  19:     cpy n1 c
    //  20:     jnz n2 d
    let part2 = part1 + factorial(12) - factorial(7);

    (part1, part2)
}

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

    #[test]
    fn test_compute_until_safe() {
        assert_eq!(compute_until_safe(0, "cpy 41 a"), 41);
    }
}