Advent of Code Day 7 Part 2

main.rs

struct Equation {
    result: u64,
    ops: Box<[u64]>
}

fn parse_input() -> Vec<Equation> {
    let mut eqs = Vec::new();
    for line in std::fs::read_to_string("input").unwrap().lines() {
        let (r, o) = line.split_once(':').unwrap();
        let result: u64 = r.parse().unwrap();
        let ops: Box<[u64]> = o.split_whitespace().map(|s| s.parse::<u64>().unwrap()).collect();
        eqs.push(Equation{result, ops});
    }
    eqs
}

fn combine(f: u64, s: u64) -> u64 {
    let mut mul = 10;
    let mut t = s;
    loop {
        t /= 10;
        if t == 0 {
            break;
        }
        mul *= 10;
    }
    f * mul + s
}

fn solve(result: u64, ops: &mut [u64]) -> bool {
    if ops.len() == 1 {
        return ops[0] == result;
    }
    let first = ops[0];
    let second = ops[1];
    ops[1] = first + second;
    if solve(result, &mut ops[1..]) {
        return true;
    }
    ops[1] = first * second;
    if solve(result, &mut ops[1..]) {
        return true;
    }
    ops[1] = combine(first, second);
    if solve(result, &mut ops[1..]) {
        return true;
    }
    ops[1] = second;
    false
}

fn main() {
    let input = parse_input();
    let mut answer = 0;
    for mut i in input {
        if solve(i.result, &mut i.ops) {
            answer += i.result;
        }
    }
    println!("Answer: {answer}");
}