use std::{fs::read_to_string, iter};

fn main() {
    let input = read_to_string("../input.txt").expect("failed to open input file");
    let mut problems = input.lines();
    let operator_line = problems.next_back().expect("file was empty");
    let numbers: Vec<_> = problems.collect();

    let operators: Vec<_> = operator_line.match_indices(['+', '*']).collect();
    let last_operator = operators.last().expect("couldn't find any operators");

    let grand_total: u64 = operators
        .windows(2)
        .map(|window| (window[0].0..window[1].0 - 1, window[0].1))
        .chain(iter::once((
            last_operator.0..operator_line.len(),
            last_operator.1,
        )))
        .map(|(problem_range, operator)| {
            let numbers = problem_range.map(|column| {
                numbers
                    .iter()
                    .filter_map(|number_line| (number_line.as_bytes()[column] as char).to_digit(10))
                    .rev()
                    .enumerate()
                    .map(|(placement, digit)| u64::from(digit) * 10u64.pow(placement as u32))
                    .sum::<u64>()
            });
            match operator {
                "+" => numbers.sum::<u64>(),
                "*" => numbers.product(),
                operator => panic!("unrecognised operator '{operator}'"),
            }
        })
        .sum();
    println!("The grand total is {grand_total}!");
}