problem_08.rs

struct Window {
    number_str: String,
    window_size: i32,
    current: i32,
}

impl Iterator for Window {
    type Item = u64;

    fn next(&mut self) -> Option<u64> {
        let sub_string: String = self.number_str
            .chars()
            .skip(self.current as usize)
            .take(self.window_size as usize)
            .collect();
        self.current = self.current + 1;
        match (sub_string.len() as i32) - (self.window_size) {
            0 => {
                let sub_string_product: u64 = sub_string
                    .chars()
                    .map(|c: char| -> u64 { c.to_digit(10).unwrap() as u64 })
                    .product();
                Some(sub_string_product)
            }
            _ => None,
        }
    }
}

fn window() -> Window {
    Window {
        number_str: "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450".to_string() ,
        window_size: 13,
        current: 0,
    }
}

fn main() {
    println!("{:?}", window().max());
}