projecteuler.net/013/main.rs

use std::fs::File;
use std::io::{ self, BufRead };

struct Num { rdigits: Vec<u8> }

impl Num {
    pub fn from(s: &String) -> Num {
        let rd: Vec<u8> = s.chars().rev()
                    .map(|c| String::from(c).parse::<u8>().unwrap())
                    .collect();
        Num { rdigits: rd }
    }
    pub fn add(&self, n: &Num) -> Num {
        let mut xs = self.rdigits.clone();
        let mut ys = n.rdigits.clone();
        let mut rs: Vec<u8> = Vec::new();

        while xs.len() < ys.len() { xs.push(0); }
        while ys.len() < xs.len() { ys.push(0); }

        let mut rm = 0u8;
        for (a, b) in xs.iter().zip(ys.iter()) {
            if a + b + rm > 9 {
                rs.push((a + b + rm) - 10);
                rm = 1u8;
            } else {
                rs.push(a + b + rm);
                rm = 0u8;
            }
        }
        if rm > 0 { rs.push(1u8); }

        Num { rdigits: rs }
    }
    pub fn print_10(&self) {
        for d in self.rdigits.iter().rev().take(10) { print!("{}", d); }
        println!("");
    }
}


fn main() {
    let file = File::open("nums.txt").unwrap();
    let mut lines = io::BufReader::new(file).lines();
    let mut ds = match lines.next() {
        Some(ln) =>  Num::from(&ln.unwrap()),
        None => Num::from(&String::from("0"))
    };

    for ln in lines {
        let dn = Num::from(&ln.unwrap());
        ds = ds.add(&dn);
        //println!("||||{}", ln.unwrap());
    }
    
    ds.print_10();
    
}