main.rs

use std::error::Error;
use std::io;
use std::io::prelude::*;
use std::process;

macro_rules! strerr {
    ($($tt:tt)*) => (Err(From::from(&*format!($($tt)*))));
}

type CliResult<T> = Result<T, Box<Error + Send>>;

fn main() {
    if let Err(err) = run() {
        writeln!(&mut io::stdout(), "{}", err).unwrap();
        process::exit(1);
    }
}

fn run() -> CliResult<()> {
    let stdin = io::stdin();
    let mut stdin = stdin.lock(); // gives us buffered access
    let num_cases: i32 = try!(try!(read_line(&mut stdin)).trim().parse());
    for i in 0..num_cases {
        let line = try!(read_line(&mut stdin));
        let pieces: Vec<&str> = line.trim().split(" ").collect();
        if pieces.len() != 2 {
            return strerr!("Parse error: '{}'", line);
        };
        let max_shyness: i32 = try!(pieces[0].parse());
        let str_digits: &str = &pieces[1];

        let mut people_sum: u32 = 0;
        let mut friends_required: u32 = 0;
        for (i, c) in str_digits.chars().enumerate() {
            let i = i as u32;
            let digit = match c.to_digit(10) {
                Some(digit) => digit,
                None => return strerr!("Not a digit: '{}'", c),
            };
            if people_sum < i {
                friends_required += i - people_sum;
                people_sum += friends_required;
            }
            people_sum += digit;
        }
        println!("Case #{}: {}", i, friends_required);
    }
    Ok(())
}

fn read_line<R: BufRead>(mut rdr: R) -> CliResult<String> {
    let mut buf = String::new();
    let n = try!(rdr.read_line(&mut buf));
    if n == 0 {
        strerr!("Expected to read new line, but got EOF.")
    } else {
        Ok(buf)
    }
}