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

#[derive(Debug)]
enum Inst{
    AddX(i64),
    Noop,
}

#[derive(Debug)]
struct ParseError(String);

fn parse_line(line: String) -> Result<Inst, ParseError> {
    match &line[..4] {
        "noop" => Ok(Inst::Noop),
        "addx" => match line[5..].parse::<i64>() {
            Ok(operand) => Ok(Inst::AddX(operand)),
            Err(e) => Err(ParseError(e.to_string())),
        },
        _ => Err(ParseError(String::from("bad instruction"))),
    }
}

fn main() {
    let file = File::open(&env::args().nth(1).unwrap()).unwrap();
    let reader = io::BufReader::new(file);
    let mut xs: Vec<i64> = vec![0, 1];

    for line in reader.lines().flatten() {
        let last_x = *xs.last().unwrap();
        match parse_line(line) {
            Ok(Inst::Noop) => { xs.push(last_x) },
            Ok(Inst::AddX(x)) => { xs.push(last_x); xs.push(last_x + x) },
            Err(e) => { eprintln!("error: {:?}", e); process::exit(1) },
        }
    }
    let res: i64 = xs.iter().enumerate()
        .skip(20).step_by(40)
        .map(|(i, &x)| (i as i64)*x)
        .sum();
    println!("{:?}", res);
}