aoc-2019-1.rs

use std::fs::File;
use std::io::{prelude::*, BufReader};
use std::cmp::max;

fn fuel_for_mass(mass: i32) -> i32 {
    let mut fuel = mass;
    fuel = fuel / 3;  // assume round down
    fuel -= 2;
    //println!("mass: {} requires: {}", mass, max(fuel, 0));
    max(fuel, 0)
}

fn all_fuel_for_mass(number: i32) -> i32 {
    let mut total = 0;
    let fuel = fuel_for_mass(number);
    total += fuel;
    let mut fuel_for_fuel = fuel_for_mass(fuel);
    while fuel_for_fuel > 0 {
        total += fuel_for_fuel;
        fuel_for_fuel = fuel_for_mass(fuel_for_fuel)
    }
    total
}


fn main() {
    assert_eq!(all_fuel_for_mass(14), 2);
    assert_eq!(all_fuel_for_mass(1969), 966);
    assert_eq!(all_fuel_for_mass(100756), 50346);

    let file = File::open("input.txt").expect("Failed to open input file.");
    let reader = BufReader::new(file);

    let mut total = 0;
    for line in reader.lines() {
        // line is a result at this point
        let numberstring = line.expect("Failed to parse line");
        let number: i32 = numberstring
            .parse()
            .unwrap_or_else(|e| panic!("Could not parse '{}': {}", numberstring, e));
        let fuel_req = all_fuel_for_mass(number);
        total += fuel_req;
        println!("Module: {}\tfuel_req: {}\tTotoal: {}", number, fuel_req, total);
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_all_fuel_for_mass() {
        assert_eq!(all_fuel_for_mass(14), 2);
        assert_eq!(all_fuel_for_mass(1969), 966);
        assert_eq!(all_fuel_for_mass(100756), 50346);
    }
}