Advent of Code 2019, Day 1

aoc_2019_01.rs

use std::env; // Get the standard library module for the process environment, so we can access arguments passed etc...
use std::fs; // Module for filesystem, so rust is aware of file objects 

// The equation specified in the description for getting the fuel required to lift a given mass
fn required_fuel(mass: i32) -> i32 {
    return mass / 3 - 2;
}

fn main(){

    let args: Vec<String> = env::args().collect(); // Get the arguments as a string vector
    let filename = &args[1]; // Where the first passed argument should be the path to the file with inputs,

    // Attempt getting the contents from the file, on panic let user know we failed, 
    // first had an .except but couldn't pass this formatted string like I wanted.
    let contents = fs::read_to_string(filename)
        .unwrap_or_else(|_| panic!("Failed to read contents of {}", filename));

    // Split the content into separate lines,
    let lines = contents.split("\n");

    // Solve the problem of the day
    let mut fuel_for_modules = 0;
    let mut total_fuel_req = 0; // So we're looking for the total sum of fuel required,
    for line in lines { // For each line in lines parse
        
        if line.is_empty(){
            continue; // Skipping any empty lines, likely the last one in this case.
        }
        
        let module_mass = line.parse::<i32>().unwrap(); // Read the value as a 32-bit interger

        let mut fuel_for_module = 0; // Sum of the required fuel for this `module`
        let mut fuel = required_fuel(module_mass); // The fuel required for the `module`
        fuel_for_modules += fuel; // Sum up the fuel needed to carry the modules to answer the first solution
        while fuel > 0 { 
            fuel_for_module += fuel; // Add the fuel to the fuel needed to carry this module,
            fuel = required_fuel(fuel); // Get the required fuel for the fuel we just added,
        }
        
        total_fuel_req += fuel_for_module; // Add the fuel to the sum total for all required fuel, for the second solution
    }

    println!("The solution to the first problem {}", fuel_for_modules);
    println!("and for the second problem {}", total_fuel_req);
}