fn fuel_for_mass1(mass: u64) -> u64 {
	mass / 3 - 2
}

fn fuel_for_modules1<I>(input: I) -> u64
where
	I: IntoIterator<Item = u64>,
{
	input.into_iter().map(fuel_for_mass1).sum()
}

fn fuel_for_mass2(mass: u64) -> u64 {
	if mass <= 8 { return 0; } /* wish really hard */
	let fuel = mass / 3 - 2;
	fuel + fuel_for_mass2(fuel)
}

fn fuel_for_modules2<I>(input: I) -> u64
where
	I: IntoIterator<Item = u64>,
{
	input.into_iter().map(fuel_for_mass2).sum()
}

fn main() {
	let input = include_str!("input.txt").split_whitespace().map(|s| s.parse::<u64>().unwrap());
	println!("Fuel 1: {}", fuel_for_modules1(input.clone()));
	println!("Fuel 2: {}", fuel_for_modules2(input));
}

#[cfg(test)]
mod day1test {
	#[test]
	fn example1() {
		// Original descriptions:
		// > For a mass of 12, divide by 3 and round down to get 4, then subtract 2 to get 2.
		// > For a mass of 14, dividing by 3 and rounding down still yields 4, so the fuel required is also 2.
		// > For a mass of 1969, the fuel required is 654.
		// > For a mass of 100756, the fuel required is 33583.
		assert_eq!(super::fuel_for_mass1(12), 2);
		assert_eq!(super::fuel_for_mass1(14), 2);
		assert_eq!(super::fuel_for_mass1(1969), 654);
		assert_eq!(super::fuel_for_mass1(100756), 33583);
	}

	#[test]
	fn example2() {
		// Original descriptions:
		// > A module of mass 14 requires 2 fuel. This fuel requires no further fuel (2 divided by 3 and rounded down is 0, which would call for a negative fuel), so the total fuel required is still just 2.
		// > At first, a module of mass 1969 requires 654 fuel. Then, this fuel requires 216 more fuel (654 / 3 - 2). 216 then requires 70 more fuel, which requires 21 fuel, which requires 5 fuel, which requires no further fuel. So, the total fuel required for a module of mass 1969 is 654 + 216 + 70 + 21 + 5 = 966.
		// > The fuel required by a module of mass 100756 and its fuel is: 33583 + 11192 + 3728 + 1240 + 411 + 135 + 43 + 12 + 2 = 50346.
		assert_eq!(super::fuel_for_mass2(14), 2);
		assert_eq!(super::fuel_for_mass2(1969), 966);
		assert_eq!(super::fuel_for_mass2(100756), 50346);
	}
}