rocket_fuel.rb

input_path = File.expand_path('../input.txt', __FILE__)
masses     = File.read(input_path).lines.map(&:to_i)

def fuel(mass)
  (mass / 3.0).floor - 2
end

p masses.reduce(0) { |s, m| s + fuel(m) }

def required_fuel(init)
  Enumerator.produce(fuel(init)) { |mass| fuel(mass) }
end

fuels = masses.flat_map do |mass|
  required_fuel(mass).take_while { |f| f > 0 }
end

p fuels.reduce(0, :+)

rocket_fuel.rs

use std::fs;
use std::iter;

fn main() {
    let content = fs::read_to_string("input.txt").unwrap();
    let masses: Vec<f64> = content.lines().flat_map(|line| line.parse()).collect();

    let fuel: f64 = masses.iter().map(|mass| fuel(*mass)).sum();
    println!("{}", fuel);

    let fuel: f64 = masses
        .iter()
        .flat_map(|mass| required_fuel(*mass).take_while(|f| *f > 0.0))
        .sum();

    println!("{}", fuel);
}

fn required_fuel(mut mass: f64) -> impl Iterator<Item = f64> {
    iter::from_fn(move || {
        mass = fuel(mass);
        Some(mass)
    })
}

fn fuel(mass: f64) -> f64 {
    (mass / 3.0).floor() - 2.0
}