From c6010439ba7398301f9bfa37079f9a0e496c4c15 Mon Sep 17 00:00:00 2001
From: Wesley Moore <wes@wezm.net>
Date: Mon, 2 Dec 2019 12:10:25 +1100
Subject: [PATCH] Day 1 part 2

---
 2019/src/bin/day1.rs | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/2019/src/bin/day1.rs b/2019/src/bin/day1.rs
index a4ec704..eb200d8 100644
--- a/2019/src/bin/day1.rs
+++ b/2019/src/bin/day1.rs
@@ -3,13 +3,38 @@ use std::io;
 use advent_of_code::input;
 
 fn main() -> io::Result<()> {
+    let module_masses = input::read_number_list("input/day1.txt")?;
+
     // Specifically, to find the fuel required for a module, take its mass, divide by three, round down, and subtract 2.
-    let result: i32 = input::read_number_list("input/day1.txt")?
+    let fuel: Vec<_> = module_masses
         .into_iter()
         .map(|module_mass| (module_mass as f64 / 3.).floor() as i32 - 2)
-        .sum();
+        .collect();
 
-    println!("Result: {}", result);
+    println!("Part 1: {}", fuel.iter().sum::<i32>());
+    part2(&fuel);
 
     Ok(())
 }
+
+fn part2(fuel: &[i32]) {
+    // Fuel itself requires fuel just like a module - take its mass, divide by three, round down, and subtract 2.
+    let result: i32 = fuel
+        .iter()
+        .map(|&fuel| {
+            let mut total = 0;
+            let mut extra = fuel_fuel(fuel);
+            while extra > 0 {
+                total += extra;
+                extra = fuel_fuel(extra);
+            }
+            total
+        })
+        .sum();
+
+    println!("Part 2: {}", result + fuel.iter().sum::<i32>());
+}
+
+fn fuel_fuel(fuel: i32) -> i32 {
+    ((fuel as f64 / 3.).floor() as i32).saturating_sub(2)
+}