day 14 task 2

src/tasks/day14.rs

@@ -1,6 +1,7 @@
 use itertools::Itertools;
 use std::collections::HashMap;
 
+#[allow(dead_code)]
 pub fn run() {
     fn parser(s: &str) -> Chemical {
         let (n, t) = s.split(" ").collect_tuple().unwrap();
@@ -25,6 +26,36 @@ pub fn run() {
             )
         })
         .collect();
+    task1(&rules);
+    task2(&rules);
+}
+
+fn task1(rules: &HashMap<&str, (usize, Vec<Chemical>)>) {
+    println!("Task 1: {} ORE is needed", calc_ore_for_fuel(1, rules));
+}
+
+fn task2(rules: &HashMap<&str, (usize, Vec<Chemical>)>) {
+    const ORE_STORAGE: usize = 1_000_000_000_000;
+    // binary search max fuel
+    let (max_fuel, _) = std::iter::successors(Some((1, ORE_STORAGE)), |(min, max)| {
+        if max - min > 1 {
+            let middle = (max + min) / 2;
+            let ore = calc_ore_for_fuel(middle, rules);
+            if ore > ORE_STORAGE {
+                Some((*min, middle))
+            } else {
+                Some((middle, *max))
+            }
+        } else {
+            None
+        }
+    })
+    .last()
+    .unwrap();
+    println!("Task 2: the maximum amount of fuel is {}", max_fuel);
+}
+
+fn calc_ore_for_fuel(fuel: usize, rules: &HashMap<&str, (usize, Vec<Chemical>)>) -> usize {
     /*
     The idea is: the rules create a DAG of requirements.
     We store for every chemical a list of the chemicals that rely on it.
@@ -52,7 +83,7 @@ pub fn run() {
     chem_used_by.insert("FUEL", vec![]);
     // the amount of a not processed chemical C, which is needed by the chemicals that rely on C
     let mut requirements = HashMap::<&str, usize>::new();
-    requirements.insert("FUEL", 1);
+    requirements.insert("FUEL", fuel);
     while let Some((name, amount)) = requirements
         .iter()
         .find(|(name, _)| chem_used_by[*name].is_empty() && **name != "ORE")
@@ -77,7 +108,7 @@ pub fn run() {
         }
         requirements.remove(name);
     }
-    println!("Task 1: {} ORE is needed", requirements["ORE"]);
+    requirements["ORE"]
 }
 
 #[derive(PartialEq, Eq, Hash, Copy, Clone, Debug)]