Year 2015 Day 21

Author: maneatingape

README.md

@@ -256,3 +256,4 @@ pie
 | 18 | [Like a GIF For Your Yard](https://adventofcode.com/2015/day/18) | [Source](src/year2015/day18.rs) | 161 |
 | 19 | [Medicine for Rudolph](https://adventofcode.com/2015/day/19) | [Source](src/year2015/day19.rs) | 188 |
 | 20 | [Infinite Elves and Infinite Houses](https://adventofcode.com/2015/day/20) | [Source](src/year2015/day20.rs) | 1671 |
+| 21 | [RPG Simulator 20XX](https://adventofcode.com/2015/day/21) | [Source](src/year2015/day21.rs) | 3 |

benches/benchmark.rs

@@ -56,6 +56,7 @@ mod year2015 {
     benchmark!(year2015, day18);
     benchmark!(year2015, day19);
     benchmark!(year2015, day20);
+    benchmark!(year2015, day21);
 }
 
 mod year2019 {

input/year2015/day21.txt

@@ -0,0 +1,3 @@
+Hit Points: 100
+Damage: 8
+Armor: 2

src/lib.rs

@@ -180,6 +180,7 @@ pub mod year2015 {
     pub mod day18;
     pub mod day19;
     pub mod day20;
+    pub mod day21;
 }
 
 /// # Rescue Santa from deep space with a solar system adventure.

src/main.rs

@@ -96,6 +96,7 @@ fn all_solutions() -> Vec<Solution> {
         solution!(year2015, day18),
         solution!(year2015, day19),
         solution!(year2015, day20),
+        solution!(year2015, day21),
         // 2019
         solution!(year2019, day01),
         solution!(year2019, day02),

src/year2015/day21.rs

@@ -0,0 +1,95 @@
+//! # RPG Simulator 20XX
+//!
+//! The trick to get the outcome of each battle quickly is to divide the hero's health by the
+//! boss's damage and vice-versa then find out how many turns each takes to win.
+use crate::util::iter::*;
+use crate::util::parse::*;
+use std::ops::Add;
+
+#[derive(Clone, Copy)]
+struct Item {
+    cost: i32,
+    damage: i32,
+    armor: i32,
+}
+
+impl Add for Item {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Item {
+            cost: self.cost + rhs.cost,
+            damage: self.damage + rhs.damage,
+            armor: self.armor + rhs.armor,
+        }
+    }
+}
+
+type Result = (bool, i32);
+
+pub fn parse(input: &str) -> Vec<Result> {
+    let [boss_health, boss_damage, boss_armor]: [i32; 3] =
+        input.iter_signed().chunk::<3>().next().unwrap();
+
+    let weapon = [
+        Item { cost: 8, damage: 4, armor: 0 },
+        Item { cost: 10, damage: 5, armor: 0 },
+        Item { cost: 25, damage: 6, armor: 0 },
+        Item { cost: 40, damage: 7, armor: 0 },
+        Item { cost: 74, damage: 8, armor: 0 },
+    ];
+
+    let armor = [
+        Item { cost: 0, damage: 0, armor: 0 },
+        Item { cost: 13, damage: 0, armor: 1 },
+        Item { cost: 31, damage: 0, armor: 2 },
+        Item { cost: 53, damage: 0, armor: 3 },
+        Item { cost: 75, damage: 0, armor: 4 },
+        Item { cost: 102, damage: 0, armor: 5 },
+    ];
+
+    let ring = [
+        Item { cost: 25, damage: 1, armor: 0 },
+        Item { cost: 50, damage: 2, armor: 0 },
+        Item { cost: 100, damage: 3, armor: 0 },
+        Item { cost: 20, damage: 0, armor: 1 },
+        Item { cost: 40, damage: 0, armor: 2 },
+        Item { cost: 80, damage: 0, armor: 3 },
+    ];
+
+    let mut combinations = Vec::with_capacity(22);
+    combinations.push(Item { cost: 0, damage: 0, armor: 0 });
+
+    for i in 0..6 {
+        combinations.push(ring[i]);
+        for j in (i + 1)..6 {
+            combinations.push(ring[i] + ring[j]);
+        }
+    }
+
+    let mut results = Vec::with_capacity(660);
+
+    for first in weapon {
+        for second in armor {
+            for &third in &combinations {
+                let Item { cost, damage, armor } = first + second + third;
+
+                let hero_turns = boss_health / (damage - boss_armor).max(1);
+                let boss_turns = 100 / (boss_damage - armor).max(1);
+                let win = hero_turns <= boss_turns;
+
+                results.push((win, cost));
+            }
+        }
+    }
+
+    results
+}
+
+pub fn part1(input: &[Result]) -> i32 {
+    *input.iter().filter_map(|(w, c)| w.then_some(c)).min().unwrap()
+}
+
+pub fn part2(input: &[Result]) -> i32 {
+    *input.iter().filter_map(|(w, c)| (!w).then_some(c)).max().unwrap()
+}

tests/test.rs

@@ -49,6 +49,7 @@ mod year2015 {
     mod day18_test;
     mod day19_test;
     mod day20_test;
+    mod day21_test;
 }
 
 mod year2019 {

tests/year2015/day21_test.rs

@@ -0,0 +1,9 @@
+#[test]
+fn part1_test() {
+    // No example data
+}
+
+#[test]
+fn part2_test() {
+    // No example data
+}