From eba60c4c97116b96b683a55d61d184b4e5447b41 Mon Sep 17 00:00:00 2001
From: Johannes <freelon@gmx.net>
Date: Wed, 30 Oct 2024 23:29:32 +0100
Subject: [PATCH] day 21

---
 src/day21.rs | 161 +++++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs   |   1 +
 2 files changed, 162 insertions(+)
 create mode 100644 src/day21.rs

diff --git a/src/day21.rs b/src/day21.rs
new file mode 100644
index 0000000..70ec190
--- /dev/null
+++ b/src/day21.rs
@@ -0,0 +1,161 @@
+use std::cmp::max;
+
+use aoc_runner_derive::{aoc, aoc_generator};
+
+#[aoc_generator(day21)]
+fn parse(input: &str) -> Fighter {
+    let ints: Vec<i32> = input
+        .lines()
+        .map(|line| line.split_once(": ").unwrap().1)
+        .map(|it| it.parse().unwrap())
+        .collect();
+    Fighter {
+        health: ints[0],
+        damage: ints[1],
+        armor: ints[2],
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+struct Fighter {
+    health: i32,
+    damage: i32,
+    armor: i32,
+}
+
+#[derive(Clone, Copy)]
+struct Item {
+    cost: i32,
+    damage: i32,
+    armor: i32,
+}
+
+impl Item {
+    fn new(cost: i32, damage: i32, armor: i32) -> Self {
+        Self {
+            cost,
+            damage,
+            armor,
+        }
+    }
+}
+#[aoc(day21, part1)]
+fn part1(boss: &Fighter) -> usize {
+    combinations()
+        .into_iter()
+        .filter(|(fighter, _)| fight(*fighter, *boss))
+        .map(|(_, cost)| cost)
+        // .count()
+        .min()
+        .unwrap() as usize
+}
+
+/// returns `true` if `fighter` wins the fight
+fn fight(mut fighter: Fighter, mut boss: Fighter) -> bool {
+    loop {
+        boss.health -= max(1, fighter.damage - boss.armor);
+        if boss.health < 1 {
+            return true;
+        }
+        fighter.health -= max(1, boss.damage - fighter.armor);
+        if fighter.health < 1 {
+            return false;
+        }
+    }
+}
+
+fn combinations() -> Vec<(Fighter, i32)> {
+    let (weapons, armors, rings) = items();
+    let mut all_rings = vec![Item::new(0, 0, 0)];
+    for r1 in 0..rings.len() {
+        let ring1 = rings[r1];
+        all_rings.push(ring1);
+        for ring2 in rings.iter().skip(r1 + 1) {
+            all_rings.push(Item::new(
+                ring1.cost + ring2.cost,
+                ring1.damage + ring2.damage,
+                ring1.armor + ring2.armor,
+            ));
+        }
+    }
+    let mut result = vec![];
+
+    for weapon in weapons {
+        for &armor in &armors {
+            for &ring in &all_rings {
+                let cost = weapon.cost + armor.cost + ring.cost;
+                let health = 100;
+                let damage = weapon.damage + ring.damage;
+                let armor = armor.armor + ring.armor;
+                result.push((
+                    Fighter {
+                        health,
+                        damage,
+                        armor,
+                    },
+                    cost,
+                ));
+            }
+        }
+    }
+
+    result
+}
+
+#[aoc(day21, part2)]
+fn part2(boss: &Fighter) -> usize {
+    combinations()
+        .into_iter()
+        .filter(|(fighter, _)| !fight(*fighter, *boss))
+        .map(|(_, cost)| cost)
+        .max()
+        .unwrap() as usize
+}
+
+fn items() -> (Vec<Item>, Vec<Item>, Vec<Item>) {
+    let weapons = vec![
+        Item::new(8, 4, 0),
+        Item::new(10, 5, 0),
+        Item::new(25, 6, 0),
+        Item::new(40, 7, 0),
+        Item::new(74, 8, 0),
+    ];
+    let armors = vec![
+        Item::new(0, 0, 0), // because we need the option to have no armor
+        Item::new(13, 0, 1),
+        Item::new(31, 0, 2),
+        Item::new(53, 0, 3),
+        Item::new(75, 0, 4),
+        Item::new(102, 0, 5),
+    ];
+    let rings = vec![
+        Item::new(25, 1, 0),
+        Item::new(50, 2, 0),
+        Item::new(100, 3, 0),
+        Item::new(20, 0, 1),
+        Item::new(40, 0, 2),
+        Item::new(80, 0, 3),
+    ];
+    (weapons, armors, rings)
+}
+
+#[cfg(test)]
+mod test {
+    use crate::day21::{fight, Fighter};
+
+    #[test]
+    fn test_case() {
+        assert!(fight(
+            Fighter {
+                health: 8,
+                damage: 5,
+                armor: 5,
+            },
+            Fighter {
+                health: 12,
+                damage: 7,
+                armor: 2,
+            }
+        ))
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index d7d8d10..bd1adc8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,5 +13,6 @@ mod day17;
 mod day18;
 mod day19;
 mod day20;
+mod day21;
 
 aoc_lib! { year = 2015 }