day22

input/2020/day22.txt

@@ -0,0 +1,53 @@
+Player 1:
+10
+21
+37
+2
+47
+13
+6
+29
+9
+3
+4
+48
+46
+25
+44
+41
+23
+20
+24
+12
+45
+43
+5
+27
+50
+
+Player 2:
+39
+42
+31
+36
+7
+1
+49
+19
+40
+35
+8
+11
+18
+30
+14
+17
+15
+34
+26
+33
+32
+38
+28
+16
+22

src/day22.rs

@@ -0,0 +1,87 @@
+use crate::split_once;
+use aoc_runner_derive::aoc;
+use std::borrow::Cow;
+use std::collections::{HashSet, VecDeque};
+
+fn deck_score(deck: VecDeque<usize>) -> usize {
+    deck.iter()
+        .rev()
+        .enumerate()
+        .map(|(idx, value)| (idx + 1) * value)
+        .sum()
+}
+
+#[aoc(day22, part1)]
+fn solve_d22_p1(input: &str) -> usize {
+    let (p1, p2) = split_once(input, "\n\n").unwrap();
+    let mut p1_deck: VecDeque<usize> = p1.split('\n').skip(1).map(|x| x.parse().unwrap()).collect();
+    let mut p2_deck: VecDeque<usize> = p2.split('\n').skip(1).map(|x| x.parse().unwrap()).collect();
+    while !p1_deck.is_empty() && !p2_deck.is_empty() {
+        let p1_value = p1_deck.pop_front().unwrap();
+        let p2_value = p2_deck.pop_front().unwrap();
+        if p1_value > p2_value {
+            p1_deck.push_back(p1_value);
+            p1_deck.push_back(p2_value);
+        } else {
+            p2_deck.push_back(p2_value);
+            p2_deck.push_back(p1_value);
+        }
+    }
+    deck_score(if p1_deck.is_empty() { p2_deck } else { p1_deck })
+}
+
+enum Winner {
+    Player1(VecDeque<usize>),
+    Player2(VecDeque<usize>),
+}
+
+#[aoc(day22, part2)]
+fn solve_d22_p2(input: &str) -> usize {
+    let (p1, p2) = split_once(input, "\n\n").unwrap();
+    let p1_deck: VecDeque<usize> = p1.split('\n').skip(1).map(|x| x.parse().unwrap()).collect();
+    let p2_deck: VecDeque<usize> = p2.split('\n').skip(1).map(|x| x.parse().unwrap()).collect();
+    match play_recursive_combat(p1_deck, p2_deck) {
+        Winner::Player1(deck) | Winner::Player2(deck) => deck_score(deck),
+    }
+}
+
+fn play_recursive_combat(mut p1_deck: VecDeque<usize>, mut p2_deck: VecDeque<usize>) -> Winner {
+    let mut previous_rounds = HashSet::new();
+    while !p1_deck.is_empty() && !p2_deck.is_empty() {
+        if previous_rounds.contains(&(Cow::Borrowed(&p1_deck), Cow::Borrowed(&p2_deck))) {
+            return Winner::Player1(p1_deck);
+        }
+        previous_rounds.insert((Cow::Owned(p1_deck.clone()), Cow::Owned(p2_deck.clone())));
+
+        let p1_value = p1_deck.pop_front().unwrap();
+        let p2_value = p2_deck.pop_front().unwrap();
+        if p1_deck.len() >= p1_value && p2_deck.len() >= p2_value {
+            match play_recursive_combat(
+                p1_deck.iter().copied().take(p1_value).collect(),
+                p2_deck.iter().copied().take(p2_value).collect(),
+            ) {
+                Winner::Player1(_) => {
+                    p1_deck.push_back(p1_value);
+                    p1_deck.push_back(p2_value);
+                }
+                Winner::Player2(_) => {
+                    p2_deck.push_back(p2_value);
+                    p2_deck.push_back(p1_value);
+                }
+            }
+        } else {
+            if p1_value > p2_value {
+                p1_deck.push_back(p1_value);
+                p1_deck.push_back(p2_value);
+            } else {
+                p2_deck.push_back(p2_value);
+                p2_deck.push_back(p1_value);
+            }
+        }
+    }
+    if p1_deck.is_empty() {
+        Winner::Player2(p2_deck)
+    } else {
+        Winner::Player1(p1_deck)
+    }
+}

src/lib.rs

@@ -12,6 +12,7 @@ pub mod day19;
 pub mod day2;
 pub mod day20;
 pub mod day21;
+pub mod day22;
 pub mod day3;
 pub mod day4;
 pub mod day5;