From 5a055b3fb194ef9186bd8b163481706af553a1bd Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Tue, 22 Dec 2020 09:11:27 +0100
Subject: [PATCH] Implementation day 22.

---
 2020/benches/days.rs |   2 +-
 2020/inputs/22.txt   |  53 ++++++++++++++++++++
 2020/samples/22.txt  |  13 +++++
 2020/src/day22.rs    | 114 ++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 179 insertions(+), 3 deletions(-)
 create mode 100644 2020/inputs/22.txt
 create mode 100644 2020/samples/22.txt

diff --git a/2020/benches/days.rs b/2020/benches/days.rs
index eed6a9e..4f10991 100644
--- a/2020/benches/days.rs
+++ b/2020/benches/days.rs
@@ -7,7 +7,7 @@ use criterion::criterion_main;
 use criterion::BenchmarkId;
 use criterion::Criterion;
 
-const DAYS_IMPLEMENTED: usize = 20;
+const DAYS_IMPLEMENTED: usize = 22;
 
 fn read_input(day: usize) -> Vec<u8> {
     let input_path = format!("inputs/{:02}.txt", day);
diff --git a/2020/inputs/22.txt b/2020/inputs/22.txt
new file mode 100644
index 0000000..546ae56
--- /dev/null
+++ b/2020/inputs/22.txt
@@ -0,0 +1,53 @@
+Player 1:
+26
+14
+6
+34
+37
+9
+17
+39
+4
+5
+1
+8
+49
+16
+18
+47
+20
+31
+23
+19
+35
+41
+28
+15
+44
+
+Player 2:
+7
+2
+10
+25
+29
+46
+40
+45
+11
+50
+42
+24
+38
+13
+36
+22
+33
+3
+43
+21
+48
+30
+32
+12
+27
diff --git a/2020/samples/22.txt b/2020/samples/22.txt
new file mode 100644
index 0000000..391cd24
--- /dev/null
+++ b/2020/samples/22.txt
@@ -0,0 +1,13 @@
+Player 1:
+9
+2
+6
+3
+1
+
+Player 2:
+5
+8
+4
+7
+10
diff --git a/2020/src/day22.rs b/2020/src/day22.rs
index f51a2b2..5d448b4 100644
--- a/2020/src/day22.rs
+++ b/2020/src/day22.rs
@@ -1,12 +1,122 @@
+use std::collections::HashSet;
+use std::collections::VecDeque;
 use std::io::Read;
 
+use crate::common::Lines;
 use crate::Solution;
 
+type Deck = VecDeque<u32>;
+
+fn read_input(input: &mut dyn Read) -> (Deck, Deck) {
+    let mut lines = Lines::new(input).skip(1);
+
+    let mut player1 = VecDeque::new();
+
+    while let Some(line) = lines.next() {
+        if line.is_empty() {
+            break;
+        }
+
+        player1.push_back(line.parse().unwrap());
+    }
+
+    let player2 = lines.skip(1).map(|l| l.parse().unwrap()).collect();
+
+    (player1, player2)
+}
+
+fn play_recursive_game(mut p1: Deck, mut p2: Deck) -> (bool, Deck) {
+    let mut seen = HashSet::new();
+
+    while !p1.is_empty() && !p2.is_empty() {
+        if !seen.insert((p1.clone(), p2.clone())) {
+            return (true, p1);
+        }
+
+        let v1 = p1.pop_front().unwrap();
+        let v2 = p2.pop_front().unwrap();
+
+        let p1_wins = if v1 as usize <= p1.len() && v2 as usize <= p2.len() {
+            let p1_copy = p1.iter().take(v1 as usize).copied().collect();
+            let p2_copy = p2.iter().take(v2 as usize).copied().collect();
+
+            play_recursive_game(p1_copy, p2_copy).0
+        } else {
+            v1 > v2
+        };
+
+        if p1_wins {
+            p1.push_back(v1);
+            p1.push_back(v2);
+        } else {
+            p2.push_back(v2);
+            p2.push_back(v1);
+        }
+    }
+
+    if p1.is_empty() {
+        (false, p2)
+    } else {
+        (true, p1)
+    }
+}
+
+fn score(winner: Deck) -> u32 {
+    winner
+        .into_iter()
+        .rev()
+        .enumerate()
+        .map(|(pos, val)| (pos as u32 + 1) * (val as u32))
+        .sum()
+}
+
 #[derive(Default)]
 pub struct Day22;
 
 impl Solution for Day22 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let (mut p1, mut p2) = read_input(input);
+
+        while !p1.is_empty() && !p2.is_empty() {
+            let v1 = p1.pop_front().unwrap();
+            let v2 = p2.pop_front().unwrap();
+
+            if v1 > v2 {
+                p1.push_back(v1);
+                p1.push_back(v2);
+            } else {
+                p2.push_back(v2);
+                p2.push_back(v1);
+            }
+        }
+
+        let winner = if p1.is_empty() { p2 } else { p1 };
+
+        score(winner).to_string()
+    }
+
+    fn part2(&mut self, input: &mut dyn Read) -> String {
+        let (p1, p2) = read_input(input);
+
+        score(play_recursive_game(p1, p2).1).to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("../samples/22.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation!(Day22, 1, SAMPLE, 306);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation!(Day22, 2, SAMPLE, 291);
     }
 }