solution: day 22

data/examples/22.txt

@@ -0,0 +1,4 @@
+1
+2
+3
+2024

src/bin/22.rs

@@ -0,0 +1,91 @@
+use aoc::parsers::to_vec_map;
+use hashbrown::{HashMap, HashSet};
+
+fn pseudo_next(mut n: usize) -> usize {
+    n = ((n << 6) ^ n) % 16777216;
+    n = ((n >> 5) ^ n) % 16777216;
+    n = ((n << 11) ^ n) % 16777216;
+    n
+}
+
+struct MonkeyTrader {
+    numbers: [usize; 2001],
+    sequences: HashMap<[isize; 4], usize>,
+}
+
+impl MonkeyTrader {
+    fn new(seed: usize) -> Self {
+        let mut numbers = [0; 2001];
+        numbers[0] = seed;
+
+        let mut n = seed;
+        for num in numbers.iter_mut().skip(1) {
+            n = pseudo_next(n);
+            *num = n;
+        }
+
+        let mut sequences = HashMap::new();
+        for i in 4..2001 {
+            let mut key = [0; 4];
+            for j in 1..=4 {
+                let d1 = numbers[i - 4 + j - 1] % 10;
+                let d2 = numbers[i - 4 + j] % 10;
+                let diff = d2 as isize - d1 as isize;
+                key[j - 1] = diff;
+            }
+            if sequences.contains_key(&key) {
+                continue;
+            }
+            *sequences.entry(key).or_default() = numbers[i] % 10;
+        }
+
+        MonkeyTrader { numbers, sequences }
+    }
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    to_vec_map(input, '\n', MonkeyTrader::new)
+        .into_iter()
+        .map(|x| x.numbers[2000])
+        .sum::<usize>()
+        .into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let traders = to_vec_map(input, '\n', MonkeyTrader::new);
+    let key_union: HashSet<[isize; 4]> = traders
+        .iter()
+        .flat_map(|x| x.sequences.keys().cloned())
+        .collect();
+
+    key_union
+        .into_iter()
+        .map(|k| {
+            traders
+                .iter()
+                .filter_map(|t| t.sequences.get(&k))
+                .sum::<usize>()
+        })
+        .max()
+}
+
+aoc::solution!(22);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(
+            part_one(&aoc::template::read_file("examples", 22)),
+            Some(37990510)
+        );
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(
+            part_two(&aoc::template::read_file("examples", 22)),
+            Some(23)
+        );
+    }
+}