day22: part 2 solution. 38s runtime but works.

src/day22.rs

@@ -1,9 +1,17 @@
 use aoc_runner_derive::{aoc, aoc_generator};
+use itertools::Itertools;
+use rayon::iter::{IntoParallelRefIterator, ParallelIterator};
+
+#[derive(Debug, Eq, PartialEq, PartialOrd, Ord)]
+struct Change {
+    price: i8,
+    delta: i8,
+}
 
 fn evolve_secret(mut n: i64) -> i64 {
     n = ((n * 64) ^ n) % 16777216;
     n = ((n / 32) ^ n) % 16777216;
-    n = ((n * 2048) ^ n)  % 16777216;
+    n = ((n * 2048) ^ n) % 16777216;
     n
 }
 
@@ -14,6 +22,54 @@ fn rounds(mut secret: i64, n: i64) -> i64 {
     secret
 }
 
+fn prices(mut secret: i64, n: i64) -> Vec<i8> {
+    let mut prices = vec![(secret % 10) as i8];
+    for _ in 1..n {
+        secret = evolve_secret(secret);
+        prices.push((secret % 10) as i8);
+    }
+    prices
+}
+
+fn changes(prices: &Vec<i8>) -> Vec<Change> {
+    prices
+        .windows(2)
+        .map(|a| Change {
+            price: a[1],
+            delta: a[1] - a[0],
+        })
+        .collect()
+}
+
+fn profit_for_sequence(changes: &Vec<Vec<Change>>, seq: &[i8]) -> i64 {
+    changes
+        .par_iter()
+        .filter_map(|inner| {
+            if let Some(buy) = inner
+                .windows(seq.len())
+                .find(|window| window.iter().zip(seq).all(|(w, z)| w.delta == *z))
+            {
+                Some(buy[seq.len() - 1].price as i64)
+            } else {
+                None
+            }
+        })
+        .sum()
+}
+
+fn find_best_sequence(changes: &Vec<Vec<Change>>) -> [i8; 4] {
+    let mut best_seq = [0, 0, 0, 0];
+    let mut best_profit = 0;
+    for seq in (0..4).map(|_| (-9..=9 as i8)).multi_cartesian_product() {
+        let profit = profit_for_sequence(changes, &seq);
+        if profit > best_profit {
+            best_seq = seq.try_into().unwrap();
+            best_profit = profit;
+        }
+    }
+    best_seq
+}
+
 fn parse(input: &str) -> Vec<i64> {
     input.lines().map(|l| l.parse().unwrap()).collect()
 }
@@ -27,9 +83,14 @@ fn part1(input: &str) -> i64 {
 
 #[aoc(day22, part2)]
 fn part2(input: &str) -> i64 {
-    todo!()
-}
+    let secrets = parse(input);
 
+    let price_changes = secrets.iter().map(|s| changes(&prices(*s, 2000))).collect_vec();
+
+    let seq = find_best_sequence(&price_changes);
+    println!("found best seq: {:?}", seq);
+    profit_for_sequence(&price_changes, &seq)
+}
 
 #[cfg(test)]
 mod tests {
@@ -37,6 +98,11 @@ mod tests {
     const EXAMPLE: &str = "1
 10
 100
+2024";
+
+    const EXAMPLE2: &str = "1
+2
+3
 2024";
 
     #[test]
@@ -44,21 +110,54 @@ mod tests {
         assert_eq!(evolve_secret(123), 15887950);
         assert_eq!(evolve_secret(15887950), 16495136);
         assert_eq!(evolve_secret(16495136), 527345);
-    }   
+    }
 
     #[test]
     fn test_rounds() {
         assert_eq!(rounds(1, 2000), 8685429);
         assert_eq!(rounds(10, 2000), 4700978);
-    }   
+    }
+
+    #[test]
+    fn test_prices() {
+        assert_eq!(prices(123, 10), vec![3, 0, 6, 5, 4, 4, 6, 4, 4, 2]);
+    }
+
+    #[test]
+    fn test_profit() {
+        assert_eq!(
+            profit_for_sequence(&vec![changes(&prices(123, 10))], &[-1, -1, 0, 2]),
+            6
+        );
+        let secrets = parse(EXAMPLE2);
+
+        let price_changes = secrets.iter().map(|s| changes(&prices(*s, 2000))).collect_vec();
+        assert_eq!(
+            profit_for_sequence(&price_changes, &[-2, 1, -1, 3]),
+            23
+        );
+    }
+
+    #[test]
+    fn test_changes() {
+        let changes = changes(&prices(123, 10));
+        assert_eq!(
+            changes.iter().map(|c| c.delta).collect_vec(),
+            vec![-3, 6, -1, -1, 0, 2, -2, 0, -2]
+        );
+        assert_eq!(
+            changes.iter().map(|c| c.price).collect_vec(),
+            vec![0, 6, 5, 4, 4, 6, 4, 4, 2]
+        );
+    }
 
     #[test]
     fn part1_example() {
-        assert_eq!(part1(EXAMPLE),37327623);
+        assert_eq!(part1(EXAMPLE), 37327623);
     }
 
     #[test]
     fn part2_example() {
-        assert_eq!(part2(EXAMPLE), 0);
+        assert_eq!(part2(EXAMPLE2), 23);
     }
-}
+}