From 20be794e885d020a3ea9d8905fab46fc64a4ae85 Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Sun, 22 Dec 2024 11:12:33 +0100
Subject: [PATCH] Solve 22

---
 src/bin/22.rs | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 src/bin/22.rs

diff --git a/src/bin/22.rs b/src/bin/22.rs
new file mode 100644
index 0000000..6d68c9a
--- /dev/null
+++ b/src/bin/22.rs
@@ -0,0 +1,84 @@
+use aoc::*;
+use itertools::Itertools;
+use std::collections::HashMap;
+use std::iter;
+
+const INPUT: &str = include_str!("../../input/22");
+
+fn main() {
+    assert_example!(part1, "22-test", 37327623);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "22-test", 24);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    parse(input)
+        .map(|secret| secret_numbers(secret).last().unwrap())
+        .sum()
+}
+
+fn part2(input: &str) -> usize {
+    let buyers = parse(input);
+    let possible_sales = buyers.flat_map(possible_sales);
+
+    let mut sequences = HashMap::new();
+    for (seq, price) in possible_sales {
+        let price = price as usize;
+        let entry = sequences.entry(seq).or_insert(0);
+        *entry += price;
+    }
+
+    sequences.into_values().max().unwrap()
+}
+
+fn possible_sales(secret: usize) -> HashMap<[i8; 4], u8> {
+    let mut sales = HashMap::new();
+    for (a, b, c, d) in price_changes(secret).tuple_windows() {
+        let seq = [a.change, b.change, c.change, d.change];
+        sales.entry(seq).or_insert(d.price);
+    }
+    sales
+}
+
+fn price_changes(secret: usize) -> impl Iterator<Item = PriceChange> {
+    let prices = secret_numbers(secret).map(|s| (s % 10) as u8);
+    prices.tuple_windows().map(|(a, b)| PriceChange {
+        price: b,
+        change: b as i8 - a as i8,
+    })
+}
+
+#[derive(Debug, Copy, Clone)]
+struct PriceChange {
+    price: u8,
+    change: i8,
+}
+
+fn secret_numbers(mut secret: usize) -> impl Iterator<Item = usize> {
+    iter::from_fn(move || {
+        let s = secret;
+        secret = mix_prune(secret);
+        Some(s)
+    })
+    .take(2001)
+}
+
+fn mix_prune(mut secret: usize) -> usize {
+    const MOD: usize = 16777216;
+
+    let n = secret * 64;
+    secret = (secret ^ n) % MOD;
+
+    let n = secret / 32;
+    secret = (secret ^ n) % MOD;
+
+    let n = secret * 2048;
+    secret = (secret ^ n) % MOD;
+
+    secret
+}
+
+fn parse(input: &str) -> impl Iterator<Item = usize> + use<'_> {
+    input.lines().map(|line| line.parse().unwrap())
+}