Day 22 Part 2 solution

bin/day22.dart

@@ -1,10 +1,11 @@
 import 'package:aoc_2024/lib.dart';
 import 'package:aoc_2024/day22/part_1.dart' as part1;
+import 'package:aoc_2024/day22/part_2.dart' as part2;
 
 Future<void> main(List<String> arguments) async {
   await runDay(
     day: Day.day22,
     part1: part1.calculate,
-    part2: (_) => Future.value(0),
+    part2: part2.calculate,
   );
 }

lib/day22/part_2.dart

@@ -0,0 +1,78 @@
+import 'dart:collection';
+import 'dart:io';
+
+import 'shared.dart';
+
+/// Continuing from part 1, determine the best instructions for selling.
+/// Monkeys can only be told to sell based on the a sequence of four
+/// price changes. Prices are the final digit of the secret number.
+///
+/// Monkeys will sell as soon as they see the specified sequence of four
+/// prices.
+///
+/// For all the secret numbers in the input (each representing a buyer),
+/// deterime the price change sequence that will result in the highest
+/// total sales. It is possible that the sequence does not appear in the
+/// first 2000 prices for certain buyers (which is ok, as long as the
+/// total value is highest).
+Future<int> calculate(File file) async {
+  final data = await loadData(file);
+  totalPriceForSequence.clear();
+
+  // First, run through all 2000 numbers per buyer, during which
+  // [totalPriceForSequence] will maintain the total value of sales
+  // given that sequence.
+  for (final start in data) {
+    _simulate(start, 2000);
+  }
+
+  // Then, iterate through all of the sequences, and find the highest
+  // total sales..
+  int max = 0;
+  for (final entry in totalPriceForSequence.entries) {
+    if (entry.value > max) {
+      max = entry.value;
+    }
+  }
+
+  return max;
+}
+
+/// Stores the total sales price for a given sequence.
+/// Each secret number simulation should only contribute once per
+/// key to this map.
+HashMap<String, int> totalPriceForSequence = HashMap();
+
+/// Generate the secret numbers up to [iterations], starting at
+/// the given [starting] secret number. The first time each
+/// sequence of four price changes is encountered, the sales price
+/// is added to [totalPriceForSequence].
+void _simulate(int starting, int iterations) {
+  // Keep track of the sequences seen for this starting number simulation.
+  Set<String> seenSequences = {};
+  List<int> trailingFourChanges = [];
+
+  int secret = starting;
+  int lastPrice = starting % 10;
+  for (int i = 0; i < iterations; i++) {
+    secret = nextSecret(secret);
+    final newPrice = secret % 10;
+
+    trailingFourChanges.add(newPrice - lastPrice);
+    if (i >= 4) {
+      trailingFourChanges.removeAt(0);
+      final key = trailingFourChanges.join(',');
+
+      // If this is the first time we have seen this sequence, add the
+      // price to the sales map.
+      if (!seenSequences.contains(key)) {
+        seenSequences.add(key);
+
+        totalPriceForSequence.update(key, (x) => x + newPrice,
+            ifAbsent: () => newPrice);
+      }
+    }
+
+    lastPrice = newPrice;
+  }
+}

test/day22_test.dart

@@ -1,5 +1,6 @@
 import 'package:aoc_2024/lib.dart';
 import 'package:aoc_2024/day22/part_1.dart' as part1;
+import 'package:aoc_2024/day22/part_2.dart' as part2;
 import 'package:test/test.dart';
 
 void main() {
@@ -12,6 +13,10 @@ void main() {
     test('part1', () async {
       expect(await part1.calculate(file), 37327623);
     });
+
+    test('part2', () async {
+      expect(await part2.calculate(file), 24);
+    });
   });
 
   group('real data', tags: 'real-data', () {
@@ -21,5 +26,9 @@ void main() {
     test('part1', () async {
       expect(await part1.calculate(file), 15608699004);
     });
+
+    test('part2', () async {
+      expect(await part2.calculate(file), 1791);
+    });
   });
 }