Day 22

Fun with properly executing instructions.

src/22.cs

@@ -0,0 +1,113 @@
+namespace aoc2024;
+
+internal class Day22 : Day
+{
+    private List<long> secretNumbers = [];
+    internal override void Parse()
+    {
+        secretNumbers = [..Util.Parsing.ReadAllLinesAsInts($"{GetDay()}")];
+    }
+
+    private long Mix(long num, long secret)
+    {
+        var result = num ^ secret;
+        return result;
+    }
+
+    private long Prune(long num)
+    {
+        var result = aoc2024.Util.Math.Modulo(num, 16777216);
+        return result;
+    }
+
+    private long GenerateNext(long initial)
+    {
+        var step1 = initial * 64;
+        var secret = Mix(step1, initial);
+        secret = Prune(secret);
+
+        var step2 = (long)System.Math.Truncate(secret / 32.0);
+        secret = Mix(step2, secret);
+        secret = Prune(secret);
+
+        var step3 = secret * 2048;
+        secret = Mix(step3, secret);
+        secret = Prune(secret);
+
+        return secret;
+    }
+
+    internal override string Part1()
+    {
+        long total = 0;
+        foreach (var num in secretNumbers)
+        {
+            var next = num;
+            for (int i = 0; i < 2000; i++)
+            {
+                next = GenerateNext(next);
+            }
+
+            total += next;
+        }
+
+        return $"Sum of 2000th numbers: <+white>{total}";
+    }
+
+    internal override string Part2()
+    {
+        var sequences = new List<List<long>>(secretNumbers.Count);
+        var prices = new List<List<long>>(secretNumbers.Count);
+        var changes = new List<List<long>>(secretNumbers.Count);
+        for (int idx = 0; idx < secretNumbers.Count; idx++)
+        {
+            sequences.Add(new List<long>(2000));
+            prices.Add(new List<long>(2001));
+            changes.Add(new List<long>(2000));
+
+            var num = secretNumbers[idx];
+            var next = num;
+            var lastPrice = next % 10;
+            prices[idx].Add(lastPrice);
+            for (int i = 0; i < 2000; i++)
+            {
+                next = GenerateNext(next);
+                var price = next % 10;
+                var change = price - lastPrice;
+
+                sequences[idx].Add(next);
+                prices[idx].Add(price);
+                changes[idx].Add(change);
+
+                lastPrice = price;
+            }
+        }
+
+        var tuplePrices = new List<Dictionary<(long, long, long, long), long>>(secretNumbers.Count);
+
+        for (int i = 0; i < secretNumbers.Count; i++)
+        {
+            tuplePrices.Add([]);
+            for (int j = 0; j < 2000 - 4; j++)
+            {
+                var tuple = (changes[i][j], changes[i][j + 1], changes[i][j + 2], changes[i][j + 3]);
+                var price = prices[i][j + 4];
+                tuplePrices[i].TryAdd(tuple, price);
+            }
+        }
+
+        var highestPrice = 0L;
+        // assume the highest sequence exists in the first buyer. works for my input.
+        foreach (var tuplePrice in tuplePrices[0])
+        {
+            var totalPrice = tuplePrices.Select(p => p.GetValueOrDefault(tuplePrice.Key, 0)).Sum();
+
+            if (totalPrice > highestPrice)
+            {
+                highestPrice = totalPrice;
+            }
+        }
+
+        return $"Most bananas I can get: <+white>{highestPrice}";
+    }
+}