2024/22

2024/Day22/Solution.cs

@@ -1,82 +1,60 @@
 namespace AdventOfCode.Y2024.Day22;
 
-using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
-using System.Text.RegularExpressions;
-using System.Text;
-using System.Numerics;
 
 [ProblemName("Monkey Market")]
 class Solution : Solver {
 
     public object PartOne(string input) {
-       return GetNums(input).Select(x => (long)Step(x).Last()).Sum();
+       return GetNums(input).Select(x => (long)SecretNumbers(x).Last()).Sum();
     }
 
     public object PartTwo(string input) {
-        var mainDict = new Dictionary<(int a, int b, int c, int d), int>();
-        foreach(var num in GetNums(input)) {
-            var banana = GetBanana(num);
-            foreach(var k in banana.Keys){
-                mainDict[k] = mainDict.GetValueOrDefault(k) + banana[k];
+        // create a dictionary of all buying options then select the one with the most banana:
+
+        var buyingOptions = new Dictionary<string, int>();
+        foreach (var num in GetNums(input)) {
+            var optionsBySeller = BuyingOptions(num);
+            foreach(var seq in optionsBySeller.Keys){
+                buyingOptions[seq] = buyingOptions.GetValueOrDefault(seq) + optionsBySeller[seq];
             }
         }
-        return mainDict.Values.Max();
+        return buyingOptions.Values.Max();
     }
 
-    Dictionary<(int a, int b, int c, int d), int> GetBanana(int num) {
-        var res = new Dictionary<(int a, int b, int c, int d), int>();
+    Dictionary<string, int> BuyingOptions(int seed) {
+        var bananasSold = Bananas(seed).ToArray();
+
+        var buyOptions = new Dictionary<string, int>();
 
-        var prices = Step(num).Select(n => n % 10).ToArray();
-        for(var i = 5;i < prices.Length; i++) {
-            var slice = prices[(i-5) .. i];
-            var diff = Diff(slice);
-            var key = (diff[0], diff[1], diff[2], diff[3]);
-            if (!res.ContainsKey(key)) {
-                res[key] = slice.Last();
+        // a sliding window of 5 elements over the sold bananas defines the sequence the monkey 
+        // will recognize. add the first occurrence of each sequence to the buyOptions dictionary 
+        // with the corresponding banana count
+        for (var i = 5;i < bananasSold.Length; i++) {
+            var slice = bananasSold[(i-5) .. i];
+            var seq = string.Join(",", Diff(slice)); 
+            if (!buyOptions.ContainsKey(seq)) {
+                buyOptions[seq] = slice.Last();
             }
         }
-        return res;
-        // var diff = Diff(steps).ToArray();
-
-        // var res = 0;
-        // for(var i = seq.Length-1; i<diff.Length;i++) {
-        //     if (diff[(i-(seq.Length-1))..i].SequenceEqual(seq)) {
-        //         res += steps[i];
-        //     }
-        // }
-        return res;
-    }
-    IEnumerable<int[]> Quads(int[] nums) {
-        for(var i = 4;i<=nums.Length;i++) {
-            yield return nums[(i-4) .. i];
-        }
-
+        return buyOptions;
     }
+    int[] Bananas(int seed)  => SecretNumbers(seed).Select(n => n % 10).ToArray();
 
+    int[] Diff(IEnumerable<int> x) => x.Zip(x.Skip(1)).Select(p => p.Second - p.First).ToArray();
 
-    int[] Diff(IEnumerable<int> x) {
-        return x.Zip(x.Skip(1)).Select(p => p.Second - p.First).ToArray();
-    }
-    IEnumerable<int> Step(int a) {
-        yield return a;
+    IEnumerable<int> SecretNumbers(int seed) {
+        var mixAndPrune = (int a, long b) => (int)((a ^ b) % 16777216);
+
+        yield return seed;
         for(var i = 0;i< 2000;i++) {
-            var b = (long)a;
-            b = Mix(b, b * 64);
-            b = Prune(b);
-            b = Mix(b, b/32);
-            b = Prune(b);
-            b = Mix(b, b*2048);
-            b = Prune(b);
-            a = (int)b;
-            yield return a;
+            seed = mixAndPrune(seed, seed * 64L);
+            seed = mixAndPrune(seed, seed / 32L);
+            seed = mixAndPrune(seed, seed * 2048L);
+            yield return seed;
         }
     }
-    long Mix(long a, long b) => a ^ b;
-    long Prune(long a) => a % 16777216;
-
-    IEnumerable<int> GetNums(string input) =>
-        input.Split("\n").Select(int.Parse);
+    IEnumerable<int> GetNums(string input) => input.Split("\n").Select(int.Parse);
 }