1 files changed, 42 insertions, 0 deletions

diff --git a/2024/src/day22.cpp b/2024/src/day22.cpp
index b103986..7fdc40e 100644
--- a/2024/src/day22.cpp
+++ b/2024/src/day22.cpp
@@ -1,6 +1,8 @@
 #include <cstdlib>
 #include <fstream>
 #include <iostream>
+#include <map>
+#include <set>
 #include <vector>
 using namespace std;
 
@@ -20,6 +22,7 @@ read_file(string_view filename)
 unsigned long
 step(unsigned long value)
 {
+        // compiler will optimize these calculations
         value = (value ^ (value * 64)) % 16777216;
         value = (value ^ (value / 32)) % 16777216;
         value = (value ^ (value * 2048)) % 16777216;
@@ -39,9 +42,48 @@ part1(const vector<unsigned long>& values)
         cout << sum << endl;
 }
 
+void
+part2(const vector<unsigned long>& values)
+{
+        using seq_type = tuple<unsigned long, unsigned long, unsigned long, unsigned long>;
+
+        map<seq_type, unsigned long> total;
+
+        for ( auto value: values ) {
+                vector<unsigned long> list{ value % 10 };
+
+                for ( int i = 0; i != 2000; ++i ) {
+                        value = step(value);
+                        list.emplace_back(value % 10);
+                }
+
+                set<seq_type> seen;
+
+                for ( size_t idx = 4; idx < list.size(); ++idx ) {
+                        const auto a = list[idx - 4];
+                        const auto b = list[idx - 3];
+                        const auto c = list[idx - 2];
+                        const auto d = list[idx - 1];
+                        const auto e = list[idx];
+
+                        seq_type seq{ b - a, c - b, d - c, e - d };
+
+                        if ( seen.contains(seq) ) {
+                                continue;
+                        }
+                        seen.insert(seq);
+
+                        total[seq] += e;
+                }
+        }
+
+        cout << ranges::max_element(total, [](const auto& lhs, const auto& rhs) { return lhs.second < rhs.second; })->second << endl;
+}
+
 int
 main()
 {
         auto data = read_file("data/day22.txt");
         part1(data);
+        part2(data);
 }