Fix invalid diff list initialization

Author: fwcd

day22/experiments/.clangd

@@ -0,0 +1,2 @@
+CompileFlags:
+  Add: [-std=c++20]

day22/experiments/Makefile

@@ -8,4 +8,4 @@ out:
 	mkdir -p $@
 
 out/day22: ./day22.cpp | out
-	$(CXX) $< -ffast-math -fopenmp -O3 -std=c++11 -o $@
+	$(CXX) $< -ffast-math -fopenmp -O3 -std=c++20 -o $@

day22/experiments/day22.cpp

@@ -2,6 +2,7 @@
 #include <cstdint>
 #include <iostream>
 #include <fstream>
+#include <optional>
 #include <vector>
 
 const int64_t LIMIT = 2000;
@@ -25,7 +26,7 @@ int64_t prng(int64_t secret, int64_t n) {
 }
 
 int64_t monkey(int64_t secret, int64_t x1, int64_t x2, int64_t x3, int64_t x4) {
-  int64_t d1 = -1, d2 = -1, d3 = -1, d4 = -1;
+  std::optional<int64_t> d1, d2, d3, d4;
   for (int64_t i = 0; i < LIMIT; i++) {
     int64_t lastPrice = secret % 10;
     secret = next(secret);

day22/src/day22.dart

@@ -21,7 +21,7 @@ int prng(int secret, int n) {
 }
 
 int monkey(int secret, int x1, int x2, int x3, int x4) {
-  int d1 = -1, d2 = -1, d3 = -1, d4 = -1;
+  int? d1 = null, d2 = null, d3 = null, d4 = null;
   for (var i = 0; i < LIMIT; i++) {
     int lastPrice = secret % 10;
     secret = next(secret);