1 files changed, 43 insertions, 0 deletions
diff --git a/day25.cc b/day25.cc
new file mode 100644
index 0000000..f1eba65
--- /dev/null
+++ b/day25.cc
@@ -0,0 +1,43 @@
+#include <iostream>
+#include <numeric>
+#include <vector>
+
+using namespace std;
+
+struct trans;
+using state = pair<trans,trans>;
+struct trans {
+	int w, m;
+	state *n;
+};
+
+enum {
+	L = -1,
+	R = +1
+};
+
+int main() {
+	int l = 12399302;
+
+	state A, B, C, D, E, F;
+	A = {{1, R, &B}, {0, R, &C}};
+	B = {{0, L, &A}, {0, R, &D}};
+	C = {{1, R, &D}, {1, R, &A}};
+	D = {{1, L, &E}, {0, L, &D}};
+	E = {{1, R, &F}, {1, L, &B}};
+	F = {{1, R, &A}, {1, R, &E}};
+
+	state *s = &A;
+
+	vector<int> v(20'000, 0);
+	auto head = begin(v) + 10'000;
+
+	for (int i = 0; i < l; i++) {
+		trans &t = *head == 0 ? s->first : s->second;
+		*head = t.w;
+		head += t.m;
+		s = t.n;
+	}
+
+	cout << accumulate(begin(v), end(v), 0) << endl;	// 2794
+}