From 1287f7d36af4cdbdc18194451538c09e800951ac Mon Sep 17 00:00:00 2001
From: Leah Neukirchen <leah@vuxu.org>
Date: Mon, 25 Dec 2017 15:34:43 +0100
Subject: day25

---
 day25    | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 day25.cc | 43 +++++++++++++++++++++++++++++++++++++++++++
 day25.k  |  5 +++++
 3 files changed, 110 insertions(+)
 create mode 100644 day25
 create mode 100644 day25.cc
 create mode 100644 day25.k

diff --git a/day25 b/day25
new file mode 100644
index 0000000..e2e4d56
--- /dev/null
+++ b/day25
@@ -0,0 +1,62 @@
+Begin in state A.
+Perform a diagnostic checksum after 12399302 steps.
+
+In state A:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state C.
+
+In state B:
+  If the current value is 0:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state D.
+
+In state C:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state D.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+
+In state D:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state E.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state D.
+
+In state E:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state F.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state B.
+
+In state F:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state E.
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
+}
diff --git a/day25.k b/day25.k
new file mode 100644
index 0000000..5bdcc3a
--- /dev/null
+++ b/day25.k
@@ -0,0 +1,5 @@
+/ d:`a`b`c`d`e`f!((1,1,`b;0,1,`c);(0,-1,`a;0,1,`d);(1,1,`d;1,1,`a);(1,-1,`e;0,-1,`d);(1,1,`f;1,-1,`b);(1,1,`a;1,1,`e))
+
+d:`a`b`c`d`e`f!0N 2#+(1 0 0 0 1 1 1 0 1 1 1 1;1 1 -1 1 1 1 -1 -1 1 -1 1 1;`b`c`a`d`d`a`e`d`f`b`a`e)
+v:20000#0;12399302{v[*x]::*s:d[*|x]v@*x;(s[1]+*x;s[2])}/10000,`a;+/v
+\\
-- 
cgit 1.4.1