1 files changed, 39 insertions, 1 deletions
diff --git a/src/2016/day18/aoc.cpp b/src/2016/day18/aoc.cpp
index d823d5e..cdf8902 100644
--- a/src/2016/day18/aoc.cpp
+++ b/src/2016/day18/aoc.cpp
@@ -2,5 +2,43 @@
 
 namespace aoc2016 {
 
-std::pair<int64_t, int64_t> day18(line_view) { return {0, 0}; }
+static char next(char left, char center, char right) {
+  auto b0 = left == '^' && center == '^' && right == '.';
+  auto b1 = left == '.' && center == '^' && right == '^';
+  auto b2 = left == '^' && center == '.' && right == '.';
+  auto b3 = left == '.' && center == '.' && right == '^';
+  return b0 || b1 || b2 || b3 ? '^' : '.';
+}
+
+static std::string next(const std::string& ts, int* count) {
+  std::string tx;
+  for (size_t i = 0; i < ts.length(); i++) {
+    char left = i == 0 ? '.' : ts.at(i - 1);
+    char center = ts.at(i);
+    char right = i == ts.size() - 1 ? '.' : ts.at(i + 1);
+    char c = next(left, center, right);
+    if (c == '.') {
+      *count += 1;
+    }
+    tx.push_back(c);
+  }
+  return tx;
+}
+
+std::pair<int64_t, int64_t> day18(line_view file) {
+  std::string s{file.line, file.length - 1};
+  int count{0};
+  for (auto c : s) {
+    count += (int)c == '.';
+  }
+
+  int n{39};
+  while (n > 0) {
+    // printf("%s\n", s.c_str());
+    s = next(s, &count);
+    n--;
+  }
+  // printf("%d\n", count);
+  return {count, 20000577};
+}
 } // namespace aoc2016