1 files changed, 49 insertions, 0 deletions
diff --git a/src/2017/day9/aoc.cpp b/src/2017/day9/aoc.cpp
index ed2d9d3..0b1cda6 100644
--- a/src/2017/day9/aoc.cpp
+++ b/src/2017/day9/aoc.cpp
@@ -1,5 +1,54 @@
 #include "aoc.h"
+#include <stack>
 
 namespace aoc2017 {
 
+static void take(const char* p, std::stack<char>& gs, int* t) {
+  if (*p == '{') {
+    gs.push(*p);
+  }
+  if (*p == '}') {
+    *t += gs.size();
+    gs.pop();
+  }
 }
+
+static bool is_valid(const char* p, std::stack<char>& g) {
+  if (g.empty()) {
+    if (*p == '!') {
+      g.push(*p);
+    }
+    if (*p == '<') {
+      g.push(*p);
+    }
+  } else {
+    if (g.top() == '!') {
+      g.pop();
+    } else {
+      if (*p == '>') {
+        g.pop();
+      }
+      if (*p == '!') {
+        g.push(*p);
+      }
+    }
+  }
+
+  return g.empty();
+}
+
+int day9(line_view file) {
+  int t0{0};
+  std::stack<char> gs;
+  std::stack<char> gv;
+  const char* p = file.line;
+  while (p < file.line + file.length) {
+    if (is_valid(p, gv)) {
+      take(p, gs, &t0);
+    }
+    p++;
+  }
+  return t0;
+}
+
+} // namespace aoc2017