2017 day9 part1

3 files changed, 63 insertions, 0 deletions

diff --git a/src/2017/day9/README.md b/src/2017/day9/README.md
index c57880c..162ef71 100644
--- a/src/2017/day9/README.md
+++ b/src/2017/day9/README.md
@@ -40,3 +40,16 @@ Your goal is to find the total score for all groups in your input. Each group is
 {{<a!>},{<a!>},{<a!>},{<ab>}}, score of 1 + 2 = 3.
 What is the total score for all groups in your input?
 
+--- Part Two ---
+Now, you're ready to remove the garbage.
+
+To prove you've removed it, you need to count all of the characters within the garbage. The leading and trailing < and > don't count, nor do any canceled characters or the ! doing the canceling.
+
+<>, 0 characters.
+<random characters>, 17 characters.
+<<<<>, 3 characters.
+<{!>}>, 2 characters.
+<!!>, 0 characters.
+<!!!>>, 0 characters.
+<{o"i!a,<{i<a>, 10 characters.
+How many non-canceled characters are within the garbage in your puzzle input?
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
diff --git a/src/2017/day9/aoc.h b/src/2017/day9/aoc.h
index 7aacc4c..7fb26e7 100644
--- a/src/2017/day9/aoc.h
+++ b/src/2017/day9/aoc.h
@@ -3,4 +3,5 @@
 
 namespace aoc2017 {
 
+int day9(line_view);
 }