2017 day17 part1

3 files changed, 54 insertions, 1 deletions

diff --git a/src/2017/day17/README.md b/src/2017/day17/README.md
index f1ce675..33fc413 100644
--- a/src/2017/day17/README.md
+++ b/src/2017/day17/README.md
@@ -30,3 +30,15 @@ What is the value after 2017 in your completed circular buffer?
 
 Your puzzle input is 356.
 
+--- Part Two ---
+
+The spinlock does not short-circuit. Instead, it gets more angry. At least, you assume that's what happened; it's spinning significantly faster than it was a moment ago.
+
+You have good news and bad news.
+
+The good news is that you have improved calculations for how to stop the spinlock. They indicate that you actually need to identify the value after 0 in the current state of the circular buffer.
+
+The bad news is that while you were determining this, the spinlock has just finished inserting its fifty millionth value (50000000).
+
+What is the value after 0 the moment 50000000 is inserted?
+
diff --git a/src/2017/day17/aoc.cpp b/src/2017/day17/aoc.cpp
index f2a9d5a..ba0d5bc 100644
--- a/src/2017/day17/aoc.cpp
+++ b/src/2017/day17/aoc.cpp
@@ -2,6 +2,40 @@
 
 namespace aoc2017 {
 
-std::pair<int64_t, int64_t> day17(line_view file) { return {0, 0}; }
+static spinlock* next(spinlock* x, int step) {
+  while (step-- > 0) {
+    x = x->next;
+  }
+  return x;
+}
+
+static void insert(spinlock* s, spinlock** current, int v) {
+  spinlock* n = s->next;
+  spinlock* x = new spinlock;
+  x->val = v;
+  s->next = x;
+  x->prev = s;
+  x->next = n;
+  n->prev = x;
+  *current = x;
+}
+
+std::pair<int64_t, int64_t> day17(line_view file) {
+  spinlock* l = new spinlock;
+  l->val = 0;
+  l->next = l;
+  l->prev = l;
+  // spinlock* l0 = l;
+
+  int step{356};
+  for (int i = 1; i < 2018; i++) {
+    l = next(l, step);
+    insert(l, &l, i);
+  }
+  int t0 = l->next->val;
+  // printf("%d\n", l0->next->val);
+
+  return {t0, 0};
+}
 
 } // namespace aoc2017
diff --git a/src/2017/day17/aoc.h b/src/2017/day17/aoc.h
index e8b7659..022f148 100644
--- a/src/2017/day17/aoc.h
+++ b/src/2017/day17/aoc.h
@@ -3,5 +3,12 @@
 #include <vector>
 
 namespace aoc2017 {
+
+struct spinlock {
+  int val;
+  spinlock* next = nullptr;
+  spinlock* prev = nullptr;
+};
+
 std::pair<int64_t, int64_t> day17(line_view);
 }