2016 day15 done

1 files changed, 34 insertions, 1 deletions

diff --git a/src/2016/day15/aoc.cpp b/src/2016/day15/aoc.cpp
index e188291..318b345 100644
--- a/src/2016/day15/aoc.cpp
+++ b/src/2016/day15/aoc.cpp
@@ -2,5 +2,38 @@
 
 namespace aoc2016 {
 
-std::pair<int64_t, int64_t> day15(line_view) { return {0, 0}; }
+size_t position_at(int t, int slot, size_t p) { return t > 0 ? (t % slot + p) % slot : p; }
+
+struct disc {
+  int slots;
+  size_t p;
+};
+
+static bool same(size_t* ps, int x) {
+  for (int i = 0; i < x; i++) {
+    if (ps[i] != ps[x]) {
+      return false;
+    }
+  }
+  return true;
+}
+
+int first_catch(disc ds[7], int x) {
+  int t{0};
+  while (true) {
+    size_t ps[7] = {0, 0, 0, 0, 0, 0, 0};
+    for (size_t i = 0; i < 7; i++) {
+      ps[i] = position_at(t + i + 1, ds[i].slots, ds[i].p);
+    }
+    if (same(ps, x)) {
+      return t;
+    }
+    t++;
+  }
+}
+
+std::pair<int64_t, int64_t> day15(line_view) {
+  disc ds[7] = {{17, 1}, {7, 0}, {19, 2}, {5, 0}, {3, 0}, {13, 5}, {11, 0}};
+  return {first_catch(ds, 5), first_catch(ds, 6)};
+}
 } // namespace aoc2016