1 files changed, 36 insertions, 0 deletions
diff --git a/src/2017/day11/aoc.h b/src/2017/day11/aoc.h
index 1270e1a..afd1cb2 100644
--- a/src/2017/day11/aoc.h
+++ b/src/2017/day11/aoc.h
@@ -6,5 +6,41 @@ namespace aoc2017 {
 
 enum class hd { n, ne, nw, se, sw, s };
 
+// a, r, c
+// n: a, r-1, c
+// nw: 1-a, r-(1-a), c-(1-a)
+// sw: 1-a, r+a, c-(1-a)
+// ne: 1-a, r-(1-a), c+a
+// se: 1-a, r+a, c+a
+// s: a, r+1, c
+struct hexagon {
+  int a;
+  int r;
+  int c;
+
+  hexagon neighbour(hd d) {
+    switch (d) {
+    case hd::n:
+      return {a, r - 1, c};
+    case hd::ne:
+      return {1 - a, r - (1 - a), c + a};
+    case hd::nw:
+      return {1 - a, r - (1 - a), c - (1 - a)};
+    case hd::se:
+      return {1 - a, r + a, c + a};
+    case hd::sw:
+      return {1 - a, r + a, c - (1 - a)};
+    case hd::s:
+      return {a, r + 1, c};
+    }
+    return *this;
+  }
+
+  friend bool operator==(hexagon h1, hexagon h2) { return h1.a == h2.a && h1.r == h2.r && h1.c == h2.c; }
+  friend bool operator<(hexagon h1, hexagon h2) {
+    return h1.a < h2.a ? true : h1.a > h2.a ? false : h1.r < h2.r ? true : h1.r > h2.r ? false : h1.c < h2.c;
+  }
+};
+
 std::pair<int64_t, int64_t> day11(line_view);
 } // namespace aoc2017