1 files changed, 127 insertions, 0 deletions

diff --git a/2016/src/day13.cpp b/2016/src/day13.cpp
new file mode 100644
index 0000000..107bc16
--- /dev/null
+++ b/2016/src/day13.cpp
@@ -0,0 +1,127 @@
+#include <bit>
+#include <iostream>
+#include <queue>
+#include <set>
+
+using namespace std;
+
+namespace {
+
+using pos_type = tuple<long, long>;
+
+bool
+is_wall(long x, long y, long number)
+{
+        auto result = (x * x) + (3 * x) + (2 * x * y) + y + (y * y);
+        result += number;
+        auto num = popcount(static_cast<unsigned long>(result));
+
+        return (num % 2) != 0;
+}
+
+int
+part1(long initial_value, pos_type dest)
+{
+        pos_type start_pos = { 1, 1 };
+
+        set<pos_type>               seen;
+        queue<tuple<pos_type, int>> queue; // pos, #steps
+
+        seen.emplace(start_pos);
+        queue.emplace(start_pos, 0);
+
+        while ( !queue.empty() ) {
+                auto [pos, steps] = queue.front();
+                queue.pop();
+
+                if ( pos == dest ) {
+                        return steps;
+                }
+
+                static const pos_type dirs[] = { { -1, 0 }, { 0, -1 }, { 1, 0 }, { 0, 1 } };
+
+                for ( const auto [dx, dy]: dirs ) {
+                        auto [x, y] = pos;
+                        x += dx;
+                        y += dy;
+
+                        if ( x < 0 || y < 0 ) {
+                                continue;
+                        }
+
+                        if ( seen.contains({ x, y }) ) {
+                                continue;
+                        }
+
+                        if ( !is_wall(x, y, initial_value) ) {
+                                seen.emplace(x, y);
+                                queue.emplace(pos_type{ x, y }, steps + 1);
+                        }
+                }
+        }
+        return -1; // Error
+}
+
+int
+part2(long initial_value, int max_steps)
+{
+        pos_type start_pos = { 1, 1 };
+
+        int count = 0;
+
+        set<pos_type>               seen;
+        queue<tuple<pos_type, int>> queue; // pos, #steps
+
+        seen.emplace(start_pos);
+        queue.emplace(start_pos, 0);
+
+        while ( !queue.empty() ) {
+                auto [pos, steps] = queue.front();
+                queue.pop();
+
+                if ( steps > max_steps ) {
+                        continue;
+                }
+
+                ++count;
+
+                static const pos_type dirs[] = { { -1, 0 }, { 0, -1 }, { 1, 0 }, { 0, 1 } };
+
+                for ( const auto [dx, dy]: dirs ) {
+                        auto [x, y] = pos;
+                        x += dx;
+                        y += dy;
+
+                        if ( x < 0 || y < 0 ) {
+                                continue;
+                        }
+
+                        if ( seen.contains({ x, y }) ) {
+                                continue;
+                        }
+
+                        if ( !is_wall(x, y, initial_value) ) {
+                                seen.emplace(x, y);
+                                queue.emplace(pos_type{ x, y }, steps + 1);
+                        }
+                }
+        }
+        return count;
+}
+
+} // namespace
+
+int
+main()
+{
+#if 0
+        static const long     number = 10;
+        static const pos_type dest   = { 7, 4 };
+#else
+        static const long     number = 1352;
+        static const pos_type dest   = { 31, 39 };
+#endif
+
+        cout << part1(number, dest) << '\n';
+        cout << part2(number, 50) << '\n';
+}