2 files changed, 135 insertions, 0 deletions

diff --git a/2016/src/day17.cpp b/2016/src/day17.cpp
new file mode 100644
index 0000000..757a8cb
--- /dev/null
+++ b/2016/src/day17.cpp
@@ -0,0 +1,132 @@
+// Standard C++
+#include <algorithm>
+#include <array>
+#include <iostream>
+#include <map>
+#include <queue>
+#include <string>
+#include <tuple>
+
+// System
+#include <md5.h>
+
+using namespace std;
+
+namespace {
+
+using pos_type = tuple<int, int, string>;
+
+string
+md5(string_view input)
+{
+        array<char, MD5_DIGEST_STRING_LENGTH> digest{};
+        MD5Data(input.data(), input.size(), digest.data());
+        return digest.data();
+}
+
+void
+part1(const string& passcode)
+{
+        static const map<size_t, tuple<char, int, int>> dirs{
+                { 0, { 'U', 0, -1 } },
+                { 1, { 'D', 0, 1 } },
+                { 2, { 'L', -1, 0 } },
+                { 3, { 'R', 1, 0 } }
+        };
+
+        auto is_open = [](auto chr) {
+                return strchr("bcdef", chr) != nullptr;
+        };
+
+        queue<pos_type> queue;
+        queue.emplace(0, 0, "");
+
+        while ( !queue.empty() ) {
+                auto [x, y, path] = queue.front();
+                queue.pop();
+
+                if ( x == 3 && y == 3 ) {
+                        cout << path << '\n';
+                        return;
+                }
+
+                auto hash = md5(passcode + path);
+
+                for ( size_t i = 0; i != 4; ++i ) {
+                        if ( !is_open(hash.at(i)) ) {
+                                continue;
+                        }
+
+                        auto [chr, dx, dy] = dirs.at(i);
+
+                        auto new_x = x + dx;
+                        auto new_y = y + dy;
+
+                        if ( new_x < 0 || new_y < 0 || new_x >= 4 || new_y >= 4 ) {
+                                continue;
+                        }
+
+                        queue.emplace(new_x, new_y, path + chr);
+                }
+        }
+}
+
+void
+part2(const string& passcode)
+{
+        static const map<size_t, tuple<char, int, int>> dirs{
+                { 0, { 'U', 0, -1 } },
+                { 1, { 'D', 0, 1 } },
+                { 2, { 'L', -1, 0 } },
+                { 3, { 'R', 1, 0 } }
+        };
+
+        auto is_open = [](auto chr) {
+                return strchr("bcdef", chr) != nullptr;
+        };
+
+        size_t max_length = 0;
+
+        queue<pos_type> queue;
+        queue.emplace(0, 0, "");
+
+        while ( !queue.empty() ) {
+                auto [x, y, path] = queue.front();
+                queue.pop();
+
+                if ( x == 3 && y == 3 ) {
+                        max_length = max(max_length, path.size());
+                        continue;
+                }
+
+                auto hash = md5(passcode + path);
+
+                for ( size_t i = 0; i != 4; ++i ) {
+                        if ( !is_open(hash.at(i)) ) {
+                                continue;
+                        }
+
+                        auto [chr, dx, dy] = dirs.at(i);
+
+                        auto new_x = x + dx;
+                        auto new_y = y + dy;
+
+                        if ( new_x < 0 || new_y < 0 || new_x >= 4 || new_y >= 4 ) {
+                                continue;
+                        }
+
+                        queue.emplace(new_x, new_y, path + chr);
+                }
+        }
+
+        cout << max_length << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+        part1("awrkjxxr");
+        part2("awrkjxxr");
+}
diff --git a/makefile b/makefile
index ffbddf4..8e6c7c2 100644
--- a/makefile
+++ b/makefile
@@ -24,6 +24,9 @@ all: $(patsubst 2015/src/%.cpp,2015/bin/%,$(wildcard 2015/src/*.cpp)) \
 2016/bin/day14: 2016/src/day14.cpp | 2016/bin
         c++ $(CPPFLAGS) $^ -lmd -o $@
 
+2016/bin/day17: 2016/src/day17.cpp | 2016/bin
+        c++ $(CPPFLAGS) $^ -lmd -o $@
+
 2016/bin/day05: 2016/src/day05.cpp | 2016/bin
         c++ $(CPPFLAGS) $^ -lmd -o $@