aoc 2024, day6

1 files changed, 139 insertions, 0 deletions

diff --git a/2024/src/day06.cpp b/2024/src/day06.cpp
new file mode 100644
index 0000000..1198d54
--- /dev/null
+++ b/2024/src/day06.cpp
@@ -0,0 +1,139 @@
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <set>
+#include <string>
+#include <tuple>
+using namespace std;
+
+using pos_type = tuple<size_t, size_t>;
+
+map<pos_type, char>
+read_file(string_view filename)
+{
+        fstream             input{ filename };
+        map<pos_type, char> data;
+
+        size_t yPos = 0;
+        for ( string line; getline(input, line); ) {
+                for ( size_t xPos = 0; xPos != line.size(); ++xPos ) {
+                        data[{ xPos, yPos }] = line[xPos];
+                }
+                ++yPos;
+        }
+
+        return data;
+}
+
+set<pos_type>
+distinct_positions(const map<pos_type, char>& data)
+{
+        static const array<pos_type, 4> deltas = {
+                make_tuple(0, -1),
+                make_tuple(1, 0),
+                make_tuple(0, 1),
+                make_tuple(-1, 0)
+        };
+
+        size_t direction = 0;
+
+        auto position = get<0>(*ranges::find_if(data, [](auto position) {
+                return get<1>(position) == '^';
+        }));
+
+        set<pos_type> positions;
+        while ( true ) {
+                positions.emplace(position);
+
+                auto next_position = make_tuple(get<0>(position) + get<0>(deltas.at(direction)),
+                                                get<1>(position) + get<1>(deltas.at(direction)));
+
+                if ( !data.contains(next_position) ) {
+                        break;
+                }
+
+                if ( data.at(next_position) == '#' ) {
+                        direction = (direction + 1) % deltas.size();
+                }
+                else {
+                        position = next_position;
+                }
+        }
+
+        return positions;
+}
+
+void
+part1(const map<pos_type, char>& data)
+{
+        auto positions = distinct_positions(data);
+
+        cout << positions.size() << endl;
+}
+
+bool
+would_loop(const map<pos_type, char>& data)
+{
+        static const array<pos_type, 4> deltas = {
+                make_tuple(0, -1),
+                make_tuple(1, 0),
+                make_tuple(0, 1),
+                make_tuple(-1, 0)
+        };
+
+        size_t direction = 0;
+
+        auto position = get<0>(*ranges::find_if(data, [](auto position) {
+                return get<1>(position) == '^';
+        }));
+
+        set<tuple<pos_type, size_t>> positions;
+
+        while ( true ) {
+                positions.emplace(position, direction);
+
+                auto next_position = make_tuple(get<0>(position) + get<0>(deltas.at(direction)),
+                                                get<1>(position) + get<1>(deltas.at(direction)));
+
+                if ( !data.contains(next_position) ) {
+                        return false;
+                }
+
+                if ( data.at(next_position) == '#' ) {
+                        direction = (direction + 1) % deltas.size();
+                }
+                else {
+                        position = next_position;
+                }
+
+                if ( positions.contains({ position, direction }) ) {
+                        return true;
+                }
+        }
+}
+
+void
+part2(map<pos_type, char> data)
+{
+        const auto positions = distinct_positions(data);
+
+        long sum = 0;
+        for ( const auto& position: positions ) {
+                if ( data.at(position) == '.' ) {
+                        data.at(position) = '#';
+                        if ( would_loop(data) ) {
+                                ++sum;
+                        }
+                        data.at(position) = '.';
+                }
+        }
+        cout << sum << endl;
+}
+
+int
+main()
+{
+        auto data = read_file("data/day06.txt");
+        part1(data);
+        part2(data);
+}