1 files changed, 216 insertions, 0 deletions

diff --git a/2024/06/day06b.cpp b/2024/06/day06b.cpp
new file mode 100644
index 0000000..2498536
--- /dev/null
+++ b/2024/06/day06b.cpp
@@ -0,0 +1,216 @@
+/*
+This is quite inefficient, but it still gives the right answer in less
+than a minute (25 seconds on my old laptop).
+*/
+
+#include <iostream>
+#include <algorithm>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+enum Direction { U = 0, D, R, L };
+Direction all_directions[] = {
+        Direction::U, Direction::D, Direction::R, Direction::L,
+};
+
+pair<int, int> step(pair<int, int> p, Direction d) {
+        auto [i, j] = p;
+
+        switch (d) {
+        case Direction::U:
+                return make_pair(i-1, j);
+        case Direction::D:
+                return make_pair(i+1, j);
+        case Direction::R:
+                return make_pair(i, j+1);
+        case Direction::L:
+                return make_pair(i, j-1);
+        }
+
+        return make_pair(-999,-999);
+}
+
+Direction turn(Direction d) {
+        switch (d) {
+        case Direction::U:
+                return Direction::R;
+        case Direction::D:
+                return Direction::L;
+        case Direction::R:
+                return Direction::D;
+        case Direction::L:
+                return Direction::U;
+        }
+
+        return Direction::U;
+}
+
+class Board {
+public:
+        const char out_of_bound = '$';
+        int M, N;
+
+        Board(vector<string> &lines) {
+                N = lines.size();
+                M = 0;
+                for (string l : lines)
+                        M = max(M, (int)l.size());
+
+                cells = new char[M * N];
+                visited = new int[M * N];
+                for (int i = 0; i < N; i++) {
+                        for (int j = 0; j < M; j++) {
+                                cells[N*i + j] = j < (int)lines[i].size() ?
+                                    lines[i][j] : out_of_bound;
+                                visited[N*i + j] = 0;
+                        }
+                }
+        }
+
+        ~Board() {
+                delete []cells;
+                delete []visited;
+        }
+
+        char operator[](pair<int, int> p) {
+                int c = coord(p);
+                return c == -1 ? out_of_bound : cells[c];
+        }
+
+        bool is_visited(pair<int, int> p, Direction d) {
+                return get_vmask(p) & (1 << d);
+        }
+
+        void set_visited(pair<int, int> p, Direction d) {
+                toggle_vmask(p, d);
+        }
+
+        void clear_visited() {
+                for (int i = 0; i < N; i++)
+                        for (int j = 0; j < M; j++)
+                                set_vmask(make_pair(i, j), 0);
+        }
+
+        bool is_obstruction(pair<int, int> p) {
+                return (*this)[p] == '#';
+        }
+
+        void set_obstruction(pair<int, int> p) {
+                set_cell(p, '#');
+        }
+
+        void set_clean(pair<int, int> p) {
+                set_cell(p, '.');
+        }
+
+private:
+        char *cells;
+        int *visited;
+
+        int coord(pair<int, int> p) {
+                auto [i, j] = p;
+                return i >= N || i < 0 || j >= M || j < 0 ? -1 : N*i + j;
+        }
+
+        void set_cell(pair<int, int> p, char c) {
+                int i = coord(p);
+                cells[i] = c;
+        }
+
+        int get_vmask(pair<int, int> p) {
+                int i = coord(p);
+                return visited[i];
+        }
+
+        void set_vmask(pair<int, int> p, int x) {
+                int i = coord(p);
+                visited[i] = x;
+        }
+
+        void toggle_vmask(pair<int, int> p, Direction d) {
+                int i = coord(p);
+                visited[i] ^= 1 << d;
+        }
+};
+
+Direction guard_direction(char c) {
+        switch (c) {
+        case '^':
+                return Direction::U;
+        case 'v':
+                return Direction::D;
+        case '>':
+                return Direction::R;
+        case '<':
+                return Direction::L;
+        }
+
+        return Direction::U;
+}
+
+pair<pair<int, int>, Direction> find_guard(Board &board) {
+        pair<int, int> p(0, 0);
+        for (p.first = 0; p.first < board.N; p.first++) {
+                for (p.second = 0; p.second < board.M; p.second++) {
+                        if (board[p] != '.' && board[p] != '#') {
+                                Direction d = guard_direction(board[p]);
+                                return make_pair(p, d);
+                        }
+                }
+        }
+
+        return make_pair(make_pair(-999, -999), Direction::U);
+}
+
+bool isloop(pair<int, int> i, pair<int, int> p, Direction d, Board &board) {
+        bool ret = false;
+
+        board.set_obstruction(i);
+
+        while (true) {
+                auto q = step(p, d);
+                if (board.is_visited(p, d)) {
+                        ret = true;
+                        break;
+                }
+                board.set_visited(p, d);
+
+                if (board[q] == board.out_of_bound)
+                        break;
+
+                if (board.is_obstruction(q)) {
+                        d = turn(d);
+                } else {
+                        p = q;
+                }
+        }
+
+        board.clear_visited();
+        board.set_clean(i);
+
+        return ret;
+}
+
+int main() {
+        string line;
+        vector<string> lines;
+        while (getline(cin, line))
+                lines.push_back(line);
+
+        Board board(lines);
+
+        int tot = 0;
+        auto [p, d] = find_guard(board);
+
+        pair i(0, 0);
+        for (i.first = 0; i.first < board.N; i.first++)
+                for (i.second = 0; i.second < board.M; i.second++)
+                        if (board[i] == '.')
+                                tot += isloop(i, p, d, board);
+
+        cout << tot << endl;
+
+        return 0;
+}