1 files changed, 113 insertions, 0 deletions

diff --git a/2024/10/day10a.cpp b/2024/10/day10a.cpp
new file mode 100644
index 0000000..fda4ab8
--- /dev/null
+++ b/2024/10/day10a.cpp
@@ -0,0 +1,113 @@
+#include <iostream>
+#include <algorithm>
+#include <set>
+#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);
+}
+
+class Board {
+public:
+        const int out_of_bound = -1;
+        int M, N;
+
+        Board(vector<string> &lines) {
+                N = lines.size();
+                M = 0;
+                for (string l : lines)
+                        M = max(M, (int)l.size());
+
+                cells = new int[M * N];
+                for (int i = 0; i < N; i++)
+                        for (int j = 0; j < M; j++)
+                                cells[N*i + j] = lines[i][j] - '0';
+        }
+
+        ~Board() {
+                delete []cells;
+        }
+
+        int operator[](pair<int, int> p) {
+                int c = coord(p);
+                return c == -1 ? out_of_bound : cells[c];
+        }
+
+private:
+        int *cells;
+
+        int coord(pair<int, int> p) {
+                auto [i, j] = p;
+                return i >= N || i < 0 || j >= M || j < 0 ? -1 : N*i + j;
+        }
+};
+
+void reachable9(pair<int, int> p, Board& board, set<pair<int, int>>& s) {
+        if (board[p] == 9) {
+                s.insert(p);
+                return;
+        }
+
+        auto q = step(p, Direction::U);
+        if (board[q] == board[p]+1)
+                reachable9(q, board, s);
+
+        q = step(p, Direction::D);
+        if (board[q] == board[p]+1)
+                reachable9(q, board, s);
+
+        q = step(p, Direction::R);
+        if (board[q] == board[p]+1)
+                reachable9(q, board, s);
+
+        q = step(p, Direction::L);
+        if (board[q] == board[p]+1)
+                reachable9(q, board, s);
+}
+
+int main() {
+        string line;
+        vector<string> lines;
+        while (getline(cin, line))
+                lines.push_back(line);
+
+        Board board(lines);
+
+        int tot = 0;
+
+        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] == 0) {
+                                set<pair<int, int>> s;
+                                reachable9(i, board, s);
+                                tot += s.size();
+                        }
+                }
+        }
+
+        cout << tot << endl;
+
+        return 0;
+}