1 files changed, 159 insertions, 0 deletions

diff --git a/2024/12/day12b.cpp b/2024/12/day12b.cpp
new file mode 100644
index 0000000..4078fa7
--- /dev/null
+++ b/2024/12/day12b.cpp
@@ -0,0 +1,159 @@
+#include <iostream>
+#include <cstdint>
+#include <algorithm>
+#include <string>
+#include <string_view>
+#include <vector>
+#include <set>
+using namespace std;
+
+class Direction {
+public:
+        const int U, R;
+
+        Direction(const int i, const int j) : U{ i }, R{ j } {}
+
+        Direction turnright() const {
+                return turn(-1, 0);
+        }
+
+        Direction turnleft() const {
+                return turn(1, 0);
+        }
+
+        bool operator<(const Direction& d) const { // For set<Direction>
+                return this->U < d.U || (this->U == d.U && this->R < d.R);
+        }
+private:
+        Direction turn(int64_t sin, int64_t cos) const {
+                return Direction(cos * U - sin * R, sin * U + cos * R);
+        }
+};
+
+class Position {
+public:
+        int64_t i, j;
+
+        Position(int64_t a, int64_t b) : i{a}, j{b} {}
+
+        Position step(const Direction d) const {
+                return Position(i+d.U, j+d.R);
+        }
+};
+
+const Direction all_directions[] = {
+        Direction(1, 0), Direction(-1, 0), Direction(0, 1), Direction(0, -1)
+};
+
+class Board {
+public:
+        int64_t N, M;
+        vector<int64_t> region_area;
+
+        Board(const vector<string>& lines) :
+        N{static_cast<int64_t>(lines.size())},
+        M{static_cast<int64_t>(lines[0].size())},
+        region_area(M*N), cells(M*N), region(M*N), visited(M*N)
+        {
+                for (int64_t i = 0; i < N; i++) {
+                        for (int64_t j = 0; j < M; j++) {
+                                region[N*i+j] = -1;
+                                cells[N*i+j] = lines[i][j];
+                        }
+                }
+        }
+
+        char& operator[](const Position p) {
+                if (const auto c = coord(p); c == -1)
+                        return out_of_bound;
+                else
+                        return cells[c];
+        }
+
+        int64_t& reg(const Position p) {
+                if (const auto c = coord(p); c == -1)
+                        return out_of_region;
+                else
+                        return region[c];
+        }
+
+        void fill(const Position p, const int64_t r) {
+                reg(p) = r;
+                region_area[r]++;
+                for (const auto d : all_directions)
+                        if (auto q = p.step(d);
+                            (*this)[q] == (*this)[p] && reg(q) != r)
+                                fill(q, r);
+        }
+
+        int64_t compute_perim(int64_t r) {
+                int count = 0;
+                for (Position p(0, 0); p.i < N; p.i++)
+                        for (p.j = 0; p.j < M; p.j++)
+                                for (Direction d : all_directions)
+                                        if (reg(p) == r && reg(p.step(d)) != r)
+                                                  count += walk(p, d);
+
+                return count;
+        }
+
+private:
+        char out_of_bound = '$';
+        int64_t out_of_region = -1;
+
+        vector<char> cells;
+        vector<int64_t> region;
+        vector<set<Direction>> visited;
+
+        int64_t coord(const Position p) const {
+                auto [i, j] = p;
+                return i >= N || i < 0 || j >= M || j < 0 ? -1 : N * i + j;
+        }
+
+        bool is_visited(const Position p, const Direction d) const {
+                if (auto c = coord(p); c == -1)
+                        return false;
+                else
+                        return visited[c].count(d) > 0;
+        }
+
+        void set_visited(const Position p, const Direction d) {
+                if (auto c = coord(p); c != -1)
+                        visited[c].insert(d);
+        }
+
+        int64_t walk(const Position p, const Direction d) {
+                const auto r = reg(p);
+                for (auto q = p; reg(q) == r && reg(q.step(d)) != r;
+                    q = q.step(d.turnleft())) {
+                        if (is_visited(q, d))
+                                return 0;
+                        set_visited(q, d);
+                }
+
+                return 1;
+        }
+};
+
+int main() {
+        string line;
+        vector<string> lines;
+        while (getline(cin, line))
+                lines.push_back(line);
+
+        Board board(lines);
+
+        int64_t r = 0;
+        for (Position p(0, 0); p.i < board.N; p.i++)
+                for (p.j = 0; p.j < board.M; p.j++)
+                        if (board.reg(p) == -1)
+                                board.fill(p, r++);
+
+        int64_t tot = 0;
+        for (int64_t i = 0; i < r; i++)
+                tot += board.region_area[i] * board.compute_perim(i);
+
+        cout << tot << endl;
+
+        return 0;
+}