1 files changed, 149 insertions, 0 deletions

diff --git a/2024/12/day12a.cpp b/2024/12/day12a.cpp
new file mode 100644
index 0000000..b972fd2
--- /dev/null
+++ b/2024/12/day12a.cpp
@@ -0,0 +1,149 @@
+/*
+This code is a bit ugly, day12b.cpp contains a nicer implementation
+(the perimeter part is of course different).
+*/
+
+#include <iostream>
+#include <algorithm>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+enum class Direction { U, 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 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());
+                region = new int[M * N];
+                cells = new char[M * N];
+                for (int i = 0; i < N; i++) {
+                        for (int j = 0; j < M; j++) {
+                                region[N*i + j] = -1;
+                                cells[N*i + j] = j < (int)lines[i].size() ?
+                                    lines[i][j] : out_of_bound;
+                        }
+                }
+                                
+        }
+
+        ~Board() {
+                delete []region;
+                delete []cells;
+        }
+
+        char operator[](pair<int, int> p) {
+                int c = coord(p);
+                return c == -1 ? out_of_bound : cells[c];
+        }
+
+        int& reg(pair<int, int> p) {
+                return region[coord(p)];
+        }
+
+private:
+        char *cells;
+        int *region;
+
+        int coord(pair<int, int> p) {
+                auto [i, j] = p;
+                return i >= N || i < 0 || j >= M || j < 0 ? -1 : N*i + j;
+        }
+};
+
+void fill(pair<int, int> p, char a, int r, Board &board) {
+        if (board[p] != a || board.reg(p) == r)
+                return;
+        board.reg(p) = r;
+
+        fill(step(p, Direction::U), a, r, board);
+        fill(step(p, Direction::D), a, r, board);
+        fill(step(p, Direction::R), a, r, board);
+        fill(step(p, Direction::L), a, r, board);
+}
+
+int cell_perim(Board &board, pair<int, int> p) {
+        int perim = 0;
+        if (board[step(p, Direction::U)] != board[p]) perim++;
+        if (board[step(p, Direction::D)] != board[p]) perim++;
+        if (board[step(p, Direction::R)] != board[p]) perim++;
+        if (board[step(p, Direction::L)] != board[p]) perim++;
+        return perim;
+}
+
+int measure_region(Board &board, int r) {
+        int area = 0;
+        int perim = 0;
+        pair<int, int> p;
+        for (p.first = 0; p.first < board.N; p.first++) {
+                for (p.second = 0; p.second < board.M; p.second++) {
+                        if (board.reg(p) == r) {
+                                area++;
+                                perim += cell_perim(board, p);
+                        }
+                }
+        }
+
+        return area * perim;
+}
+
+int scan(Board &board, int maxr) {
+        int tot = 0;
+        for (int r = 0; r < maxr; r++)
+                tot += measure_region(board, r);
+
+        return tot;
+}
+
+int main() {
+        string line;
+        vector<string> lines;
+        while (getline(cin, line))
+                lines.push_back(line);
+
+        Board board(lines);
+
+        pair<int, int> p;
+        int r = 0;
+        for (p.first = 0; p.first < board.N; p.first++) {
+                for (p.second = 0; p.second < board.M; p.second++) {
+                        if (board.reg(p) == -1) {
+                                fill(p, board[p], r, board);
+                                r++;
+                        }
+                }
+        }
+
+        auto tot = scan(board, r);
+
+        cout << tot << endl;
+
+        return 0;
+}