1 files changed, 90 insertions, 0 deletions
diff --git a/day21.cc b/day21.cc
new file mode 100644
index 0000000..b6bf177
--- /dev/null
+++ b/day21.cc
@@ -0,0 +1,90 @@
+#include <algorithm>
+#include <iostream>
+#include <map>
+#include <vector>
+
+using namespace std;
+
+using pic = vector<vector<char>>;
+
+pic parse(string s) {
+	int n = s.find("/");
+	pic p(n, vector<char>(n, '?'));
+	int y = 0, x = 0;
+	for (char c : s)
+		if (c == '/') {
+			y++;
+			x = 0;
+		} else {
+			p[y][x++] = c;
+		}
+	return p;
+}
+
+pic rotate(pic p) {
+	pic np = p;
+	const int s = size(p);
+	for (int y = 0; y < s; y++)
+		for (int x = 0; x < s; x++)
+			np[x][s-1-y] = p[y][x];
+	return np;
+}
+
+int
+main() {
+	string line;
+
+	vector<pair<pic, pic>> book;
+
+	while (getline(cin, line)) {
+		int i = line.find(" => ");
+		pic k = parse(line.substr(0, i));
+		pic v = parse(line.substr(i + 4));
+
+		for (int i = 0; i < 4; i++, k = rotate(k)) {
+			book.emplace_back(k, v);
+			reverse(begin(k), end(k));
+			book.emplace_back(k, v);
+			reverse(begin(k), end(k));
+		}
+	}
+
+	pic p{{'.','#','.'}, {'.','.','#'}, {'#','#','#'}};
+
+	for (int n = 1; n <= 18; n++) {
+		int s = 2 + size(p) % 2;
+
+		pic np{(size(p)/s)*(s+1), vector<char>((size(p)/s)*(s+1), '?')};
+
+		for (int y = 0; y < size(p); y += s) {
+		for (int x = 0; x < size(p); x += s) {
+			for (auto &[k, v] : book) {
+				if (size(k) != s)
+					goto next;
+
+				for (int iy = 0; iy < s; iy++)
+				for (int ix = 0; ix < s; ix++)
+					if (p[y+iy][x+ix] != k[iy][ix])
+						goto next;
+
+				for (int iy = 0; iy < size(v); iy++)
+				for (int ix = 0; ix < size(v); ix++)
+					np[(y/s)*(s+1)+iy][(x/s)*(s+1)+ix] = v[iy][ix];
+				goto done;
+			next:;
+			}
+		done:;
+		}
+		}
+
+		if (n == 5 || n == 18) {
+			int cnt = 0;
+			for (auto row : np)
+				cnt += count(begin(row), end(row), '#');
+			cout << cnt << endl;
+		}
+
+		p.swap(np);
+	}
+
+}