1 files changed, 58 insertions, 0 deletions
diff --git a/2020/day11.c b/2020/day11.c
new file mode 100644
index 0000000..f8552ca
--- /dev/null
+++ b/2020/day11.c
@@ -0,0 +1,58 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+struct Grid {
+	int h, w;
+	char c[128][128];
+};
+static char at(const struct Grid *grid, int y, int x) {
+	if (x < 0 || y < 0 || x >= grid->w || y >= grid->h) return '.';
+	return grid->c[y][x];
+}
+static int adj(const struct Grid *grid, int y, int x) {
+	return (
+		(at(grid, y - 1, x - 1) == '#') +
+		(at(grid, y - 1, x + 0) == '#') +
+		(at(grid, y - 1, x + 1) == '#') +
+		(at(grid, y + 0, x - 1) == '#') +
+		(at(grid, y + 0, x + 1) == '#') +
+		(at(grid, y + 1, x - 1) == '#') +
+		(at(grid, y + 1, x + 0) == '#') +
+		(at(grid, y + 1, x + 1) == '#')
+	);
+}
+static struct Grid gen(const struct Grid *prev) {
+	struct Grid next = { .h = prev->h, .w = prev->w };
+	for (int y = 0; y < next.h; ++y) {
+		for (int x = 0; x < next.w; ++x) {
+			if (prev->c[y][x] == 'L' && !adj(prev, y, x)) {
+				next.c[y][x] = '#';
+			} else if (prev->c[y][x] == '#' && adj(prev, y, x) >= 4) {
+				next.c[y][x] = 'L';
+			} else {
+				next.c[y][x] = prev->c[y][x];
+			}
+		}
+	}
+	return next;
+}
+int main(void) {
+	struct Grid init = {0};
+	while (EOF != scanf("%s\n", init.c[init.h])) {
+		init.h++;
+	}
+	init.w = strlen(init.c[0]);
+	struct Grid prev = init;
+	for (;;) {
+		struct Grid next = gen(&prev);
+		if (!memcmp(&next, &prev, sizeof(next))) break;
+		prev = next;
+	}
+	int occ = 0;
+	for (int y = 0; y < prev.h; ++y) {
+		for (int x = 0; x < prev.w; ++x) {
+			occ += prev.c[y][x] == '#';
+		}
+	}
+	printf("%d\n", occ);
+}