Opened 12th door

1 files changed, 145 insertions, 0 deletions

diff --git a/2020/12/solve.c b/2020/12/solve.c
new file mode 100644
index 0000000..111f689
--- /dev/null
+++ b/2020/12/solve.c
@@ -0,0 +1,145 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+enum dir { N, E, S, W };
+
+// I <3 recursion
+enum dir rotate(enum dir dir, int cnt, int r)
+{
+	if (!cnt)
+		return dir;
+
+	if (r) { // right
+		if (dir == W)
+			dir = N;
+		else
+			dir++;
+	} else { // left
+		if (dir == N)
+			dir = W;
+		else
+			dir--;
+	}
+	return rotate(dir, cnt - 1, r);
+}
+
+// Quite inefficient but I like this approach.
+int *rotate_wp(int dirs[], int cnt, int r)
+{
+	if (!cnt)
+		return dirs;
+
+	if (r) {
+		int tmp = dirs[W];
+		dirs[W] = dirs[S];
+		dirs[S] = dirs[E];
+		dirs[E] = dirs[N];
+		dirs[N] = tmp;
+	} else {
+		int tmp = dirs[N];
+		dirs[N] = dirs[E];
+		dirs[E] = dirs[S];
+		dirs[S] = dirs[W];
+		dirs[W] = tmp;
+	}
+
+	return rotate_wp(dirs, cnt - 1, r);
+}
+
+int part_one(FILE *fp)
+{
+	char *line = NULL;
+	size_t len = 0;
+	int coords[4] = { 0 };
+	enum dir cur = E;
+	while (getline(&line, &len, fp) != -1) {
+		char dir;
+		int cnt;
+		sscanf(line, "%c%d\n", &dir, &cnt);
+		switch (dir) {
+		case 'N':
+			coords[N] += cnt;
+			break;
+		case 'E':
+			coords[E] += cnt;
+			break;
+		case 'S':
+			coords[S] += cnt;
+			break;
+		case 'W':
+			coords[W] += cnt;
+			break;
+		case 'L':
+			cur = rotate(cur, cnt / 90, 0);
+			break;
+		case 'R':
+			cur = rotate(cur, cnt / 90, 1);
+			break;
+		case 'F':
+			coords[cur] += cnt;
+			break;
+		default:
+			break;
+		}
+	}
+
+	return abs((coords[E] - coords[W]) + (coords[S] - coords[N]));
+}
+
+int part_two(FILE *fp)
+{
+	char *line = NULL;
+	size_t len = 0;
+	int wp[4] = { 1, 10, 0, 0 };
+	int coords[4] = { 0 };
+	enum dir cur = E;
+	while (getline(&line, &len, fp) != -1) {
+		char dir;
+		int cnt;
+		sscanf(line, "%c%d\n", &dir, &cnt);
+		switch (dir) {
+		case 'N':
+			wp[N] += cnt;
+			break;
+		case 'E':
+			wp[E] += cnt;
+			break;
+		case 'S':
+			wp[S] += cnt;
+			break;
+		case 'W':
+			wp[W] += cnt;
+			break;
+		case 'L':
+			rotate_wp(wp, cnt / 90, 0);
+			break;
+		case 'R':
+			rotate_wp(wp, cnt / 90, 1);
+			break;
+		case 'F':
+			coords[N] += cnt * wp[N];
+			coords[E] += cnt * wp[E];
+			coords[S] += cnt * wp[S];
+			coords[W] += cnt * wp[W];
+			break;
+		default:
+			break;
+		}
+	}
+
+	return abs((coords[E] - coords[W])) + abs((coords[S] - coords[N]));
+}
+
+int main(int argc, char *argv[])
+{
+	FILE *fp = fopen("input", "r");
+	if (!fp)
+		exit(EXIT_FAILURE);
+
+	printf("%d\n", part_one(fp));
+	rewind(fp);
+	printf("%d\n", part_two(fp));
+
+	fclose(fp);
+	return 0;
+}