1 files changed, 79 insertions, 0 deletions
diff --git a/2020/12/puzzle-2.c b/2020/12/puzzle-2.c
new file mode 100644
index 0000000..8ac34b4
--- /dev/null
+++ b/2020/12/puzzle-2.c
@@ -0,0 +1,79 @@
+#include <err.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct coords {
+	int north;
+	int east;
+};
+
+int
+main(void)
+{
+	FILE *fp;
+	char cl[6];
+	struct coords waypoint = {1, 10};
+	struct coords ship = {0, 0};
+
+	if (!(fp = fopen("input", "r")))
+		err(EXIT_FAILURE, "fopen");
+
+	while (fgets(cl, sizeof(cl), fp) != NULL) {
+		switch (cl[0]) {
+		case 'N':
+			waypoint.north += atoi(&cl[1]);
+			break;
+		case 'E':
+			waypoint.east += atoi(&cl[1]);
+			break;
+		case 'S':
+			waypoint.north -= atoi(&cl[1]);
+			break;
+		case 'W':
+			waypoint.east -= atoi(&cl[1]);
+			break;
+		case 'F':;
+			unsigned int const val = atoi(&cl[1]);
+			ship.north += val * waypoint.north;
+			ship.east += val * waypoint.east;
+			break;
+		/* Left and right */
+		default:;
+			int direction = atoi(&cl[1]);
+
+			/* (x, y) -> (-x, -y) */
+			if (direction == 180) {
+				waypoint.north = -waypoint.north;
+				waypoint.east = -waypoint.east;
+				break;
+			}
+
+			/* 0 <= direction < 360 */
+			if (cl[0] == 'R')
+				direction = -direction + 360;
+
+			int const temp = waypoint.east;
+			if (direction == 90) {
+				/* (x, y) -> (-y, x) */
+				waypoint.east = -waypoint.north;
+				waypoint.north = temp;
+			} else {
+				/* (x, y) -> (y, -x) */
+				waypoint.east = waypoint.north;
+				waypoint.north = -temp;
+			}
+			break;
+		}
+	}
+	fclose(fp);
+
+	/* Absolute value */
+	if (ship.north < 0)
+		ship.north = -ship.north;
+	if (ship.east < 0)
+		ship.east = -ship.east;
+
+	printf("%d\n", ship.north + ship.east);
+	return EXIT_SUCCESS;
+}