1 files changed, 68 insertions, 0 deletions
diff --git a/day09.zig b/day09.zig
new file mode 100644
index 0000000..6f44ab0
--- /dev/null
+++ b/day09.zig
@@ -0,0 +1,68 @@
+const std = @import("std");
+const data = @embedFile("day09");
+
+var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
+const gpa = general_purpose_allocator.allocator();
+
+const Pair = struct { x: i32, y: i32 };
+
+fn signum(n: i32) i32 {
+    if (n > 0) return 1;
+    if (n < 0) return -1;
+    return 0;
+}
+
+fn move_after(head: Pair, tail: Pair) Pair {
+    if (std.math.absInt(head.x - tail.x) catch unreachable <= 1 and
+        std.math.absInt(head.y - tail.y) catch unreachable <= 1)
+        return tail;
+    return .{ .x = tail.x + signum(head.x - tail.x),
+              .y = tail.y + signum(head.y - tail.y) };
+}
+
+fn solve(comptime ropelen: usize) !usize {
+    var lines = std.mem.tokenize(u8, data, "\n");
+
+    var visited = std.AutoHashMap(Pair, void).init(gpa);
+    defer visited.deinit();
+
+    var rope = [_]Pair{.{.x = 0, .y = 0}} ** ropelen;
+
+    try visited.put(rope[ropelen-1], {});
+
+    while (lines.next()) |line| {
+        if (line.len == 0)
+            break;
+
+        var parts = std.mem.tokenize(u8, line, " ");
+        var dir = parts.next().?[0];
+        var amt = try std.fmt.parseInt(usize, parts.next().?, 10);
+
+        var n: usize = 0;
+        while (n < amt) : (n += 1) {
+            switch (dir) {
+                'L' => rope[0].x -= 1,
+                'R' => rope[0].x += 1,
+                'U' => rope[0].y -= 1,
+                'D' => rope[0].y += 1,
+                else => unreachable,
+            }
+
+            var i: usize = 1;
+            while (i < ropelen) : (i += 1) {
+                rope[i] = move_after(rope[i-1], rope[i]);
+            }
+
+            try visited.put(rope[ropelen-1], {});
+        }
+    }
+
+    return visited.count();
+}
+
+pub fn main() !void {
+    defer std.debug.assert(!general_purpose_allocator.deinit());
+
+    std.debug.print("{}\n", .{try solve(2)});
+    std.debug.print("{}\n", .{try solve(10)});
+}