swap Vec2 up/down

going down on the grid increases the int for y

src/day9.ml

@@ -24,7 +24,7 @@ let next_pos head tail =
   then tail
   else (
     let x_dir = if dx > 0 then Vec2.right else Vec2.left in
-    let y_dir = if dy > 0 then Vec2.up else Vec2.down in
+    let y_dir = if dy > 0 then Vec2.down else Vec2.up in
     let dir =
       if abs dx = abs dy
       then Vec2.(x_dir + y_dir)

src/vec2.ml

@@ -7,10 +7,10 @@ type t =
 [@@deriving show]
 
 let origin = { x = 0; y = 0 }
-let up = { x = 0; y = 1 }
+let up = { x = 0; y = -1 }
-let down = { x = 0; y = -1 }
+let down = { x = 0; y = 1 }
-let left = { x = -1; y = 0 }
 let right = { x = 1; y = 0 }
+let left = { x = -1; y = 0 }
 let directions = [ up; down; left; right ]
 let of_tuple (x, y) = { x; y }
 let to_tuple { x; y } = x, y