✨ add Coord#directionTo and -(Direction)

src/main/scala/org/lemon/advent/lib/2d/Coord.scala

@@ -67,4 +67,13 @@ case class Coord(x: Int, y: Int):
   def +(rhs: Coord): Coord = (x + rhs.x, y + rhs.y)
   def +(direction: Direction): Coord = move(direction)
   def -(rhs: Coord): Coord = (x - rhs.x, y - rhs.y)
+  def -(direction: Direction): Coord = move(direction.turnAround)
   def *(n: Int): Coord = (x * n, y * n)
+
+  def directionTo(rhs: Coord): Option[Direction] =
+    val Coord(dx, dy) = rhs - this
+    if dx == 0 && dy < 0 then Some(Direction.Up)
+    else if dx == 0 && dy > 0 then Some(Direction.Down)
+    else if dx < 0 && dy == 0 then Some(Direction.Left)
+    else if dx > 0 && dy == 0 then Some(Direction.Right)
+    else None

src/test/scala/org/lemon/advent/lib/2d/CoordTest.scala

@@ -88,6 +88,34 @@ class CoordTest extends UnitTest:
     check((coord: Coord, n: Int) => coord.shift(Direction.Right, n) == coord.shiftRight(n))
   }
 
+  test("left is left direction") {
+    check((coord: Coord) => coord.directionTo(coord.left) == Some(Direction.Left))
+  }
+
+  test("right is right direction") {
+    check((coord: Coord) => coord.directionTo(coord.right) == Some(Direction.Right))
+  }
+
+  test("up is up direction") {
+    check((coord: Coord) => coord.directionTo(coord.up) == Some(Direction.Up))
+  }
+
+  test("down is down direction") {
+    check((coord: Coord) => coord.directionTo(coord.down) == Some(Direction.Down))
+  }
+
+  test("direction to self is None") {
+    check((coord: Coord) => coord.directionTo(coord) == None)
+  }
+
+  test("direction off line is None") {
+    check((coord: Coord, rhs: Coord) =>
+      coord.x != rhs.x && coord.y != rhs.y ==> {
+        coord.directionTo(rhs) == None
+      }
+    )
+  }
+
   test("manhattan distance of adjacent is 1") {
     check((coord: Coord) => coord.adjacent.forall(_.manhattan(coord) == 1))
   }