[core] Add an Option<i8> argument to Square::neighbor

This argument allows computing the neighbor n squares away along the direction.
Technically, squares n>1 are not neighbors… but this is a convenient calculation.

core/src/coordinates.rs

@@ -308,9 +308,12 @@ impl Square {
     }
 
     #[must_use]
-    pub fn neighbor(self, direction: Direction) -> Option<Square> {
+    pub fn neighbor(self, direction: Direction, n: Option<i8>) -> Option<Square> {
         let index: u8 = self as u8;
-        let dir: i8 = direction.to_offset();
+
+        let n = n.unwrap_or(1);
+        let dir: i8 = direction.to_offset() * n;
+
         match direction {
             Direction::North | Direction::NorthEast => {
                 Square::try_from(index.wrapping_add_signed(dir)).ok()
@@ -558,37 +561,37 @@ mod tests {
     fn valid_neighbors() {
         let sq = Square::E4;
 
-        assert_eq!(sq.neighbor(Direction::North), Some(Square::E5));
-        assert_eq!(sq.neighbor(Direction::NorthEast), Some(Square::F5));
-        assert_eq!(sq.neighbor(Direction::East), Some(Square::F4));
-        assert_eq!(sq.neighbor(Direction::SouthEast), Some(Square::F3));
-        assert_eq!(sq.neighbor(Direction::South), Some(Square::E3));
-        assert_eq!(sq.neighbor(Direction::SouthWest), Some(Square::D3));
-        assert_eq!(sq.neighbor(Direction::West), Some(Square::D4));
-        assert_eq!(sq.neighbor(Direction::NorthWest), Some(Square::D5));
+        assert_eq!(sq.neighbor(Direction::North, None), Some(Square::E5));
+        assert_eq!(sq.neighbor(Direction::NorthEast, None), Some(Square::F5));
+        assert_eq!(sq.neighbor(Direction::East, None), Some(Square::F4));
+        assert_eq!(sq.neighbor(Direction::SouthEast, None), Some(Square::F3));
+        assert_eq!(sq.neighbor(Direction::South, None), Some(Square::E3));
+        assert_eq!(sq.neighbor(Direction::SouthWest, None), Some(Square::D3));
+        assert_eq!(sq.neighbor(Direction::West, None), Some(Square::D4));
+        assert_eq!(sq.neighbor(Direction::NorthWest, None), Some(Square::D5));
     }
 
     #[test]
     fn invalid_neighbors() {
         let sq = Square::A1;
-        assert!(sq.neighbor(Direction::West).is_none());
-        assert!(sq.neighbor(Direction::SouthWest).is_none());
-        assert!(sq.neighbor(Direction::South).is_none());
+        assert!(sq.neighbor(Direction::West, None).is_none());
+        assert!(sq.neighbor(Direction::SouthWest, None).is_none());
+        assert!(sq.neighbor(Direction::South, None).is_none());
 
         let sq = Square::H1;
-        assert!(sq.neighbor(Direction::East).is_none());
-        assert!(sq.neighbor(Direction::SouthEast).is_none());
-        assert!(sq.neighbor(Direction::South).is_none());
+        assert!(sq.neighbor(Direction::East, None).is_none());
+        assert!(sq.neighbor(Direction::SouthEast, None).is_none());
+        assert!(sq.neighbor(Direction::South, None).is_none());
 
         let sq = Square::A8;
-        assert!(sq.neighbor(Direction::North).is_none());
-        assert!(sq.neighbor(Direction::NorthWest).is_none());
-        assert!(sq.neighbor(Direction::West).is_none());
+        assert!(sq.neighbor(Direction::North, None).is_none());
+        assert!(sq.neighbor(Direction::NorthWest, None).is_none());
+        assert!(sq.neighbor(Direction::West, None).is_none());
 
         let sq = Square::H8;
-        assert!(sq.neighbor(Direction::North).is_none());
-        assert!(sq.neighbor(Direction::NorthEast).is_none());
-        assert!(sq.neighbor(Direction::East).is_none());
+        assert!(sq.neighbor(Direction::North, None).is_none());
+        assert!(sq.neighbor(Direction::NorthEast, None).is_none());
+        assert!(sq.neighbor(Direction::East, None).is_none());
     }
 
     #[test]