Make the is_contiguous check more robust and readable

board-shared/src/board.rs

@@ -29,31 +29,29 @@ impl Board {
         diff
     }
 
+    /// Determines whether the given positions are contiguous.
+    ///
+    /// Contiguous means that the positions are adjacent in a straight line, either
+    /// horizontally or vertically.
     pub fn is_contiguous(positions: &[Position2d]) -> Option<bool> {
         let mut it = positions.iter();
         let first = *it.next()?;
         let mut second = *it.next()?;
-        if first.x == second.x {
-            // Vertical
-            for &pos in it {
-                if pos.x != first.x || pos.y != second.y + 1 {
-                    return Some(false);
-                }
-                second = pos;
-            }
-            Some(true)
-        } else if first.y == second.y {
-            // Horizontal
-            for &pos in it {
-                if pos.y != first.y || pos.y != second.y + 1 {
-                    return Some(false);
-                }
-                second = pos;
+
+        let orientation = match (second.x.checked_sub(first.x), second.y.checked_sub(first.y)) {
+            (Some(0), Some(1)) => (0, 1),
+            (Some(1), Some(0)) => (1, 0),
+            (_, _) => return Some(false),
+        };
+
+        for &pos in it {
+            if pos.x != second.x + orientation.0 || pos.y != second.y + orientation.1 {
+                return Some(false);
             }
-            Some(true)
-        } else {
-            Some(false)
+            second = pos;
         }
+
+        Some(true)
     }
 }
 
@@ -80,10 +78,22 @@ mod tests {
     fn test_is_contiguous() {
         assert_eq!(Board::is_contiguous(&[]), None);
         assert_eq!(Board::is_contiguous(&positions(&[(0, 0)])), None);
+        assert_eq!(
+            Board::is_contiguous(&positions(&[(0, 0), (0, 2)])),
+            Some(false)
+        );
+        assert_eq!(
+            Board::is_contiguous(&positions(&[(0, 0), (2, 0)])),
+            Some(false)
+        );
         assert_eq!(
             Board::is_contiguous(&positions(&[(0, 0), (0, 1), (0, 2)])),
             Some(true)
         );
+        assert_eq!(
+            Board::is_contiguous(&positions(&[(1, 0), (2, 0), (3, 0), (4, 0)])),
+            Some(true)
+        );
         assert_eq!(
             Board::is_contiguous(&positions(&[(0, 0), (0, 1), (1, 3)])),
             Some(false)