[board] Implement a BitBoard::rank constructor

Also implement BitBoard::shift_north() to support rank().

board/src/bitboard/bitboard.rs

@@ -17,6 +17,10 @@ impl BitBoard {
         BitBoard(bits)
     }
 
+    pub fn rank(rank: u8) -> BitBoard {
+        BitBoard(0xFF).shift_north(rank)
+    }
+
     pub fn is_empty(&self) -> bool {
         self.0 == 0
     }
@@ -38,6 +42,11 @@ impl BitBoard {
     const NOT_A_FILE: u64 = 0xfefefefefefefefe;
     const NOT_H_FILE: u64 = 0x7f7f7f7f7f7f7f7f;
 
+    #[inline]
+    pub fn shift_north(&self, n: u8) -> BitBoard {
+        BitBoard(self.0 << (8 * n))
+    }
+
     #[inline]
     pub fn shift_north_one(&self) -> BitBoard {
         BitBoard(self.0 << 8)
@@ -139,6 +148,18 @@ mod tests {
     use super::*;
     use crate::Square;
 
+    #[test]
+    fn rank() {
+        assert_eq!(BitBoard::rank(0).0, 0xFF, "Rank 1");
+        assert_eq!(BitBoard::rank(1).0, 0xFF00, "Rank 2");
+        assert_eq!(BitBoard::rank(2).0, 0xFF0000, "Rank 3");
+        assert_eq!(BitBoard::rank(3).0, 0xFF000000, "Rank 4");
+        assert_eq!(BitBoard::rank(4).0, 0xFF00000000, "Rank 5");
+        assert_eq!(BitBoard::rank(5).0, 0xFF0000000000, "Rank 6");
+        assert_eq!(BitBoard::rank(6).0, 0xFF000000000000, "Rank 7");
+        assert_eq!(BitBoard::rank(7).0, 0xFF00000000000000, "Rank 8");
+    }
+
     #[test]
     fn is_empty() {
         assert!(BitBoard(0).is_empty());
@@ -210,6 +231,14 @@ mod tests {
         );
     }
 
+    #[test]
+    fn shift_n() {
+        assert_eq!(
+            BitBoard(0x0008_0000_0000).shift_north(2),
+            BitBoard(0x0008_0000_0000_0000)
+        );
+    }
+
     #[test]
     fn pieces() {
         let bb = BitBoard(0x1001_1010); // e4