[board] Implement directional shifts by 1 on BitBoard

Add some positive tests for these shifts. Negative tests, and boundary tests are still to come.

Implement at custom fmt::Debug for BitBoard so it prints binary.

board/src/bitboard.rs

@@ -1,9 +1,10 @@
 // Eryn Wells <eryn@erynwells.me>
 
 use crate::square::Square;
+use std::fmt;
 use std::ops::{BitAnd, BitOr, Not};
 
-#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct BitBoard(u64);
 
 impl BitBoard {
@@ -46,6 +47,58 @@ impl BitBoard {
     }
 }
 
+impl BitBoard {
+    const NOT_A_FILE: u64 = 0xfefefefefefefefe;
+    const NOT_H_FILE: u64 = 0x7f7f7f7f7f7f7f7f;
+
+    #[inline]
+    pub fn shift_north_one(&self) -> BitBoard {
+        BitBoard(self.0 << 8)
+    }
+
+    #[inline]
+    pub fn shift_north_east_one(&self) -> BitBoard {
+        BitBoard(self.0 << 9 & BitBoard::NOT_A_FILE)
+    }
+
+    #[inline]
+    pub fn shift_east_one(&self) -> BitBoard {
+        BitBoard(self.0 << 1 & BitBoard::NOT_A_FILE)
+    }
+
+    #[inline]
+    pub fn shift_south_east_one(&self) -> BitBoard {
+        BitBoard(self.0 >> 7 & BitBoard::NOT_A_FILE)
+    }
+
+    #[inline]
+    pub fn shift_south_one(&self) -> BitBoard {
+        BitBoard(self.0 >> 8)
+    }
+
+    #[inline]
+    pub fn shift_south_west_one(&self) -> BitBoard {
+        BitBoard(self.0 >> 9 & BitBoard::NOT_H_FILE)
+    }
+
+    #[inline]
+    pub fn shift_west_one(&self) -> BitBoard {
+        BitBoard(self.0 >> 1 & BitBoard::NOT_H_FILE)
+    }
+
+    #[inline]
+    pub fn shift_north_west_one(&self) -> BitBoard {
+        BitBoard(self.0 << 7 & BitBoard::NOT_H_FILE)
+    }
+}
+
+impl fmt::Debug for BitBoard {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+        let bits = self.0;
+        write!(f, "BitBoard({bits:064b})")
+    }
+}
+
 impl BitAnd for BitBoard {
     type Output = BitBoard;
 
@@ -139,6 +192,43 @@ mod tests {
         assert!(!bb.has_piece_at(&sq));
     }
 
+    #[test]
+    fn cardinal_direction_shifts() {
+        let bb = BitBoard(0x1000_0000); // e4
+
+        assert_eq!(bb.shift_north_one(), BitBoard(0x0010_0000_0000), "North");
+        assert_eq!(bb.shift_east_one(), BitBoard(0x2000_0000), "East");
+        assert_eq!(bb.shift_south_one(), BitBoard(0x0010_0000), "South");
+        assert_eq!(bb.shift_west_one(), BitBoard(0x0800_0000), "West");
+    }
+
+    #[test]
+    fn intercardinal_direction_shifts() {
+        let bb = BitBoard(0x1000_0000); // e4
+        println!("    bb: {:?}", bb);
+
+        assert_eq!(
+            bb.shift_north_east_one(),
+            BitBoard(0x0020_0000_0000),
+            "North East"
+        );
+        assert_eq!(
+            bb.shift_south_east_one(),
+            BitBoard(0x0020_0000),
+            "South East"
+        );
+        assert_eq!(
+            bb.shift_south_west_one(),
+            BitBoard(0x0008_0000),
+            "South West"
+        );
+        assert_eq!(
+            bb.shift_north_west_one(),
+            BitBoard(0x0008_0000_0000),
+            "North West"
+        );
+    }
+
     #[cfg(target_arch = "arm")]
     #[test]
     fn arm_count_leading_zeros() {