[board] Add a "count leading zeros" implementation for ARM machines

Call the `clz` instruction on the bitboard via `asm!`.

board/src/bitboard.rs

@@ -1,6 +1,7 @@
 // Eryn Wells <eryn@erynwells.me>
 
 use crate::square::Square;
+use std::arch::asm;
 
 #[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct BitBoard(pub u64);
@@ -17,6 +18,20 @@ impl BitBoard {
     fn place_piece_at(&mut self, sq: &Square) {
         self.0 |= 1 << sq.index
     }
+
+    #[cfg(target_arch = "arm")]
+    fn _arm_count_leading_zeros(&self) -> u8 {
+        let mut number_of_leading_zeros: u8 = 0;
+        unsafe {
+            asm!(
+                "clz {count}, {bitfield}",
+                count = out(reg) number_of_leading_zeros,
+                bitfield = in(reg) self.0,
+            );
+        }
+
+        number_of_leading_zeros
+    }
 }
 
 #[cfg(test)]
@@ -47,4 +62,13 @@ mod tests {
         bb.place_piece_at(&Square::from_index(34).expect("Invalid square index"));
         assert!(bb.has_piece_at(&Square::from_index(34).expect("Invalid square index")));
     }
+
+    #[cfg(target_arch = "arm")]
+    #[test]
+    fn arm_count_leading_zeros() {
+        assert_eq!(BitBoard(0)._arm_count_leading_zeros(), 0);
+        assert_eq!(BitBoard(0b10)._arm_count_leading_zeros(), 62);
+        assert_eq!(BitBoard(0b1010)._arm_count_leading_zeros(), 60);
+        assert_eq!(BitBoard(0x80000000)._arm_count_leading_zeros(), 0);
+    }
 }