[board] Implement BitBoard::file and fmt::Display for BitBoard

BitBoard::file returns a BitBoard representing the 0-indexed file.
fmt::Display prints a grid of bits in the standard orientation (white on bottom, left to right)
Add asserts to the rank and file constructors to catch out of bounds arguments.

board/src/bitboard/bitboard.rs

@@ -18,9 +18,15 @@ impl BitBoard {
     }
 
     pub fn rank(rank: u8) -> BitBoard {
+        assert!(rank < 8);
         BitBoard(0xFF).shift_north(rank)
     }
 
+    pub fn file(file: u8) -> BitBoard {
+        assert!(file < 8);
+        BitBoard(0x0101010101010101).shift_east(file)
+    }
+
     pub fn is_empty(&self) -> bool {
         self.0 == 0
     }
@@ -68,6 +74,29 @@ impl fmt::UpperHex for BitBoard {
     }
 }
 
+impl fmt::Display for BitBoard {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let binary_ranks = format!("{:064b}", self.0)
+            .chars()
+            .rev()
+            .map(|c| String::from(c))
+            .collect::<Vec<String>>();
+
+        let mut ranks_written = 0;
+        for rank in binary_ranks.chunks(8).rev() {
+            let joined_rank = rank.join(" ");
+            write!(f, "{}", joined_rank)?;
+
+            ranks_written += 1;
+            if ranks_written < 8 {
+                write!(f, "\n")?;
+            }
+        }
+
+        Ok(())
+    }
+}
+
 impl fmt::Debug for BitBoard {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
         f.debug_tuple("BitBoard")
@@ -136,6 +165,13 @@ mod tests {
     use super::*;
     use crate::Square;
 
+    #[test]
+    fn display_and_debug() {
+        let bb = BitBoard::file(0) | BitBoard::file(3) | BitBoard::rank(7) | BitBoard::rank(4);
+        println!("{}", &bb);
+        println!("{:?}", &bb);
+    }
+
     #[test]
     fn rank() {
         assert_eq!(BitBoard::rank(0).0, 0xFF, "Rank 1");