Add FEN en-passant target square parsing

src/board/square.rs

@@ -1,5 +1,5 @@
-use super::{Bitboard, File, Rank};
-use crate::utils::static_assert;
+use super::{Bitboard, File, FromFen, Rank};
+use crate::{error::Error, utils::static_assert};
 
 /// Represent a square on a chessboard. Defined in the same order as the
 /// [Bitboard] squares.
@@ -107,6 +107,23 @@ impl Square {
     }
 }
 
+/// Convert an en-passant target square segment of a FEN string to an optional [Square].
+impl FromFen for Option<Square> {
+    type Err = Error;
+
+    fn from_fen(s: &str) -> Result<Self, Self::Err> {
+        let res = match s.as_bytes() {
+            [b'-'] => None,
+            [file @ b'a'..=b'h', rank @ b'1'..=b'8'] => Some(Square::new(
+                File::from_index((file - b'a') as usize),
+                Rank::from_index((rank - b'1') as usize),
+            )),
+            _ => return Err(Error::InvalidFen),
+        };
+        Ok(res)
+    }
+}
+
 /// Shift the square's index left by the amount given.
 impl std::ops::Shl<usize> for Square {
     type Output = Square;