Add 'TryInto<Square>' for 'Bitboard'

src/board/bitboard/error.rs

@@ -0,0 +1,19 @@
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum IntoSquareError {
+    /// The board is empty.
+    EmptyBoard,
+    /// The board contains more than one square.
+    TooManySquares,
+}
+
+impl std::fmt::Display for IntoSquareError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let error_msg = match self {
+            Self::EmptyBoard => "The board is empty",
+            Self::TooManySquares => "The board contains more than one square",
+        };
+        write!(f, "{}", error_msg)
+    }
+}
+
+impl std::error::Error for IntoSquareError {}

src/board/bitboard/mod.rs

@@ -1,6 +1,8 @@
 use super::Square;
 use crate::utils::static_assert;
 
+mod error;
+use error::*;
 mod iterator;
 use iterator::*;
 mod superset;
@@ -102,6 +104,21 @@ impl IntoIterator for Bitboard {
     }
 }
 
+/// If the given [Bitboard] is a singleton piece on a board, return the [Square] that it is
+/// occupying. Otherwise return `None`.
+impl TryInto<Square> for Bitboard {
+    type Error = IntoSquareError;
+
+    fn try_into(self) -> Result<Square, Self::Error> {
+        let index = match self.count() {
+            1 => self.0.trailing_zeros() as usize,
+            0 => return Err(IntoSquareError::EmptyBoard),
+            _ => return Err(IntoSquareError::TooManySquares),
+        };
+        Ok(Square::from_index(index))
+    }
+}
+
 /// Treat bitboard as a set of squares, shift each square's index left by the amount given.
 impl std::ops::Shl<usize> for Bitboard {
     type Output = Bitboard;
@@ -461,4 +478,23 @@ mod test {
             1 << 8
         );
     }
+
+    #[test]
+    fn into_square() {
+        for square in Square::iter() {
+            assert_eq!(square.into_bitboard().try_into(), Ok(square));
+        }
+    }
+
+    #[test]
+    fn into_square_invalid() {
+        assert_eq!(
+            TryInto::<Square>::try_into(Bitboard::EMPTY),
+            Err(IntoSquareError::EmptyBoard)
+        );
+        assert_eq!(
+            TryInto::<Square>::try_into(Square::A1 | Square::A2),
+            Err(IntoSquareError::TooManySquares)
+        )
+    }
 }