[board] Remove Flags struct, replace it with Castle and supporting structs

Encapsulate castling rights within a small module. Castle provides the API to the
rest of the board package. Rights encodes the castling rights for each player.
Parameters defines castling parameters for each player.

board/src/castle/rights.rs

@@ -0,0 +1,92 @@
+use super::Castle;
+use chessfriend_core::Color;
+use std::fmt;
+
+#[derive(Clone, Copy, Eq, Hash, PartialEq)]
+pub struct Rights(u8);
+
+impl Rights {
+    /// Returns `true` if the player has the right to castle on the given side
+    /// of the board.
+    ///
+    /// A player retains the right to castle on a particular side of the board
+    /// as long as they have not moved their king, or the rook on that side of
+    /// the board.
+    pub fn player_has_right_to_castle(self, color: Color, castle: Castle) -> bool {
+        (self.0 & (1 << Self::_player_has_right_to_castle_flag_offset(color, castle))) != 0
+    }
+
+    pub fn set_player_has_right_to_castle_flag(&mut self, color: Color, castle: Castle) {
+        self.0 |= 1 << Self::_player_has_right_to_castle_flag_offset(color, castle);
+    }
+
+    pub fn clear_player_has_right_to_castle_flag(&mut self, color: Color, castle: Castle) {
+        self.0 &= !(1 << Self::_player_has_right_to_castle_flag_offset(color, castle));
+    }
+
+    pub fn clear_all(&mut self) {
+        self.0 &= 0b1111_1100;
+    }
+
+    fn _player_has_right_to_castle_flag_offset(color: Color, castle: Castle) -> usize {
+        ((color as usize) << 1) & castle as usize
+    }
+}
+
+impl fmt::Debug for Rights {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Flags({:08b})", self.0)
+    }
+}
+
+impl Default for Rights {
+    fn default() -> Self {
+        Self(0b0000_1111)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn castling_rights() {
+        assert_eq!(
+            Rights::_player_has_right_to_castle_flag_offset(Color::White, Castle::KingSide),
+            0
+        );
+        assert_eq!(
+            Rights::_player_has_right_to_castle_flag_offset(Color::White, Castle::QueenSide),
+            1
+        );
+        assert_eq!(
+            Rights::_player_has_right_to_castle_flag_offset(Color::Black, Castle::KingSide),
+            2
+        );
+        assert_eq!(
+            Rights::_player_has_right_to_castle_flag_offset(Color::Black, Castle::QueenSide),
+            3
+        );
+    }
+
+    #[test]
+    fn default_rights() {
+        let mut rights = Rights::default();
+        assert!(rights.player_has_right_to_castle(Color::White, Castle::KingSide));
+        assert!(rights.player_has_right_to_castle(Color::White, Castle::QueenSide));
+        assert!(rights.player_has_right_to_castle(Color::Black, Castle::KingSide));
+        assert!(rights.player_has_right_to_castle(Color::Black, Castle::QueenSide));
+
+        rights.clear_player_has_right_to_castle_flag(Color::White, Castle::QueenSide);
+        assert!(rights.player_has_right_to_castle(Color::White, Castle::KingSide));
+        assert!(!rights.player_has_right_to_castle(Color::White, Castle::QueenSide));
+        assert!(rights.player_has_right_to_castle(Color::Black, Castle::KingSide));
+        assert!(rights.player_has_right_to_castle(Color::Black, Castle::QueenSide));
+
+        rights.set_player_has_right_to_castle_flag(Color::White, Castle::QueenSide);
+        assert!(rights.player_has_right_to_castle(Color::White, Castle::KingSide));
+        assert!(rights.player_has_right_to_castle(Color::White, Castle::QueenSide));
+        assert!(rights.player_has_right_to_castle(Color::Black, Castle::KingSide));
+        assert!(rights.player_has_right_to_castle(Color::Black, Castle::QueenSide));
+    }
+}