[bitboard] Replace separate methods for leading and trailing iteration

Add chessfriend_bitboard::IterationDirection
Make BitBoard::occupied_squares() take an IterationDirection and return an iterator
corresponding to the direction.
Do the same for ::first_occupied_square().

bitboard/src/bitboard.rs

@@ -1,7 +1,8 @@
 // Eryn Wells <eryn@erynwells.me>
 
+use crate::bit_scanner::{LeadingBitScanner, TrailingBitScanner};
+use crate::direction::IterationDirection;
 use crate::library;
-use crate::{LeadingBitScanner, TrailingBitScanner};
 use chessfriend_core::{Color, Direction, File, Rank, Square};
 use forward_ref::{forward_ref_binop, forward_ref_op_assign, forward_ref_unop};
 use std::fmt;
@@ -223,27 +224,39 @@ impl BitBoard {
         self.0.is_power_of_two()
     }
 
-    /// Return an Iterator over the occupied squares. The Iterator yields
-    /// squares starting from the leading (most-significant bit) end of the
-    /// board.
+    /// Return an Iterator over the occupied squares.
     #[must_use]
-    pub fn occupied_squares(&self) -> impl Iterator<Item = Square> {
-        LeadingBitScanner::new(self.0).map(|idx| unsafe { Square::from_index_unchecked(idx as u8) })
+    pub fn occupied_squares(
+        &self,
+        direction: IterationDirection,
+    ) -> Box<dyn Iterator<Item = Square>> {
+        fn index_to_square(index: usize) -> Square {
+            unsafe { Square::from_index_unchecked(index as u8) }
+        }
+
+        match direction {
+            IterationDirection::Leading => {
+                Box::new(LeadingBitScanner::new(self.0).map(index_to_square))
+            }
+            IterationDirection::Trailing => {
+                Box::new(TrailingBitScanner::new(self.0).map(index_to_square))
+            }
+        }
     }
 
-    /// Return an Iterator over the occupied squares. The Iterator yields
-    /// squares starting from the trailing (least-significant bit) end of the
-    /// board.
-    pub fn occupied_squares_trailing(&self) -> impl Iterator<Item = Square> {
-        TrailingBitScanner::new(self.0)
-            .map(|idx| unsafe { Square::from_index_unchecked(idx as u8) })
+    #[must_use]
+    pub fn first_occupied_square(&self, direction: IterationDirection) -> Option<Square> {
+        match direction {
+            IterationDirection::Leading => self.first_occupied_square_leading(),
+            IterationDirection::Trailing => self.first_occupied_square_trailing(),
+        }
     }
 
     /// If the board is not empty, returns the first occupied square on the
     /// board, starting at the leading (most-significant) end of the board.  If
     /// the board is empty, returns `None`.
     #[must_use]
-    pub fn first_occupied_square(&self) -> Option<Square> {
+    fn first_occupied_square_leading(&self) -> Option<Square> {
         let leading_zeros = self.0.leading_zeros() as u8;
         if leading_zeros < Square::NUM as u8 {
             unsafe {
@@ -260,7 +273,7 @@ impl BitBoard {
     /// board, starting at the trailing (least-significant) end of the board.
     /// If the board is empty, returns `None`.
     #[must_use]
-    pub fn first_occupied_square_trailing(&self) -> Option<Square> {
+    fn first_occupied_square_trailing(&self) -> Option<Square> {
         let trailing_zeros = self.0.trailing_zeros() as u8;
         if trailing_zeros < Square::NUM as u8 {
             unsafe { Some(Square::from_index_unchecked(trailing_zeros)) }
@@ -458,7 +471,10 @@ mod tests {
     fn single_rank_occupancy() {
         let bb = BitBoard(0b01010100);
         let expected_squares = [Square::G1, Square::E1, Square::C1];
-        for (a, b) in bb.occupied_squares().zip(expected_squares.iter().cloned()) {
+        for (a, b) in bb
+            .occupied_squares(IterationDirection::Leading)
+            .zip(expected_squares.iter().cloned())
+        {
             assert_eq!(a, b);
         }
     }
@@ -470,7 +486,10 @@ mod tests {
 
         let expected_squares = [Square::H8, Square::F6, Square::C5, Square::E2, Square::D1];
 
-        for (a, b) in bb.occupied_squares().zip(expected_squares.iter().cloned()) {
+        for (a, b) in bb
+            .occupied_squares(IterationDirection::Leading)
+            .zip(expected_squares.iter().cloned())
+        {
             assert_eq!(a, b);
         }
     }
@@ -514,7 +533,7 @@ mod tests {
     #[test]
     fn first_occupied_squares() {
         let bb = bitboard![A8 E1];
-        assert_eq!(bb.first_occupied_square(), Some(Square::A8));
+        assert_eq!(bb.first_occupied_square_leading(), Some(Square::A8));
         assert_eq!(bb.first_occupied_square_trailing(), Some(Square::E1));
 
         let bb = bitboard![D6 E7 F8];

bitboard/src/direction.rs

@@ -0,0 +1,6 @@
+#[derive(Default)]
+pub enum IterationDirection {
+    #[default]
+    Leading,
+    Trailing,
+}

bitboard/src/lib.rs

@@ -2,12 +2,12 @@
 
 mod bit_scanner;
 mod bitboard;
+mod direction;
 mod library;
 mod shifts;
 
 pub use bitboard::BitBoard;
-
-pub(crate) use bit_scanner::{LeadingBitScanner, TrailingBitScanner};
+pub use direction::IterationDirection;
 
 #[macro_export]
 macro_rules! bitboard {