[board] Implement iter() on board

This iterator yields (Square, Piece) tuples for all pieces on the board.

board/src/board.rs

@@ -90,6 +90,10 @@ impl Board {
 }
 
 impl Board {
+    pub fn iter(&self) -> impl Iterator<Item = (Square, Piece)> {
+        self.pieces.iter()
+    }
+
     /// A [`BitBoard`] of squares occupied by pieces of all colors.
     pub fn occupancy(&self) -> BitBoard {
         self.pieces.occpuancy()

board/src/piece_sets.rs

@@ -60,6 +60,10 @@ impl PieceSet {
         }
     }
 
+    pub fn iter(&self) -> impl Iterator<Item = (Square, Piece)> {
+        self.mailbox.iter()
+    }
+
     /// A [`BitBoard`] representing all the pieces on the board.
     pub fn occpuancy(&self) -> BitBoard {
         self.color_occupancy

board/src/piece_sets/mailbox.rs

@@ -22,6 +22,13 @@ impl Mailbox {
     pub fn remove(&mut self, square: Square) {
         self.0[square as usize] = None;
     }
+
+    pub fn iter(&self) -> impl Iterator<Item = (Square, Piece)> {
+        Square::ALL
+            .into_iter()
+            .zip(self.0)
+            .filter_map(|(square, piece)| piece.map(|piece| (square, piece)))
+    }
 }
 
 impl Default for Mailbox {
@@ -45,3 +52,30 @@ impl FromIterator<(Square, Piece)> for Mailbox {
             })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use chessfriend_core::piece;
+    use std::collections::HashSet;
+
+    #[test]
+    fn iter_returns_all_pieces() {
+        let mut mailbox = Mailbox::new();
+        mailbox.set(piece!(White Queen), Square::C3);
+        mailbox.set(piece!(White Rook), Square::H8);
+        mailbox.set(piece!(Black Bishop), Square::E4);
+        mailbox.set(piece!(Black Pawn), Square::F6);
+
+        let pieces = mailbox.iter().collect::<HashSet<_>>();
+        assert_eq!(
+            pieces,
+            HashSet::from([
+                (Square::C3, piece!(White Queen)),
+                (Square::H8, piece!(White Rook)),
+                (Square::E4, piece!(Black Bishop)),
+                (Square::F6, piece!(Black Pawn)),
+            ])
+        );
+    }
+}