Rename en passant square method on Position and implement a getter for the capture square

Position::en_passant_square → en_passant_target_square
Position::en_passant_capture_square

position/src/fen.rs

@@ -95,7 +95,7 @@ impl ToFen for Position {
         write!(
             fen_string,
             " {}",
-            if let Some(en_passant_square) = self.en_passant_square() {
+            if let Some(en_passant_square) = self.en_passant_target_square() {
                 en_passant_square.to_string()
             } else {
                 "-".to_string()

position/src/position/builders/move_builder.rs

@@ -292,7 +292,7 @@ mod tests {
             new_position.piece_on_square(Square::E4),
             Some(piece!(White Pawn on E4))
         );
-        assert_eq!(new_position.en_passant_square(), Some(Square::E3));
+        assert_eq!(new_position.en_passant_target_square(), Some(Square::E3));
 
         Ok(())
     }

position/src/position/builders/position_builder.rs

@@ -52,7 +52,7 @@ impl Builder {
             flags: position.flags(),
             pieces,
             kings: [Some(white_king), Some(black_king)],
-            en_passant_square: position.en_passant_square(),
+            en_passant_square: position.en_passant_target_square(),
             ply_counter: position.ply_counter(),
             move_number: position.move_number(),
         }

position/src/position/position.rs

@@ -9,7 +9,7 @@ use crate::{
     sight::SightExt,
 };
 use chessfriend_bitboard::BitBoard;
-use chessfriend_core::{Color, Piece, PlacedPiece, Shape, Square};
+use chessfriend_core::{Color, Piece, PlacedPiece, Rank, Shape, Square};
 use std::{cell::OnceCell, fmt};
 
 #[derive(Clone, Debug, Eq)]
@@ -182,10 +182,25 @@ impl Position {
         Pieces::new(&self, color)
     }
 
-    pub fn en_passant_square(&self) -> Option<Square> {
+    /// If en passant is available in this position, the square a pawn will move
+    /// to if it captures en passant.
+    pub fn en_passant_target_square(&self) -> Option<Square> {
         self.en_passant_square
     }
 
+    /// If en passant is available in this position, the square on which the
+    /// captured pawn is sitting.
+    pub fn en_passant_capture_square(&self) -> Option<Square> {
+        let target_square = self.en_passant_square?;
+
+        let file = target_square.file();
+        Some(match target_square.rank() {
+            Rank::THREE => Square::from_file_rank(file, Rank::FOUR),
+            Rank::SIX => Square::from_file_rank(file, Rank::SEVEN),
+            _ => unreachable!(),
+        })
+    }
+
     fn _sight_of_player(&self, player: Color, pieces: &PieceBitBoards) -> BitBoard {
         let en_passant_square = self.en_passant_square;