From cc23ee2d90148d3c32f6fcd84d6091220b0b7cf6 Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Sat, 10 Feb 2024 11:40:27 -0700
Subject: [PATCH] Rename en passant square method on Position and implement a
 getter for the capture square
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Position::en_passant_square → en_passant_target_square
Position::en_passant_capture_square
---
 position/src/fen.rs                           |  2 +-
 .../src/position/builders/move_builder.rs     |  2 +-
 .../src/position/builders/position_builder.rs |  2 +-
 position/src/position/position.rs             | 19 +++++++++++++++++--
 4 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/position/src/fen.rs b/position/src/fen.rs
index d6c3568..9a8369a 100644
--- a/position/src/fen.rs
+++ b/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()
diff --git a/position/src/position/builders/move_builder.rs b/position/src/position/builders/move_builder.rs
index c926c08..2de56d4 100644
--- a/position/src/position/builders/move_builder.rs
+++ b/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(())
     }
diff --git a/position/src/position/builders/position_builder.rs b/position/src/position/builders/position_builder.rs
index 5011f21..cba432f 100644
--- a/position/src/position/builders/position_builder.rs
+++ b/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(),
         }
diff --git a/position/src/position/position.rs b/position/src/position/position.rs
index 01de793..b8c193a 100644
--- a/position/src/position/position.rs
+++ b/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;