From 4efb48244f1334658e278b2d2e287d79c26ce223 Mon Sep 17 00:00:00 2001
From: dogeystamp <dogeystamp@disroot.org>
Date: Sun, 6 Oct 2024 21:28:11 -0400
Subject: [PATCH] feat: make en-passant moves

---
 src/lib.rs     | 20 ++++++++++++++++----
 src/movegen.rs | 27 ++++++++++++++++++++++++---
 2 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 3999641..d6aa55e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -168,7 +168,7 @@ impl Square {
             'f' => 5,
             'g' => 6,
             'h' => 7,
-            _ => {return Err(SquareError::InvalidCharacter(bytes[0] as char))}
+            _ => return Err(SquareError::InvalidCharacter(bytes[0] as char)),
         };
         if let Some(row) = (bytes[1] as char).to_digit(10) {
             Square::from_row_col(row as usize - 1, col as usize)
@@ -187,7 +187,10 @@ mod tests {
         let test_cases = [("a1", 0), ("a8", 56), ("h1", 7), ("h8", 63)];
         for (sqr, idx) in test_cases {
             assert_eq!(Square::try_from(idx).unwrap().to_algebraic(), sqr);
-            assert_eq!(Square::from_algebraic(sqr).unwrap(), Square::try_from(idx).unwrap());
+            assert_eq!(
+                Square::from_algebraic(sqr).unwrap(),
+                Square::try_from(idx).unwrap()
+            );
         }
     }
 }
@@ -339,6 +342,10 @@ pub struct BoardState {
     turn: Color,
 }
 
+/// Piece missing where there should be one.
+#[derive(Debug)]
+struct NoPieceError;
+
 impl BoardState {
     /// Get mutable reference to a player.
     fn pl_mut(&mut self, col: Color) -> &mut Player {
@@ -352,12 +359,17 @@ impl BoardState {
         *self.mail.sq_mut(idx) = Some(pc);
     }
 
-    /// Delete the piece in a location, if it exists.
-    fn del_piece(&mut self, idx: Square) {
+    /// Delete the piece in a location.
+    ///
+    /// Returns an error if there is no piece in the location.
+    fn del_piece(&mut self, idx: Square) -> Result<(), NoPieceError> {
         if let Some(pc) = *self.mail.sq_mut(idx) {
             let pl = self.pl_mut(pc.col);
             pl.board(pc.into()).off_idx(idx);
             *self.mail.sq_mut(idx) = None;
+            Ok(())
+        } else {
+            Err(NoPieceError)
         }
     }
 
diff --git a/src/movegen.rs b/src/movegen.rs
index 8c89960..8f73afb 100644
--- a/src/movegen.rs
+++ b/src/movegen.rs
@@ -98,7 +98,7 @@ impl Move {
                 pc_asserts!(pc_src, self);
                 debug_assert_eq!(pc_src.pc, Piece::Pawn);
 
-                node.pos.del_piece(self.src);
+                node.pos.del_piece(self.src).expect("Move source should have piece.");
                 node.pos.set_piece(
                     self.dest,
                     ColPiece {
@@ -127,12 +127,22 @@ impl Move {
                             Color::Black => { self.src.0 - BOARD_WIDTH },
                         };
                         node.pos.ep_square = Some(
-                        // TODO: fix the en passant targetsquare
                             Square::try_from(new_idx)
                                 .expect("En-passant target should be valid."),
                         )
                     } else {
                         node.pos.ep_square = None;
+                        if pc_dest.is_none() && src_col != dest_col {
+                            // we took en passant
+                            debug_assert!(src_row.abs_diff(dest_row) == 1);
+                            debug_assert_eq!(self.dest, old_pos.ep_square.unwrap());
+                            // square to actually capture at
+                            let ep_capture = Square::try_from(match pc_src.col {
+                                Color::White => { self.dest.0 - BOARD_WIDTH },
+                                Color::Black => { self.dest.0 + BOARD_WIDTH },
+                            }).expect("En-passant capture square should be valid.");
+                            node.pos.del_piece(ep_capture).expect("En-passant capture square should have piece.");
+                        }
                     }
                 } else {
                     node.pos.half_moves += 1;
@@ -168,7 +178,7 @@ impl Move {
                     }
                 }
 
-                node.pos.del_piece(self.src);
+                node.pos.del_piece(self.src).expect("Move source should have piece.");
                 node.pos.set_piece(self.dest, pc_src);
             }
         }
@@ -331,6 +341,17 @@ mod tests {
                     ("b2b1q", "4k1N1/8/8/8/8/8/8/1q2K3 w - - 0 2"),
                 ],
             ),
+            // en passant test
+            (
+                "k7/4p3/8/3P4/3p4/8/4P3/K7 w - - 0 1",
+                vec![
+                    ("e2e4", "k7/4p3/8/3P4/3pP3/8/8/K7 b - e3 0 1"),
+                    ("d4e3", "k7/4p3/8/3P4/8/4p3/8/K7 w - - 0 2"),
+                    ("a1b1", "k7/4p3/8/3P4/8/4p3/8/1K6 b - - 1 2"),
+                    ("e7e5", "k7/8/8/3Pp3/8/4p3/8/1K6 w - e6 0 3"),
+                    ("d5e6", "k7/8/4P3/8/8/4p3/8/1K6 b - - 0 3"),
+                ],
+            ),
         ];
 
         for (i, test_case) in test_cases.iter().enumerate() {