[board] Implement a pawn move generator

Holy heck I went on a *journey* here. Ultimately, I needed to implement my own
index-based iterator instead of using the Vec's Iterator.

This type establishes some patterns I want to carry forward to other move
generators.

1. The use of a Parameters struct to fully parameterize the move generation
   per-color. That lets these types only need a single color-based branch
2. A list of move lists, one list for each of captures, promotions, and quiet
   moves.
3. An index-based move iterator.
4. Separate impl for generating bitboard representations of these moves

Additional changes:

- Implement BitBoard::from_square()
- Implement a Square::e5() for tests

This class doesn't implement en passant yet. It also doesn't yet have tests for
the bitboard stuff.

board/src/bitboard/bitboard.rs

@@ -28,6 +28,10 @@ impl BitBoard {
         FILES[file]
     }
 
+    pub fn from_square(sq: Square) -> BitBoard {
+        BitBoard(1 << sq.index())
+    }
+
     pub fn is_empty(&self) -> bool {
         self.0 == 0
     }

board/src/moves/pawn.rs

@@ -1,83 +1,251 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use crate::bitboard::BitBoard;
-use crate::piece::{Color, Piece};
-use crate::Position;
+use crate::{
+    bitboard::BitBoard,
+    piece::{Color, Piece, Shape},
+    Move, Position,
+};
 
-pub(super) struct PawnMoveGenerator<'a> {
-    position: &'a Position,
+enum MoveList {
+    Quiet = 0,
+    Promotions = 1,
+    Captures = 2,
 }
 
-impl<'a> PawnMoveGenerator<'a> {
-    pub(super) fn new(position: &Position) -> PawnMoveGenerator {
-        PawnMoveGenerator { position }
-    }
+#[derive(Debug)]
+struct MoveIterator(usize, usize);
 
-    pub(super) fn pawn_moves(&self, color: Color) -> BitBoard {
-        match color {
-            Color::White => self.white_pawn_pushes() & self.white_pawn_attacks(),
-            Color::Black => self.black_pawn_pushes() & self.black_pawn_attacks(),
+struct MoveGenerationParameters {
+    starting_rank: BitBoard,
+    promotion_rank: BitBoard,
+    push_shift: fn(&BitBoard) -> BitBoard,
+    left_capture_shift: fn(&BitBoard) -> BitBoard,
+    right_capture_shift: fn(&BitBoard) -> BitBoard,
+}
+
+pub(super) struct PawnMoveGenerator<'pos> {
+    color: Color,
+    position: &'pos Position,
+
+    did_populate_move_lists: bool,
+
+    pushes: BitBoard,
+    attacks: BitBoard,
+
+    move_lists: [Vec<Move>; 3],
+    move_iterator: MoveIterator,
+}
+
+impl<'pos> PawnMoveGenerator<'pos> {
+    pub(super) fn new(position: &Position, color: Color) -> PawnMoveGenerator {
+        PawnMoveGenerator {
+            position,
+            color,
+            did_populate_move_lists: false,
+            pushes: BitBoard::empty(),
+            attacks: BitBoard::empty(),
+            move_lists: [Vec::new(), Vec::new(), Vec::new()],
+            move_iterator: MoveIterator(0, 0),
         }
     }
 
-    pub(super) fn pawn_attacks(&self, color: Color) -> BitBoard {
-        match color {
-            Color::White => self.white_pawn_attacks(),
-            Color::Black => self.black_pawn_attacks(),
+    pub(super) fn generate_moves(&mut self) {
+        self.generate_move_bitboards();
+        self.populate_move_lists();
+    }
+
+    fn move_generation_parameters(&self) -> MoveGenerationParameters {
+        match self.color {
+            Color::White => MoveGenerationParameters {
+                starting_rank: BitBoard::rank(1),
+                promotion_rank: BitBoard::rank(7),
+                push_shift: BitBoard::shift_north_one,
+                left_capture_shift: BitBoard::shift_north_west_one,
+                right_capture_shift: BitBoard::shift_north_east_one,
+            },
+            Color::Black => MoveGenerationParameters {
+                starting_rank: BitBoard::rank(6),
+                promotion_rank: BitBoard::rank(0),
+                push_shift: BitBoard::shift_south_one,
+                left_capture_shift: BitBoard::shift_south_east_one,
+                right_capture_shift: BitBoard::shift_south_west_one,
+            },
         }
     }
 
-    pub(super) fn pawn_pushes(&self, color: Color) -> BitBoard {
-        match color {
-            Color::White => self.white_pawn_pushes(),
-            Color::Black => self.black_pawn_pushes(),
-        }
+    #[inline]
+    fn quiet_move_list(&mut self) -> &mut Vec<Move> {
+        &mut self.move_lists[MoveList::Quiet as usize]
     }
 
-    fn white_pawn_pushes(&self) -> BitBoard {
+    #[inline]
+    fn promotion_move_list(&mut self) -> &mut Vec<Move> {
+        &mut self.move_lists[MoveList::Promotions as usize]
+    }
+
+    #[inline]
+    fn capture_move_list(&mut self) -> &mut Vec<Move> {
+        &mut self.move_lists[MoveList::Captures as usize]
+    }
+}
+
+impl<'pos> PawnMoveGenerator<'pos> {
+    fn generate_move_bitboards(&mut self) {
+        let parameters = self.move_generation_parameters();
+        self.generate_pushes_bitboard(&parameters);
+        self.generate_attacks_bitboard(&parameters);
+    }
+
+    fn generate_pushes_bitboard(&mut self, parameters: &MoveGenerationParameters) {
         let empty_squares = self.position.empty_squares();
+        let bb = self.position.bitboard_for_piece(Piece::pawn(self.color));
+
+        let legal_1square_pushes = (parameters.push_shift)(bb) & empty_squares;
+        let legal_2square_pushes =
+            (parameters.push_shift)(&(legal_1square_pushes & BitBoard::rank(2))) & empty_squares;
+
+        self.pushes = legal_1square_pushes | legal_2square_pushes;
+    }
+
+    fn generate_attacks_bitboard(&mut self, parameters: &MoveGenerationParameters) {
+        let opponent_pieces = self.position.bitboard_for_color(self.color.other());
         let bb = self.position.bitboard_for_piece(Piece::pawn(Color::White));
 
-        let legal_1square_pushes = bb.shift_north_one() & empty_squares;
-        let legal_2square_pushes =
-            (legal_1square_pushes & BitBoard::rank(2)).shift_north_one() & empty_squares;
+        self.attacks = ((parameters.left_capture_shift)(bb) | (parameters.right_capture_shift)(bb))
+            & opponent_pieces;
 
-        legal_1square_pushes | legal_2square_pushes
+        #[allow(unused_variables)]
+        if let Some(en_passant) = self.en_passant() {
+            // TODO: Add en passant move to the attacks bitboard.
+        }
+    }
+}
+
+impl<'pos> PawnMoveGenerator<'pos> {
+    fn populate_move_lists(&mut self) {
+        let parameters = self.move_generation_parameters();
+
+        self._populate_move_lists(&parameters);
+        self.did_populate_move_lists = true;
     }
 
-    fn white_pawn_attacks(&self) -> BitBoard {
-        let black_pieces = self.position.bitboard_for_color(Color::Black);
-        let bb = self.position.bitboard_for_piece(Piece::pawn(Color::White));
+    fn _populate_move_lists(&mut self, parameters: &MoveGenerationParameters) {
+        let piece = Piece::pawn(self.color);
 
-        (bb.shift_north_east_one() | bb.shift_north_west_one()) & black_pieces
-    }
+        let bb = self.position.bitboard_for_piece(piece);
+        if bb.is_empty() {
+            return;
+        }
 
-    fn black_pawn_pushes(&self) -> BitBoard {
         let empty_squares = self.position.empty_squares();
-        let bb = self.position.bitboard_for_piece(Piece::pawn(Color::Black));
+        let black_pieces = self.position.bitboard_for_color(self.color.other());
 
-        let legal_1square_pushes = bb.shift_south_one() & empty_squares;
-        let legal_2square_pushes =
-            (legal_1square_pushes & BitBoard::rank(5)).shift_south_one() & empty_squares;
+        for from_sq in bb.occupied_squares() {
+            let pawn = BitBoard::from_square(from_sq);
 
-        legal_1square_pushes | legal_2square_pushes
+            let push = (parameters.push_shift)(&pawn);
+            if !(push & empty_squares).is_empty() {
+                let to_sq = push.occupied_squares().next().unwrap();
+
+                let r#move = Move::new(piece, from_sq, to_sq);
+                if !(push & parameters.promotion_rank).is_empty() {
+                    for shape in Shape::promotable() {
+                        self.promotion_move_list()
+                            .push(r#move.clone().promoting_to(*shape));
+                    }
+                } else {
+                    self.quiet_move_list().push(r#move);
+                }
+
+                if !(pawn & parameters.starting_rank).is_empty() {
+                    let push = (parameters.push_shift)(&push);
+                    if !(push & empty_squares).is_empty() {
+                        let to_sq = push.occupied_squares().next().unwrap();
+                        self.quiet_move_list()
+                            .push(Move::new(piece, from_sq, to_sq));
+                    }
+                }
+            }
+
+            for attack in [
+                (parameters.left_capture_shift)(&pawn),
+                (parameters.right_capture_shift)(&pawn),
+            ] {
+                if !(attack & black_pieces).is_empty() {
+                    let to_sq = attack.occupied_squares().next().unwrap();
+                    let captured_piece = self.position.piece_on_square(to_sq).unwrap();
+
+                    let r#move = Move::new(piece, from_sq, to_sq).capturing(captured_piece);
+                    if !(attack & parameters.promotion_rank).is_empty() {
+                        for shape in Shape::promotable() {
+                            self.capture_move_list()
+                                .push(r#move.clone().promoting_to(*shape));
+                        }
+                    } else {
+                        self.capture_move_list().push(r#move);
+                    }
+                }
+            }
+
+            if let Some(en_passant) = self.en_passant() {
+                self.capture_move_list().push(en_passant);
+            }
+        }
     }
 
-    fn black_pawn_attacks(&self) -> BitBoard {
-        let white_pieces = self.position.bitboard_for_color(Color::White);
-        let bb = self.position.bitboard_for_piece(Piece::pawn(Color::Black));
+    fn en_passant(&self) -> Option<Move> {
+        // TODO: En passant. I think the way to do this is to have the position mark
+        // an en passant square when the conditions are correct, i.e. when the
+        // opposing player has pushed a pawn two squares off the initial rank, and
+        // then check in these routines if a pawn of this color attacks that square.
 
-        (bb.shift_south_east_one() | bb.shift_south_west_one()) & white_pieces
+        None
     }
+}
 
-    // TODO: En passant
+impl<'pos> Iterator for PawnMoveGenerator<'pos> {
+    type Item = Move;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if !self.did_populate_move_lists {
+            self.generate_moves();
+        }
+
+        let iter = &mut self.move_iterator;
+
+        // Find the next non-empty list.
+        loop {
+            if iter.0 >= self.move_lists.len() || !self.move_lists[iter.0].is_empty() {
+                break;
+            }
+
+            iter.0 += 1;
+        }
+
+        if let Some(move_list) = self.move_lists.get(iter.0) {
+            let next_move = move_list[iter.1].clone();
+
+            iter.1 += 1;
+            if iter.1 >= move_list.len() {
+                // Increment the list index here. On the next iteration, find the next non-empty list.
+                iter.0 += 1;
+                iter.1 = 0;
+            }
+
+            Some(next_move)
+        } else {
+            None
+        }
+    }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::piece::PlacedPiece;
     use crate::position::DiagramFormatter;
     use crate::{Position, Square};
+    use std::collections::HashSet;
 
     #[test]
     fn one_2square_push() {
@@ -85,14 +253,19 @@ mod tests {
         pos.place_piece(&Piece::pawn(Color::White), &Square::e2())
             .expect("Failed to place pawn on e2");
 
-        let generator = PawnMoveGenerator::new(&pos);
-        let pawn_pushes = generator.pawn_pushes(Color::White);
+        let generator = PawnMoveGenerator::new(&pos, Color::White);
 
-        let mut expected = BitBoard::empty();
-        expected.place_piece_at(&Square::e3());
-        expected.place_piece_at(&Square::e4());
+        let expected_moves = HashSet::from_iter(
+            [
+                Move::new(Piece::pawn(Color::White), Square::e2(), Square::e3()),
+                Move::new(Piece::pawn(Color::White), Square::e2(), Square::e4()),
+            ]
+            .into_iter(),
+        );
 
-        assert_eq!(pawn_pushes, expected);
+        let generated_moves: HashSet<Move> = generator.collect();
+
+        assert_eq!(generated_moves, expected_moves);
     }
 
     #[test]
@@ -101,13 +274,20 @@ mod tests {
         pos.place_piece(&Piece::pawn(Color::White), &Square::e3())
             .expect("Failed to place pawn on e3");
 
-        let generator = PawnMoveGenerator::new(&pos);
-        let pawn_pushes = generator.pawn_pushes(Color::White);
+        let generator = PawnMoveGenerator::new(&pos, Color::White);
 
-        let mut expected = BitBoard::empty();
-        expected.place_piece_at(&Square::e4());
+        let expected_moves = HashSet::from_iter(
+            [Move::new(
+                Piece::pawn(Color::White),
+                Square::e3(),
+                Square::e4(),
+            )]
+            .into_iter(),
+        );
 
-        assert_eq!(pawn_pushes, expected);
+        let generated_moves: HashSet<Move> = generator.collect();
+
+        assert_eq!(generated_moves, expected_moves);
     }
 
     #[test]
@@ -120,13 +300,37 @@ mod tests {
 
         println!("{}", DiagramFormatter::new(&pos));
 
-        let generator = PawnMoveGenerator::new(&pos);
-        let pawn_pushes = generator.white_pawn_pushes();
+        let generator = PawnMoveGenerator::new(&pos, Color::White);
 
-        let mut expected = BitBoard::empty();
-        expected.place_piece_at(&Square::e3());
+        let expected_moves = HashSet::from_iter(
+            [Move::new(
+                Piece::pawn(Color::White),
+                Square::e2(),
+                Square::e3(),
+            )]
+            .into_iter(),
+        );
 
-        assert_eq!(pawn_pushes, expected);
+        let generated_moves: HashSet<Move> = generator.collect();
+
+        assert_eq!(generated_moves, expected_moves);
+    }
+
+    #[test]
+    fn one_obstructed_1square_push() {
+        let mut pos = Position::empty();
+        pos.place_piece(&Piece::pawn(Color::White), &Square::e2())
+            .expect("Failed to place pawn on e2");
+        pos.place_piece(&Piece::knight(Color::White), &Square::e3())
+            .expect("Failed to place knight on e4");
+
+        println!("{}", DiagramFormatter::new(&pos));
+
+        let generator = PawnMoveGenerator::new(&pos, Color::White);
+
+        let generated_moves: HashSet<Move> = generator.collect();
+
+        assert_eq!(generated_moves, HashSet::new());
     }
 
     #[test]
@@ -134,18 +338,26 @@ mod tests {
         let mut pos = Position::empty();
         pos.place_piece(&Piece::pawn(Color::White), &Square::e4())
             .expect("Failed to place pawn on e4");
+        pos.place_piece(&Piece::bishop(Color::White), &Square::e5())
+            .expect("Failed to place pawn on e4");
         pos.place_piece(&Piece::knight(Color::Black), &Square::d5())
             .expect("Failed to place knight on d5");
 
         println!("{}", DiagramFormatter::new(&pos));
 
-        let generator = PawnMoveGenerator::new(&pos);
-        let pawn_attacks = generator.white_pawn_attacks();
+        let generator = PawnMoveGenerator::new(&pos, Color::White);
 
-        let mut expected = BitBoard::empty();
-        expected.place_piece_at(&Square::d5());
+        let expected_moves = HashSet::from_iter(
+            [
+                Move::new(Piece::pawn(Color::White), Square::e4(), Square::d5())
+                    .capturing(PlacedPiece::new(Piece::knight(Color::Black), Square::d5())),
+            ]
+            .into_iter(),
+        );
 
-        assert_eq!(pawn_attacks, expected);
+        let generated_moves: HashSet<Move> = generator.collect();
+
+        assert_eq!(generated_moves, expected_moves);
     }
 
     #[test]
@@ -153,6 +365,8 @@ mod tests {
         let mut pos = Position::empty();
         pos.place_piece(&Piece::pawn(Color::White), &Square::e4())
             .expect("Failed to place pawn on e4");
+        pos.place_piece(&Piece::bishop(Color::White), &Square::e5())
+            .expect("Failed to place pawn on e4");
         pos.place_piece(&Piece::knight(Color::Black), &Square::d5())
             .expect("Failed to place knight on d5");
         pos.place_piece(&Piece::queen(Color::Black), &Square::f5())
@@ -160,13 +374,24 @@ mod tests {
 
         println!("{}", DiagramFormatter::new(&pos));
 
-        let generator = PawnMoveGenerator::new(&pos);
-        let pawn_attacks = generator.white_pawn_attacks();
+        let generator = PawnMoveGenerator::new(&pos, Color::White);
 
-        let mut expected = BitBoard::empty();
-        expected.place_piece_at(&Square::d5());
-        expected.place_piece_at(&Square::f5());
+        let expected_moves = HashSet::from_iter(
+            [
+                Move::new(Piece::pawn(Color::White), Square::e4(), Square::d5())
+                    .capturing(PlacedPiece::new(Piece::knight(Color::Black), Square::d5())),
+                Move::new(Piece::pawn(Color::White), Square::e4(), Square::f5())
+                    .capturing(PlacedPiece::new(Piece::queen(Color::Black), Square::f5())),
+            ]
+            .into_iter(),
+        );
 
-        assert_eq!(pawn_attacks, expected);
+        let generated_moves: HashSet<Move> = generator.collect();
+
+        assert_eq!(
+            generated_moves, expected_moves,
+            "generated: {:#?}\nexpected: {:#?}",
+            generated_moves, expected_moves
+        );
     }
 }

board/src/tests.rs

@@ -18,5 +18,6 @@ impl Square {
     sq_constructor!(e2);
     sq_constructor!(e3);
     sq_constructor!(e4);
+    sq_constructor!(e5);
     sq_constructor!(f5);
 }