path: root/src/board/engine.rs

use std::{time::{Instant, Duration}, f32::INFINITY, cmp::Ordering};
use crate::{bitboard::pop_count, board::*};

use super::ttable::{NodeType, TranspositionTableItem};

static A_FILE:     Bitboard = 0x0101010101010101;

#[derive(Debug, Default, PartialEq)]
pub struct PerftResult {
    leaf_nodes: u64,
    captures: u64,
    en_passants: u64,
    castles: u64,
    checks: u64,
}

impl Board {
    pub fn perft(&mut self, depth: u8, print: bool) -> PerftResult {
        let mut result = PerftResult::default();

        if depth == 0 {
            result.leaf_nodes = 1;
            return result;
        }
        let color = self.color();

        let moves = self.generate_pseudolegal_moves(color);

        if print {
            println!("Running perft for depth {}. Color to move is {:?}\n{} moves available", depth, color, moves.len());
            println!("{} moves available", moves.len());
        }

        for mov in moves {
            let ep_target_before = self.ep_target.clone();
            let castling_rights_before = self.castling_rights.clone();
            let hash_before = self.hash.clone();
            let captured_piece = self.make_move(mov);
            // King can not be in check after our own move
            if !self.is_king_in_check(color) {
                if depth == 1 {
                    match mov.kind {
                        MoveKind::Capture => {
                            result.captures += 1;
                        }
                        MoveKind::EnPassant => {
                            result.en_passants += 1;
                            result.captures += 1;
                        }
                        MoveKind::Castle => {
                            result.castles += 1;
                        }
                        _ => {}
                    }
                    if self.is_king_in_check(color.flip()) {
                        result.checks += 1;
                    }
                }

                if print {
                    println!("{:?}", mov);
                    self.print();
                }
                let subtree_result = self.perft(depth - 1, print);

                result.leaf_nodes += subtree_result.leaf_nodes;
                result.captures += subtree_result.captures;
                result.checks += subtree_result.checks;
                result.castles += subtree_result.castles;
                result.en_passants += subtree_result.en_passants;

            }
            self.unmake_move(mov, captured_piece, ep_target_before, castling_rights_before, hash_before);
        }

        if print {
            println!("Found {} leaf nodes in this subtree (depth {})", result.leaf_nodes, depth);
        }

        result
    }

    /// Compute material advantage relative to the current player
    pub fn material_advantage(&self) -> f32 {
        let mut eval = 0f32;
        for (piece_index, bitboard) in self.pieces.iter().enumerate() {
            let piece_type = PieceType::from(piece_index);
            let sign = if Color::from_piece(piece_type) == self.color() {
                1.
            } else {
                -1.
            };

            eval += sign * match piece_type {
                PieceType::Pawn => {
                    serialize_bitboard(*bitboard).iter().fold(0., |acc, square| {
                        acc + match (*square).rank() {
                            6 => 3.,
                            5 => 2.,
                            _ => 1.,
                        }
                    })
                }
                PieceType::PawnBlack => {
                    serialize_bitboard(*bitboard).iter().fold(0., |acc, square| {
                        acc + match (*square).rank() {
                            1 => 3.,
                            2 => 2.,
                            _ => 1.,
                        }
                    })
                }
                _ => {
                    piece_type.static_eval() * pop_count(*bitboard) as f32
                }
            };
        }
        eval
    }

    /// Returns sum of the doubled, blocked and isolated pawns
    /// The greater result is, the worse is the pawn structure
    pub fn pawn_structure_penalty(&self, color: Color) -> f32 {
        let mut result = 0.0;

        let pawns = match color {
            Color::White => self.pieces[PieceType::Pawn as usize],
            Color::Black => self.pieces[PieceType::PawnBlack as usize],
        };

        for file in 0..8 {
            let file_mask = A_FILE << file;
            let pawns_on_file = pop_count(pawns & file_mask) as f32;

            // Doubled pawns (-1 because one pawn on a file is ok)
            result += (pawns_on_file - 1.).max(0.0);

            // Isolated pawns (no pawns on neighbor files)
            if [
                A_FILE << (file - 1).max(0), // File to the left (if any)
                A_FILE << (file + 1).min(7), // File to the right (if any)
            ].iter().all(|file| file & pawns == 0) {
                result += pawns_on_file;
            }
        }

        // Blocked pawns
        let blocked_mask = match color {
            Color::White => self.occupancy >> 8,
            Color::Black => self.occupancy << 8,
        };
        result += pop_count(pawns & blocked_mask) as f32;

        result
    }

    /// Evaluate a position relative to the current player
    pub fn evaluate(&self, precomputed_mobility: Option<f32>) -> f32 {
        let opponent_mobility = self.mobility(self.color().flip());
        let player_mobility = match precomputed_mobility {
            Some(m) => m,
            None => self.mobility(self.color()),
        };
        let mobility_advantage = player_mobility - opponent_mobility as f32;

        let material_advantage = self.material_advantage();

        let pawn_structure_penalty = self.pawn_structure_penalty(self.color()) - self.pawn_structure_penalty(self.color().flip());

        material_advantage + 0.1 * mobility_advantage - 0.5 * pawn_structure_penalty
    }

    /// Evaluate move for move ordering, prioritizing efficient captures
    /// where low-value pieces capture high-value pieces
    fn eval_move(&self, m: Move) -> f32 {
        if m.is_tactical() {
            let [source_eval, target_eval] = [m.source, m.target]
                .map(|sq| self.piece_by_square(sq))
                .map(|p| {
                    match p {
                        Some(p) => p.static_eval(),
                        None => 0.,
                    }
                });
            return 2. * target_eval - source_eval
        }
        0.0
    }

    pub fn order_moves(&self, moves: Vec<Move>) -> Vec<Move> {
        let mut moves_with_eval: Vec<(Move, f32)> = moves
            .iter()
            .map(|m| (*m, self.eval_move(*m)))
            .collect();

        moves_with_eval.sort_unstable_by(|(a, a_eval), (b, b_eval)| {
            if *a_eval == 0.0 && *b_eval == 0.0 {
                // Prioritize equal captures over non-captures
                if a.is_tactical() && !b.is_tactical() {
                    return Ordering::Less
                }
                if b.is_tactical() && !a.is_tactical() {
                    return Ordering::Greater
                }
            }
            a_eval.total_cmp(b_eval).reverse()
        });

        moves_with_eval.iter_mut().map(|(m, _)| *m).collect()
    }

    pub fn negamax_search(&mut self, mut alpha: f32, beta: f32, depth_left: u8, deadline: Instant) -> (f32, Vec<Move>) {
        let mut principal_variation = Vec::new();
        let color = self.color();

        let mut moves = self.generate_pseudolegal_moves(color);
        moves = self.order_moves(moves);

        match self.transposition_table[(self.hash % TTABLE_SIZE) as usize] {
            Some(item) => {
                if item.hash == self.hash {
                    moves.insert(0, item.best_move);
                }
            }
            None => {},
        }

        if depth_left == 0 {
            return (self.quiscence(alpha, beta), principal_variation);
        }

        for mov in moves {
            let ep_target_before = self.ep_target.clone();
            let castling_rights_before = self.castling_rights.clone();
            let hash_before = self.hash.clone();
            let captured_piece = self.make_move(mov);

            if !self.is_king_in_check(color) {
                let (mut score, mut subtree_pv) = self.negamax_search(-beta, -alpha, depth_left - 1, deadline);
                score *= -1.;
                self.unmake_move(mov, captured_piece, ep_target_before, castling_rights_before, hash_before);

                if score >= beta {
                    self.transposition_table[(self.hash % TTABLE_SIZE) as usize] = Some(TranspositionTableItem {
                        hash: self.hash,
                        best_move: mov,
                        depth: depth_left, // TODO: should be actual depth searched
                        node_type: NodeType::Cut,
                        score,
                    });

                    return (beta, principal_variation);
                }
                if score > alpha {
                    alpha = score;
                    principal_variation = Vec::with_capacity(depth_left as usize);
                    principal_variation.push(mov);
                    principal_variation.append(&mut subtree_pv);

                    self.transposition_table[(self.hash % TTABLE_SIZE) as usize] = Some(TranspositionTableItem {
                        hash: self.hash,
                        best_move: mov,
                        depth: depth_left, // TODO: should be actual depth searched
                        node_type: NodeType::PV,
                        score,
                    });
                } else {
                    self.transposition_table[(self.hash % TTABLE_SIZE) as usize] = Some(TranspositionTableItem {
                        hash: self.hash,
                        best_move: mov,
                        depth: depth_left, // TODO: should be actual depth searched
                        node_type: NodeType::All,
                        score,
                    });
                }
            } else {
                self.unmake_move(mov, captured_piece, ep_target_before, castling_rights_before, hash_before);
            }

            // Could not finish in time, return what we have so far
            if Instant::now() > deadline {
                println!("Returning early!");
                return (alpha, principal_variation)
            }
        }

        (alpha, principal_variation)
    }

    pub fn quiscence(&mut self, mut alpha: f32, beta: f32) -> f32 {
        let color = self.color();
        let mut moves = self.generate_pseudolegal_moves(color);
        moves = self.order_moves(moves);

        match self.transposition_table[(self.hash % TTABLE_SIZE) as usize] {
            Some(item) => {
                if item.hash == self.hash {
                    moves.insert(0, item.best_move);
                }
            }
            None => {},
        }

        let stand_pat = self.evaluate(Some(moves.len() as f32));

        if stand_pat >= beta {
            return beta;
        }
        if alpha < stand_pat {
            alpha = stand_pat;
        }

        let tactical_moves = moves.iter().filter(|m| m.is_tactical());

        for mov in tactical_moves {
            let ep_target_before = self.ep_target.clone();
            let castling_rights_before = self.castling_rights.clone();
            let hash_before = self.hash.clone();
            let captured_piece = self.make_move(*mov);

            if !self.is_king_in_check(color) {
                let evaluation = -self.quiscence(-beta, -alpha);
                self.unmake_move(*mov, captured_piece, ep_target_before, castling_rights_before, hash_before);

                if evaluation >= beta {
                    return beta; // Fail-hard beta-cutoff
                }
                if evaluation > alpha {
                    alpha = evaluation;
                }
            } else {
                self.unmake_move(*mov, captured_piece, ep_target_before, castling_rights_before, hash_before);
            }
        }

        alpha
    }

    pub fn iterative_deepening(&mut self, max_depth: u8, duration: Duration) -> (f32, Vec<Move>) {
        let start = Instant::now();
        let deadline = start + duration;
        let mut result = None;
        let mut depth = 1;
        let mut alpha = -INFINITY;
        let mut beta = INFINITY;
        let window_size = 0.25;

        while depth <= max_depth {
            let search_result = self.negamax_search(alpha, beta, depth, deadline);
            println!("Finished depth({}) {:?} [{:?} left]", depth, search_result, deadline - Instant::now());

            if Instant::now() > deadline {
                break;
            }
            if search_result.1.len() > 0 {
                depth += 1;
                alpha = search_result.0 - window_size;
                beta = search_result.0 + window_size;
            } else if search_result.0 <= alpha { // Alpha-cutoff
                println!("Alpha cutoff {} <= {:?}", search_result.0, (alpha, beta));
                beta = alpha;
                alpha = search_result.0 - window_size * 4.0;
                continue;
            } else if search_result.0 >= beta { // Beta-cutoff
                println!("Beta cutoff {:?} <= {}", (alpha, beta), search_result.0);
                alpha = beta;
                beta = search_result.0 + window_size * 4.0;
                continue;
            } else {
                panic!("Can this ever be possible? (probably not)");
            }

            result = Some(search_result);
        }

        match result {
            Some(r) => return r,
            None => panic!("Could not find a move in time"),
        }
    }
}


#[cfg(test)]
mod tests {
    use crate::board::{Board, engine::PerftResult};

    #[test]
    fn perft() {
        let mut board = Board::new();

        assert_eq!(board.perft(0, false), PerftResult { leaf_nodes: 1, captures: 0, en_passants: 0, castles: 0 , checks: 0 });
        assert_eq!(board.perft(1, false), PerftResult { leaf_nodes: 20, captures: 0, en_passants: 0, castles: 0 , checks: 0 });
        assert_eq!(board.perft(2, false), PerftResult { leaf_nodes: 400, captures: 0, en_passants: 0, castles: 0 , checks: 0 });
        assert_eq!(board.perft(3, false), PerftResult { leaf_nodes: 8902, captures: 34, en_passants: 0, castles: 0 , checks: 12 });
        assert_eq!(board.perft(4, false), PerftResult { leaf_nodes: 197281, captures: 1576, en_passants: 0, castles: 0 , checks: 469 });
        // assert_eq!(board.perft(5, false), PerftResult { leaf_nodes: 4865609, captures: 82719, en_passants: 258, castles: 0, checks: 27351 });
    }

    #[test]
    fn position_perft() {
        let fen = String::from("r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq - ");
        let mut board = Board::from_FEN(fen);
        assert_eq!(board.perft(0, false), PerftResult { leaf_nodes: 1, captures: 0, en_passants: 0, castles: 0 , checks: 0 });
        assert_eq!(board.perft(1, false), PerftResult { leaf_nodes: 48, captures: 8, en_passants: 0, castles: 2 , checks: 0 });
        assert_eq!(board.perft(2, false), PerftResult { leaf_nodes: 2039, captures: 351, en_passants: 1, castles: 91 , checks: 3 });
        assert_eq!(board.perft(3, false), PerftResult { leaf_nodes: 97862, captures: 17102, en_passants: 45, castles: 3162, checks: 993 });
        // assert_eq!(board.perft(4, false), PerftResult { leaf_nodes: 4085603, captures: 757163, en_passants: 1929, castles: 128013, checks: 25523 });
    }

    #[test]
    fn endgame_perft() {
        let fen = String::from("8/2p5/3p4/KP5r/1R3p1k/8/4P1P1/8 w - - ");
        let mut board = Board::from_FEN(fen);

        assert_eq!(board.perft(1, false), PerftResult { leaf_nodes: 14, captures: 1, en_passants: 0, castles: 0 , checks: 2 });
        assert_eq!(board.perft(2, false), PerftResult { leaf_nodes: 191, captures: 14, en_passants: 0, castles: 0 , checks: 10 });
        // assert_eq!(board.perft(3, false), PerftResult { leaf_nodes: 2182, captures: 209, en_passants: 2, castles: 0 , checks: 267 });
    }

    #[test]
    fn material_advantage() {
        let board = Board::new();
        assert_eq!(board.material_advantage(), 0.0, "Material advantage should be 0 at starting position");

    }
}