Split up search and eval

1 files changed, 233 insertions, 0 deletions

diff --git a/engine/src/search.rs b/engine/src/search.rs
new file mode 100644
index 0000000..b9cb5e6
--- /dev/null
+++ b/engine/src/search.rs
@@ -0,0 +1,233 @@
+use std::num::NonZeroU8;
+use std::sync::{atomic::AtomicBool, Arc};
+use std::time::Instant;
+
+use model::{CheckersBitBoard, Move, PieceColor, PossibleMoves};
+
+use crate::{
+	eval::{eval_position, Evaluation},
+	lazysort::LazySort,
+	EvaluationTask, Frontend, TranspositionTableRef,
+};
+
+unsafe fn sort_moves(
+	a: &Move,
+	board: CheckersBitBoard,
+	table: TranspositionTableRef,
+) -> Evaluation {
+	table
+		.get_any_depth(a.apply_to(board))
+		.unwrap_or(Evaluation::DRAW)
+}
+
+pub fn negamax(
+	depth: u8,
+	mut alpha: Evaluation,
+	beta: Evaluation,
+	board: CheckersBitBoard,
+	allowed_moves: Option<Arc<[Move]>>,
+	cancel_flag: &AtomicBool,
+	task: &EvaluationTask,
+) -> (Evaluation, Option<Move>) {
+	task.nodes_explored
+		.fetch_add(1, std::sync::atomic::Ordering::Release);
+
+	if depth < 1 {
+		if board.turn() == PieceColor::Dark {
+			(eval_position(board), None)
+		} else {
+			(-eval_position(board), None)
+		}
+	} else {
+		let table = task.transposition_table;
+		if let Some(entry) = table.get(board, depth) {
+			return (entry, None);
+		}
+
+		let turn = board.turn();
+		let mut best_eval = Evaluation::NULL_MIN;
+		let mut best_move = None;
+
+		let sort_fn = |m: &Move| unsafe { sort_moves(m, board, table) };
+		let sorter: LazySort<Move, _, Evaluation, { PossibleMoves::MAX_POSSIBLE_MOVES }> =
+			if let Some(moves) = allowed_moves {
+				LazySort::new(moves.iter().cloned(), sort_fn)
+			} else {
+				let moves = PossibleMoves::moves(board);
+				LazySort::new(moves, sort_fn)
+			};
+
+		if sorter.is_empty() {
+			return (Evaluation::LOSS, None);
+		}
+
+		for current_move in sorter.into_iter() {
+			if cancel_flag.load(std::sync::atomic::Ordering::Acquire) {
+				return (best_eval, best_move);
+			}
+
+			let board = unsafe { current_move.apply_to(board) };
+			let current_eval = if board.turn() == turn {
+				negamax(depth - 1, alpha, beta, board, None, cancel_flag, task)
+					.0
+					.increment()
+			} else {
+				-negamax(depth - 1, -beta, -alpha, board, None, cancel_flag, task)
+					.0
+					.increment()
+			};
+
+			if best_eval < current_eval {
+				best_eval = current_eval;
+				best_move = Some(current_move);
+			}
+
+			if alpha < best_eval {
+				alpha = best_eval;
+			}
+
+			if alpha >= beta {
+				return (best_eval, best_move);
+			}
+		}
+
+		table.insert(board, best_eval, unsafe { NonZeroU8::new_unchecked(depth) });
+
+		(best_eval, best_move)
+	}
+}
+
+pub fn search(task: Arc<EvaluationTask>, frontend: &dyn Frontend) -> Evaluation {
+	let board = task.position;
+	let cancel_flag = &task.cancel_flag;
+
+	let allowed_moves = task.allowed_moves.clone();
+	let limits = task.limits;
+	let max_depth = limits.depth;
+	let max_nodes = limits.nodes;
+	let max_time = limits.time.map(|d| Instant::now() + d.div_f32(2.0));
+
+	let mut alpha = Evaluation::NULL_MIN;
+	let mut beta = Evaluation::NULL_MAX;
+	let mut depth = 0;
+	let mut eval = Evaluation::DRAW;
+	let mut best_move = None;
+	loop {
+		if let Some(max_depth) = max_depth {
+			if depth > max_depth.get() {
+				break;
+			}
+		}
+
+		if let Some(max_time) = max_time {
+			if Instant::now() > max_time {
+				break;
+			}
+		}
+
+		if let Some(max_nodes) = max_nodes {
+			if task
+				.nodes_explored
+				.load(std::sync::atomic::Ordering::Acquire)
+				> max_nodes.get()
+			{
+				break;
+			}
+		}
+
+		let em = negamax(
+			depth,
+			alpha,
+			beta,
+			board,
+			allowed_moves.clone(),
+			cancel_flag,
+			&task,
+		);
+
+		// prevent incomplete search from overwriting evaluation
+		if cancel_flag.load(std::sync::atomic::Ordering::Acquire) {
+			break;
+		}
+
+		eval = em.0;
+		best_move = em.1;
+
+		while (eval <= alpha) || (eval >= beta) {
+			let em = negamax(
+				depth,
+				alpha,
+				beta,
+				board,
+				allowed_moves.clone(),
+				cancel_flag,
+				&task,
+			);
+
+			// prevent incomplete search from overwriting evaluation
+			if cancel_flag.load(std::sync::atomic::Ordering::Acquire) {
+				break;
+			}
+
+			eval = em.0;
+			best_move = em.1;
+
+			if eval <= alpha {
+				alpha = Evaluation::NULL_MIN;
+			} else if eval >= beta {
+				beta = Evaluation::NULL_MAX;
+			}
+		}
+
+		if alpha.is_force_loss() {
+			alpha = Evaluation::NULL_MIN;
+		} else {
+			alpha = eval.add(-0.125);
+		}
+
+		if beta.is_force_win() {
+			beta = Evaluation::NULL_MAX;
+		} else {
+			beta = eval.add(0.125);
+		}
+
+		depth += 1;
+	}
+
+	// ponder
+	if let Some(best_move) = best_move {
+		// If the best move has not been found yet, then no move will be
+		// reported. This should be very rare. This technically is not allowed
+		// by the UCI specification, but if someone stops it this quickly, they
+		// probably didn't care about the best move anyway.
+		frontend.report_best_move(best_move);
+
+		if task.ponder {
+			let board = unsafe { best_move.apply_to(board) };
+
+			let mut depth = 0;
+			loop {
+				if task
+					.end_ponder_flag
+					.load(std::sync::atomic::Ordering::Acquire)
+				{
+					break;
+				}
+
+				negamax(
+					depth,
+					Evaluation::NULL_MIN,
+					Evaluation::NULL_MAX,
+					board,
+					None,
+					&task.end_ponder_flag,
+					&task,
+				);
+
+				depth += 1;
+			}
+		}
+	}
+
+	eval
+}