use std::{
fmt::{Debug, Display},
num::NonZeroU8,
ops::Neg,
};
use model::{CheckersBitBoard, Move, PieceColor, PossibleMoves};
use crate::transposition_table::TranspositionTableRef;
const KING_WORTH: u32 = 2;
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Evaluation(i16);
impl Display for Evaluation {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
if self.is_force_win() {
write!(f, "+M{}", self.force_sequence_length().unwrap())
} else if self.is_force_loss() {
write!(f, "-M{}", self.force_sequence_length().unwrap())
} else {
write!(f, "{:+}", self.to_f32().unwrap())
}
}
}
impl Neg for Evaluation {
type Output = Self;
fn neg(self) -> Self::Output {
Self(-self.0)
}
}
impl Evaluation {
const NULL_MAX: Self = Self(i16::MAX);
const NULL_MIN: Self = Self(i16::MIN + 1);
pub const WIN: Self = Self(i16::MAX - 1);
pub const DRAW: Self = Self(0);
pub const LOSS: Self = Self(i16::MIN + 2);
// last fourteen bits set to 1
const FORCE_WIN_THRESHOLD: i16 = 0x3FFF;
pub fn new(eval: f32) -> Self {
if eval >= 1.0 {
return Self::WIN;
} else if eval <= -1.0 {
return Self::LOSS;
}
Self((eval * 16384.0) as i16)
}
pub fn to_f32(self) -> Option<f32> {
if self.is_force_sequence() {
return None;
}
Some(self.0 as f32 / 16384.0)
}
pub fn is_force_win(self) -> bool {
self.0 > Self::FORCE_WIN_THRESHOLD
}
pub fn is_force_loss(self) -> bool {
self.0 < -Self::FORCE_WIN_THRESHOLD
}
pub fn is_force_sequence(self) -> bool {
self.is_force_win() || self.is_force_loss()
}
pub fn force_sequence_length(self) -> Option<u8> {
if self == Self::NULL_MAX || self == Self::NULL_MIN {
return None;
}
if self.is_force_win() {
Some((Self::WIN.0 - self.0) as u8)
} else if self.is_force_loss() {
Some((self.0 - Self::LOSS.0) as u8)
} else {
None
}
}
fn increment(self) -> Self {
if self.is_force_win() {
Self(self.0 - 1)
} else if self.is_force_loss() {
Self(self.0 + 1)
} else {
self
}
}
fn add(self, rhs: f32) -> Self {
let Some(eval) = self.to_f32() else {
return self;
};
Self::new(eval + rhs)
}
}
fn eval_position(board: CheckersBitBoard) -> Evaluation {
let light_pieces = board.pieces_bits() & !board.color_bits();
let dark_pieces = board.pieces_bits() & board.color_bits();
let light_peasants = light_pieces & !board.king_bits();
let dark_peasants = dark_pieces & !board.king_bits();
let light_kings = light_pieces & board.king_bits();
let dark_kings = dark_pieces & board.king_bits();
// if we assume the black player doesn't exist, how good is this for white?
let light_eval =
(light_peasants.count_ones() as f32) + ((light_kings.count_ones() * KING_WORTH) as f32);
let dark_eval =
(dark_peasants.count_ones() as f32) + ((dark_kings.count_ones() * KING_WORTH) as f32);
// avoiding a divide by zero error
if dark_eval + light_eval != 0.0 {
Evaluation::new((dark_eval - light_eval) / (dark_eval + light_eval))
} else {
Evaluation::DRAW
}
}
fn eval_jumps(
mut alpha: Evaluation,
beta: Evaluation,
board: CheckersBitBoard,
table: TranspositionTableRef,
) -> Evaluation {
// todo stop checking for jumps twice, but also don't look for slides if there are no jumps
if PossibleMoves::has_jumps(board) {
// todo check if this is useful
// todo make a board for the other player's turn reusable
let turn = board.turn();
let mut best_eval = Evaluation::LOSS;
let moves = PossibleMoves::moves(board);
if moves.is_empty() {
return Evaluation::LOSS;
}
for current_move in moves {
let board = unsafe { current_move.apply_to(board) };
let current_eval = if board.turn() != turn {
-eval_jumps(-beta, -alpha, board, table).increment()
} else {
eval_jumps(alpha, beta, board, table).increment()
};
table.insert(board, current_eval, unsafe { NonZeroU8::new_unchecked(1) });
if current_eval >= beta {
return beta;
}
if best_eval < current_eval {
best_eval = current_eval;
}
if alpha < best_eval {
alpha = best_eval;
}
}
best_eval
} else if board.turn() == PieceColor::Dark {
eval_position(board)
} else {
-eval_position(board)
}
}
unsafe fn sort_moves(
a: &Move,
b: &Move,
board: CheckersBitBoard,
table: TranspositionTableRef,
) -> std::cmp::Ordering {
let a_entry = table
.get_any_depth(a.apply_to(board))
.unwrap_or(Evaluation::DRAW);
let b_entry = table
.get_any_depth(b.apply_to(board))
.unwrap_or(Evaluation::DRAW);
a_entry.cmp(&b_entry)
}
pub fn negamax(
depth: u8,
mut alpha: Evaluation,
beta: Evaluation,
board: CheckersBitBoard,
table: TranspositionTableRef,
) -> Evaluation {
if depth < 1 {
if board.turn() == PieceColor::Dark {
eval_position(board)
} else {
-eval_position(board)
}
} else {
if let Some(entry) = table.get(board, depth) {
return entry;
}
let turn = board.turn();
let mut best_eval = Evaluation::LOSS;
let mut moves: Vec<Move> = PossibleMoves::moves(board).into_iter().collect();
if moves.is_empty() {
return Evaluation::LOSS;
}
moves.sort_unstable_by(|a, b| unsafe { sort_moves(a, b, board, table) });
for current_move in moves {
let board = unsafe { current_move.apply_to(board) };
let current_eval = if board.turn() == turn {
negamax(depth - 1, alpha, beta, board, table).increment()
} else {
-negamax(depth - 1, -beta, -alpha, board, table).increment()
};
if best_eval < current_eval {
best_eval = current_eval;
}
if alpha < best_eval {
alpha = best_eval;
}
if alpha >= beta {
return best_eval;
}
}
table.insert(board, best_eval, unsafe { NonZeroU8::new_unchecked(depth) });
best_eval
}
}
pub fn current_evaluation(
depth: u8,
board: CheckersBitBoard,
table: TranspositionTableRef,
) -> Evaluation {
let mut alpha = Evaluation::NULL_MIN;
let mut beta = Evaluation::NULL_MAX;
for i in 0..depth {
let mut eval = negamax(i, alpha, beta, board, table);
while (eval <= alpha) || (eval >= beta) {
eval = negamax(i, alpha, beta, board, table);
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);
}
}
let mut eval = negamax(depth, alpha, beta, board, table);
if (eval <= alpha) || (eval >= beta) {
eval = negamax(
depth,
Evaluation::NULL_MIN,
Evaluation::NULL_MAX,
board,
table,
);
}
eval
}
pub fn best_move(depth: u8, board: CheckersBitBoard, table: TranspositionTableRef) -> Move {
let moves = PossibleMoves::moves(board).into_iter();
let mut best_move = None;
let mut best_eval = Evaluation::NULL_MIN;
for current_move in moves {
let current_board = unsafe { current_move.apply_to(board) };
let current_eval = if board.turn() == current_board.turn() {
current_evaluation(depth - 1, current_board, table)
} else {
-current_evaluation(depth - 1, current_board, table)
};
if current_eval >= best_eval {
best_eval = current_eval;
best_move = Some(current_move);
}
}
best_move.unwrap()
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn zero_eval() {
let draw = Evaluation::new(0.0);
assert_eq!(draw, Evaluation::DRAW);
assert_eq!(draw.to_f32(), Some(0.0));
assert_eq!(draw.to_string(), "+0");
}
#[test]
fn comparisons() {
assert!(Evaluation::NULL_MAX > Evaluation::WIN);
assert!(Evaluation::WIN > Evaluation::new(0.5));
assert!(Evaluation::new(0.5) > Evaluation::DRAW);
assert!(Evaluation::DRAW > Evaluation::new(-0.5));
assert!(Evaluation::new(-0.5) > Evaluation::LOSS);
assert!(Evaluation::LOSS > Evaluation::NULL_MIN);
}
#[test]
fn negations() {
assert_eq!(-Evaluation::NULL_MAX, Evaluation::NULL_MIN);
assert_eq!(-Evaluation::NULL_MIN, Evaluation::NULL_MAX);
assert_eq!(-Evaluation::WIN, Evaluation::LOSS);
assert_eq!(-Evaluation::LOSS, Evaluation::WIN);
assert_eq!(-Evaluation::DRAW, Evaluation::DRAW);
assert_eq!(-Evaluation::new(0.5), Evaluation::new(-0.5));
}
}
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