Use 'arrayvec' crate for our MoveList implementation

Elo   | 1.05 +- 3.35 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.95 (-2.94, 2.94) [-5.00, 0.00]
Games | N: 17274 W: 4955 L: 4903 D: 7416
Penta | [437, 1838, 4036, 1888, 438]

Cargo.toml

@@ -8,6 +8,7 @@ rust-version = "1.81"
 release = []
 
 [dependencies]
+arrayvec = "0.7.6"
 colored = "2.1.0"
 nom = "7.1.1"
 rand = "0.8.5"

src/chess/game.rs

@@ -1,5 +1,5 @@
 use crate::chess::bitboard::bitboards;
-use crate::chess::movelist::MoveList;
+use crate::chess::moves::MoveList;
 use crate::chess::piece::Piece;
 use crate::chess::square::squares;
 use crate::chess::zobrist::ZobristHash;
@@ -135,7 +135,7 @@ impl Game {
         if self.halfmove_clock >= 100 {
             let mut movelist = MoveList::new();
             generate_legal_moves(self, &mut movelist);
-            return movelist.has_moves();
+            return !movelist.is_empty();
         }
 
         false

src/chess/mod.rs

@@ -4,7 +4,6 @@ pub mod direction;
 pub mod fen;
 pub mod game;
 pub mod movegen;
-pub mod movelist;
 pub mod moves;
 pub mod perft;
 pub mod piece;

src/chess/movegen/gen.rs

@@ -1,6 +1,6 @@
 use crate::chess::bitboard::{bitboards, Bitboard};
 use crate::chess::movegen::{attackers, pins, tables};
-use crate::chess::movelist::MoveList;
+use crate::chess::moves::MoveList;
 use crate::chess::square::{squares, Square};
 use crate::chess::{game::Game, moves::Move, piece::PromotionPieceKind};
 

src/chess/moves.rs

@@ -1,4 +1,9 @@
 use crate::chess::{piece::PromotionPieceKind, square::Square};
+use arrayvec::ArrayVec;
+
+const MAX_LEGAL_MOVES: usize = 218;
+
+pub type MoveList = ArrayVec<Move, MAX_LEGAL_MOVES>;
 
 #[derive(PartialEq, Eq, Clone, Copy)]
 pub struct Move {

src/engine/search/move_picker.rs

@@ -1,8 +1,7 @@
 use crate::chess::game::Game;
 use crate::chess::movegen;
 use crate::chess::movegen::MovegenCache;
-use crate::chess::movelist::MoveList;
-use crate::chess::moves::Move;
+use crate::chess::moves::{Move, MoveList};
 use crate::engine::search::move_ordering::score_move;
 use crate::engine::search::{move_ordering, PersistentState, SearchState};
 
@@ -135,7 +134,7 @@ impl MovePicker {
 
             if let Some(killer1) = state.killer_moves.get_0(plies) {
                 for i in self.first_quiet..self.moves.len() {
-                    if self.moves.get(i) == killer1 {
+                    if self.moves.get(i).map_or(false, |m| *m == killer1) {
                         self.moves.swap(self.first_quiet, i);
                         self.first_quiet += 1;
 
@@ -161,7 +160,7 @@ impl MovePicker {
 
             if let Some(killer2) = state.killer_moves.get_1(plies) {
                 for i in self.first_quiet..self.moves.len() {
-                    if self.moves.get(i) == killer2 {
+                    if self.moves.get(i).map_or(false, |m| *m == killer2) {
                         self.moves.swap(self.first_quiet, i);
                         self.first_quiet += 1;
 
@@ -226,7 +225,7 @@ impl MovePicker {
                 }
             }
 
-            let best_move = self.moves.get(best_move_idx);
+            let best_move = *self.moves.get(best_move_idx).unwrap();
 
             // Move our best move to the start of the moves we haven't tried
             self.moves.swap(self.idx, best_move_idx);
@@ -252,7 +251,11 @@ impl MovePicker {
         persistent_state: &PersistentState,
     ) {
         for i in start..end {
-            self.scores[i] = score_move(game, self.moves.get(i), &persistent_state.history_table);
+            self.scores[i] = score_move(
+                game,
+                *self.moves.get(i).unwrap(),
+                &persistent_state.history_table,
+            );
         }
     }
 }

src/tests/perft_tests.rs

@@ -2,7 +2,7 @@ use crate::chess::fen::START_POS;
 use crate::chess::game::Game;
 use crate::chess::movegen;
 use crate::chess::movegen::MovegenCache;
-use crate::chess::movelist::MoveList;
+use crate::chess::moves::MoveList;
 use crate::chess::perft::perft;
 use crate::engine::search::move_picker::MovePicker;
 use crate::engine::search::{PersistentState, SearchState};
@@ -107,17 +107,21 @@ fn movepicker_perft(
         // To get good coverage, we use the length of the move list to determine whether to try captures/quiets
         // in best move/killers.
         if captures_movelist.len() >= 3 {
-            best_move = Some(captures_movelist.get(0));
+            best_move = Some(*captures_movelist.first().unwrap());
         } else if quiets_movelist.len() >= 3 {
-            best_move = Some(quiets_movelist.get(0));
+            best_move = Some(*quiets_movelist.first().unwrap());
         }
 
         if quiets_movelist.len() >= 3 {
-            state.killer_moves.try_push(depth, quiets_movelist.get(2));
+            state
+                .killer_moves
+                .try_push(depth, *quiets_movelist.get(2).unwrap());
         }
 
         if quiets_movelist.len() >= 4 {
-            state.killer_moves.try_push(depth, quiets_movelist.get(3));
+            state
+                .killer_moves
+                .try_push(depth, *quiets_movelist.get(3).unwrap());
         }
     }