check drop is valid before drawing piece

Author: JamesHeppell

package.json

@@ -1,6 +1,6 @@
 {
   "name": "chessground",
-  "version": "7.11.1-pstrat2.43",
+  "version": "7.11.1-pstrat2.44",
   "description": "playstrategy.org chess ui, forked from lichess.org",
   "type": "commonjs",
   "main": "chessground.js",

src/board.ts

@@ -367,7 +367,7 @@ export function dropNewPiece(state: HeadlessState, orig: cg.Key, dest: cg.Key, f
   } else {
     unsetPremove(state);
     unsetPredrop(state);
-    cancelDropMode(state);
+    if (!state.onlyDropsVariant) cancelDropMode(state);
   }
   state.pieces.delete(orig);
   unselect(state);
@@ -467,9 +467,14 @@ function canDrop(state: HeadlessState, orig: cg.Key, dest: cg.Key): boolean {
   const piece = state.pieces.get(orig);
   return (
     !!piece &&
-    (orig === dest || !state.pieces.has(dest)) &&
+    (orig === dest || !state.pieces.has(dest) || state.variant === 'backgammon' || state.variant === 'nackgammon') &&
     (state.movable.playerIndex === 'both' ||
-      (state.movable.playerIndex === piece.playerIndex && state.turnPlayerIndex === piece.playerIndex))
+      (state.movable.playerIndex === piece.playerIndex && state.turnPlayerIndex === piece.playerIndex)) &&
+    !!(
+      state.dropmode.dropDests &&
+      state.dropmode.dropDests.has(piece.role) &&
+      state.dropmode.dropDests.get(piece.role)?.includes(dest)
+    )
   );
 }
 

src/drop.ts

@@ -38,7 +38,7 @@ export function drop(s: State, e: cg.MouchEvent): void {
       dropFromPocket(s, piece.role);
     } else {
       const dest = position && board.getKeyAtDomPos(position, s.orientation, s.dom.bounds(), s.dimensions, s.variant);
-      if (dest) board.dropNewPiece(s, 'a0', dest, s.variant === 'backgammon' || s.variant === 'nackgammon');
+      if (dest) board.dropNewPiece(s, 'a0', dest);
     }
   }
   //stop processing event clicks, as click drops will trigger a screen click and select a piece.
@@ -59,5 +59,5 @@ function dropFromPocket(s: State, role: cg.Role): void {
         ? activeDiceKey
         : s.dropmode.dropDests.get(role)![0]
       : false;
-  if (dest) board.dropNewPiece(s, 'a0', dest, s.variant === 'backgammon' || s.variant === 'nackgammon');
+  if (dest) board.dropNewPiece(s, 'a0', dest);
 }

src/predrop.ts

@@ -4,6 +4,7 @@ import * as cg from './types';
 type DropMobility = (x: number, y: number) => boolean;
 
 const wholeBoard = (_x: number, _y: number) => true;
+const noSquares = (_x: number, _y: number) => false;
 
 /**
  *
@@ -142,6 +143,10 @@ export default function predrop(
       mobility = emptysquares(pieces); // cant drop on current pieces (either side) as they cant move
       break;
 
+    case 'nackgammon':
+    case 'backgammon':
+      mobility = noSquares; // dont allow predrop for backgammon
+      break;
     default:
       console.warn('Unknown drop variant', variant);
   }