Some refactoring to reduce complexity

src/main/java/com/leokom/chess/engine/PositionGenerator.java

@@ -26,29 +26,11 @@ final class PositionGenerator {
 	 * @return new position, after move from squareFrom
 	 */
 	Position generate( Move move ) {
-		if ( move == null ) {
-			throw new IllegalArgumentException( "Move must be not null" );
-		}
-
 		if ( move == Move.RESIGN ) {
-			//TODO: technically side to move is useless for such terminal position?
-			//it indicates a side of possible move IF the position wouldn't be terminal
-			final Position result = new Position( source.getSideToMove().opposite() );
-			source.copyStateTo( result );
-			//TODO: should checkmate move also set this flag?
-			result.setTerminal( source.getSideToMove().opposite() );
-			return result;
+			return getResignPosition();
 		}
 
-
-
-		if ( source.getPiece( move.getFrom() ) == null ) {
-			throw new IllegalArgumentException( "Source square is empty : " + move.getFrom() );
-		}
-
-		if ( source.getPiece( move.getFrom() ).getSide() != source.getSideToMove() ) {
-			throw new IllegalArgumentException( "Wrong side to move : " + move + ". Currently it's turn of " + source.getSideToMove() );
-		}
+		validateStandardMove( move );
 
 		String squareFrom = move.getFrom();
 		String moveTo = move.getTo();
@@ -69,6 +51,30 @@ Position generate( Move move ) {
 		}
 	}
 
+	private void validateStandardMove( Move move ) {
+		if ( move == null ) {
+			throw new IllegalArgumentException( "Move must be not null" );
+		}
+
+		if ( source.getPiece( move.getFrom() ) == null ) {
+			throw new IllegalArgumentException( "Source square is empty : " + move.getFrom() );
+		}
+
+		if ( source.getPiece( move.getFrom() ).getSide() != source.getSideToMove() ) {
+			throw new IllegalArgumentException( "Wrong side to move : " + move + ". Currently it's turn of " + source.getSideToMove() );
+		}
+	}
+
+	private Position getResignPosition() {
+		//TODO: technically side to move is useless for such terminal position?
+		//it indicates a side of possible move IF the position wouldn't be terminal
+		final Position result = new Position( source.getSideToMove().opposite() );
+		source.copyStateTo( result );
+		//TODO: should checkmate move also set this flag?
+		result.setTerminal( source.getSideToMove().opposite() );
+		return result;
+	}
+
 	private Position processRookMove( String squareFrom, String move ) {
 		final Position result = processMoveWithoutSideEffects( squareFrom, move );
 		//TODO: do I need setting this flag after castling?