将 Java while 循环转换为 Stream 以提高性能

Question

我有以下代码，由于必须将长流转换为数组，因此存在性能问题。 Board.getReachables() 方法返回一个流，因此能够使用流执行此 while 循环会提高性能。然而，我不确定如何继续。

 int counter = 1;
 while (check(board, counter)) {
   for (int i = 0; i < board.length; i++) {
     if (board[i] == counter) {
       for (Pos reach2 :
            Board.getReachables(currentTurn.getBoard().pos(i),
                                currentTurn.getBoard())
                 .toArray(Pos[]::new)) {
         if (board[currentTurn.getBoard().index(reach2)] == 0) {
           board[currentTurn.getBoard().index(reach2)] = counter + 1;
         }
       }
     }
   }
   counter++;
 }

Answer 1

将循环转换为流：

AtomicInteger counter = new AtomicInteger(1);
while (check(board, counter.get())) {
   for (int i = 0; i < board.length; i++) {
     if (board[i] == counter.get()) {
         Board.getReachables(currentTurn.getBoard().pos(i), currentTurn.getBoard())
         .filter(reach2 ->  board[currentTurn.getBoard().index(reach2)] == 0)
         .forEach(reach2 -> board[currentTurn.getBoard().index(reach2)] = counter.get() + 1);
     }
   }
   counter.incrementAndGet();
 }

counter

转换为

AtomicInteger

，因为它是在 lambda 中引用的，因此必须是 有效的最终。

其他重构留给读者。

Answer 2

未经测试，只是一个想法：

    while (check(board, counter)) {
        final int finalCounter = counter; //counter has to be final,
        //otherwise you get a "Local variable counter defined in an enclosing scope must be final or effectively final" error

        Arrays.stream(board) //IntStream
        .filter(b -> b == finalCounter) //  if (board[i] == counter) {
        .mapToObj(i ->Board.getReachables(currentTurn.getBoard().pos(i), currentTurn.getBoard())) //returns Stream<Stream<Pos>>
        .flatMap(Function.identity()) // flatmap maps Stream<Stream<Pos>> to a single (combined) Stream<Pos>
        .filter(pos -> board[currentTurn.getBoard().index(pos)] == 0)  //board[currentTurn.getBoard().index(reach2)] == 0
        .forEach(fp -> board[currentTurn.getBoard().index(fp)] = finalCounter +1); //board[currentTurn.getBoard().index(reach2)] = counter + 1;
        counter++;
    }

我认为有人不应该这样做，但这是一个有趣的练习。

Answer 3

有两件事可以提高性能：

1. 不要创建数组并对其进行循环，而只需在

   .forEach

   上调用

   Stream

   。
2. 仅计算一次指数。 不幸的是，这意味着使用

   if

   而不是

   .filter()

        int counter = 1;
        while (check(board, counter)) {
            for (int i = 0; i < board.length; i++) {
                if (board[i] == counter) {
                    Board currentTurnBoard = currentTurn.getBoard();
                    int   nextCounter      = counter + 1;
                    Board.getReachables(currentTurnBoard.pos(i), currentTurnBoard)
                         .forEach(reach2 -> {
                             int index = currentTurnBoard.index(reach2);
                             if (board[index] == 0) {
                                 board[index] = nextCounter;
                             }
                             return;
                         });
                }
            }
            counter++;
        }