转换Branch and Bound循环以使用Java Stream API

Question

我有一个简单的分支定界算法,适用于旅行商问题的变体,我认为尝试将其转换为使用Java 8 Stream API会很有趣.然而,我很难在不依赖副作用的情况下弄清楚如何做到这一点.

初始代码

int bound = Integer.MAX_VALUE;
List<Location> bestPath = null;

while(!queue.isEmpty()) {
    Node curr = queue.poll();
    //bound exceeds best, bail
    if (curr.getBound() >= bound) { 
        return bestPath;
    }
    //have a complete path, save it
    if(curr.getPath().size() == locations.size()) {
        bestPath = curr.getPath();
        bound = curr.getBound();
        continue;
    }
    //incomplete path - add all possible next steps
    Set<Location> unvisited = new HashSet<>(locations);
    unvisited.removeAll(curr.getPath());
    for (Location l : unvisited) {
        List<Location> newPath = new ArrayList<>(curr.getPath());
        newPath.add(l);
        Node newNode = new Node(newPath, getBoundForPath(newPath));
        if (newNode.getBound() <= bound){
            queue.add(newNode);
        }
    }
}

我第一次尝试将其转换为Stream API,并提出以下建议:

Java 8版

Consumer<Node> nodeConsumer = node -> {
    if(node.getPath().size() == locations.size() ) {
        bestPath = node.getPath();
        bound = node.getBound();
    } else {
        locations.stream()
            .filter(l -> !node.getPath().contains(l))
            .map(l -> {
                List<Location> newPath = new ArrayList<>(node.getPath());
                newPath.add(s);
                return new Node(newPath, getBoundForPath(newPath));
            })
            .filter(newNode -> newNode.getBound() <= bound)
            .forEach(queue::add);
    }
};

Stream.generate(() -> queue.poll())
    .peek(nodeConsumer)
    .filter(s -> s.getBound() > bound)
    .findFirst();

return bestPath;

主要问题是nodeConsumer必须引用bestPath和bound,它们不是最终变量.我可以让他们最终的AtomicReference变量解决这个问题,但我觉得这样违反了流API的精神.任何人都可以帮助我将初始算法提炼成更惯用的实现吗？

Answer 1

我想知道使用是否reduce是解决此问题的方法，因为它允许您在不需要外部变量的情况下跟踪值。

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类似于以下内容（我不得不猜测上面代码的一些细节，但希望我走在正确的轨道上）。

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    final BiFunction<Entry<Integer, List<Location>>, Node, Entry<Integer, List<Location>>> accumulator\n        = (identity, node) -> {\n            if (node.getPath().size() == locations.size() ) {\n                return new SimpleEntry<>(node.getBound(), node.getPath());\n            } else {\n                locations.stream()\n                    .filter(l -> !node.getPath().contains(l))\n                    .map(l -> {\n                        List<Location> newPath = new ArrayList<>(node.getPath());\n                        newPath.add(l);\n                        return new Node(newPath, getBoundForPath(newPath));\n                    })\n                    .filter(newNode -> newNode.getBound() <= identity.getKey())\n                    .forEach(queue::add);\n                return identity;\n            }\n        };\n\n    final BinaryOperator<Entry<Integer, List<Location>>> combiner\n        = (left, right) -> left.getKey() < right.getKey() ? left : right;\n\n    final Entry<Integer, List<Location>> identity\n        = new SimpleEntry<>(Integer.MAX_VALUE, null);\n\n    final List<Location> bestValue = Stream.generate(queue::poll)\n        .reduce(identity, accumulator, combiner)\n        .getValue();\n

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或者，您可以查看 using Seqin jOO\xce\xbb（Streams 的顺序扩展），并foldLeft改为使用。

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