swift - 使用 dispatch_async 进行并行搜索

Question

我正在尝试实现并行搜索算法。这个概念是这样的:

1. 从候选人开始并测试它是否是所需的值
2. 如果没有，生成更多候选人并将他们添加到队列中。
3. 重复直到达到想要的值

作为一个简化示例:我想在 0..<n 范围内运行一个随机数生成器直到它给我 0。我想减少 n每次迭代都会保证成功。到目前为止，这是我的代码:

let queue = dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)
let work : dispatch_function_t = { arg in
    let upper = UnsafeMutablePointer<UInt32>(arg).memory
    let random = arc4random_uniform(upper)
    if random == 0 {
        // do things
    } else {
        dispatch_async_f(queue, &(upper - 1), work)
        // Error: Variable used within its own initial value
    }
}

dispatch_async_f(queue, &1000, work)
// Error: '&' used for non inout argument of type 'UnsafeMutablePointer<Void>'

我有两个错误:

Variable used within its own initial value
'&' used for noninout argument of type 'UnsafeMutablePointer<Void>'

我该如何修复它们？非常感谢!

Answer 1

声明和初始化分两步解决“在其自身初始值内使用”的问题。

let work: dispatch_function_t
work  = { arg in
    let upper = UnsafeMutablePointer<UInt32>(arg).memory
    let random = arc4random_uniform(upper)
    if random == 0 {
        // do things
    } else {
        dispatch_async_f(queue, &(upper - 1), work)
        // Error: Variable used within its own initial value
    }
}

你可以像这样修复另一个。

var n = 1000
dispatch_async_f(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), &n, work)