BoundedQueue.swift

//
//  BoundedQueue.swift
//
//  Copyright © 2017 Doug Russell. All rights reserved.
//

import Foundation

public class BoundedQueue {
    private let group = DispatchGroup()
    private let semaphore: DispatchSemaphore
    private let work: DispatchQueue
    private let waiting: DispatchQueue
    public convenience init(label: String,
                            count: Int) {
        self.init(label: label,
                  count: count,
                  qos: .`default`,
                  autoreleaseFrequency: .workItem)
    }
    public init(label: String,
                count: Int,
                qos: DispatchQoS,
                autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency) {
        work = DispatchQueue(label: label,
                             qos: qos,
                             attributes: [.concurrent],
                             autoreleaseFrequency: autoreleaseFrequency,
                             target: nil)
        waiting = DispatchQueue(label: "\(label).waiting",
                                qos: qos,
                                attributes: [],
                                autoreleaseFrequency: .workItem,
                                target: work)
        semaphore = DispatchSemaphore(value: count)
    }
    public func async(execute work: @escaping () -> Void) {
        group.enter()
        waiting.async {
            self.semaphore.wait()
            self.work.async {
                work()
                self.semaphore.signal()
                self.group.leave()
            }
        }
    }
    public func sync<T>(execute work: () throws -> T) rethrows -> T {
        group.enter()
        return try waiting.sync {
            semaphore.wait()
            return try self.work.sync {
                let result = try work()
                semaphore.signal()
                group.leave()
                return result
            }
        }
    }
}

//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//