Custom map implementation performance test in Swift

map.swift

import Foundation

// used in map5
let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
let group = dispatch_group_create()

extension Array {
    func map2<U>(transform: Element ->U) -> [U] {
        return reduce([]) {
            $0 + [transform($1)]
        }
    }
    func map3<U>(transform: Element ->U) -> [U] {
        var result = [U]()
        for e in self {
            result.append(transform(e))
        }
        return result
    }
    func map4<U>(transform: Element ->U) -> [U] {
        var result = [U](count: self.count, repeatedValue: transform(self[0]))
        for (idx,item) in enumerate() {
            result.withUnsafeMutableBufferPointer{(inout p:UnsafeMutableBufferPointer<U>) in
                p[idx] = transform(item)
            }
        }
        return result
    }
    /// @see http://blog.scottlogic.com/2014/10/29/concurrent-functional-swift.html
    func map5<U>(transform: Element ->U, callback:[U]->()) {
        var result = [U](count: self.count, repeatedValue: transform(self[0]))
        for (idx,item) in enumerate() {
            result.withUnsafeMutableBufferPointer{(inout p:UnsafeMutableBufferPointer<U>) in
                dispatch_group_async(group, queue) {
                    p[idx] = transform(item)
                }
            }
        }
        dispatch_group_notify(group, queue) {
            callback(result)
        }
    }
}

var time = NSDate().timeIntervalSince1970
func measure(message: String? = nil) -> Double {
    let now = NSDate().timeIntervalSince1970
    let diff = now - time
    if let message = message {
      print("\(message): \(diff)")
    }
    time = now
    return diff
}

let a:[Int] = (1...1000).map{$0}
measure()
let b = a.map{$0 + 1}
measure("map ")
let c = a.map2{$0 + 1}
measure("map2")
let d = a.map3{$0 + 1}
measure("map3")
let e = a.map4{$0 + 1}
measure("map4")
var f:[Int] = []
measure()
a.map5({$0 + 1}, callback: {f = $0;measure("map5")})

// $ swift map.swift
// map : 0.000206947326660156
// map2: 0.00310802459716797
// map3: 0.000529050827026367
// map4: 0.00117397308349609
// map5: 0.00284814834594727