ForLoopComparison.swift

import Foundation

extension Array
    {
    func median() -> Element
        { return self[count / 2] }
    }

func time(name: String, iters: Int, closure: Void -> Void) -> NSTimeInterval
    {
    print(name)
    print("")
    
    let reps = 9
    let times: [NSTimeInterval] =
        (0..<reps).map
            {
            iter in
            
            let startTime = NSDate.timeIntervalSinceReferenceDate()
            
            for _ in 0..<iters
                { closure() }
            
            let time = NSDate.timeIntervalSinceReferenceDate() - startTime
            print("  Iter \(iter): \(time)")
            return time
            }
    let median = times.median()
    
    print("         ——————————————")
    print("  Median: \(median)")
    print("")
    return median
    }

func timeForLoops(name: String, iters: Int, zipStride: Void -> Void, cStyle: Void -> Void)
    {
    print("—————— \(name) ——————")
    print("")

    let zipStrideTime = time("zip+stride", iters: iters, closure: zipStride)
    let cStyleTime    = time("C-style",    iters: iters, closure: cStyle)

    print("Zip+stride takes \(zipStrideTime / cStyleTime)x the time of C-style for")
    print("")
    }

timeForLoops(
    "Paul’s comparison", iters: 5000000,
    zipStride:
        {
        var sum = 0
        for (i, j) in zip(100.stride(to: 0, by: -1), 200.stride(to: 0, by: -2))
            {
            if i % 2 == 0 { continue }
            sum += i - j
            }
        },
    cStyle:
        {
        var sum = 0
        for var i = 100, j = 200; i > 0 && j > 0; i -= 1, j -= 2
            {
            if i % 2 == 0 { continue }
            sum += i - j
            }
        }
    )

let first = 10000000
let second = 20000000

timeForLoops(
    "David’s comparison", iters: 40,
    zipStride:
        {
        var sum = 0
        for (i, j) in zip(first.stride(to: 0, by: -1), second.stride(to: 0, by: -2))
            {
            if i % 2 == 0 { continue }
            sum += 1
            }
        },
    cStyle:
        {
        var sum = 0
        for var i = first, j = second; i > 0 && j > 0; i -= 1, j -= 2
            {
            if i % 2 == 0 { continue }
            sum += 1
            }
        }
    )

timeForLoops(
    "David’s comparison, actually using indices in the loop", iters: 40,
    zipStride:
        {
        var sum = 0
        for (i, j) in zip(first.stride(to: 0, by: -1), second.stride(to: 0, by: -2))
            {
            if i % 2 == 0 { continue }
            sum += i-j
            }
        },
    cStyle:
        {
        var sum = 0
        for var i = first, j = second; i > 0 && j > 0; i -= 1, j -= 2
            {
            if i % 2 == 0 { continue }
            sum += i-j
            }
        }
    )