COW Problem

cow.swift

import Foundation
let arraySize = 1000000
let loopCount = 1000

class Values {
    var values = [Int](repeating: 0, count: arraySize)
    func foo1() {
        values[0] += values[0]
    }
    func foo2() {
        let v = values[0]
        values[0] += v
    }
}

func measure(_ body: () -> Void) {
    let time1 = Date.timeIntervalSinceReferenceDate
    for _ in 0 ..< loopCount {
        body()
    }
    let time2 = Date.timeIntervalSinceReferenceDate
    print(time2 - time1)
}
func f1() {
    let v = Values()
    measure { v.foo1() }
}
func f2() {
    let v = Values()
    measure { v.foo2() }
}
f1()
f2()

// 0.8569329977035522
// 0.001692056655883789