代碼地址
QuickCheck是一個用於隨機測試的Haskell庫,相對於獨立的單元測試,QuickCheck描述函數抽象特性並生成測試來驗證這些特性。
生成隨機數
定義一個表達如何生成隨機數的協議:
protocol Arbitrary {
static func arbitrary() -> Self
}
擴展Int類型,實現Arbitrary協議,在這裏使用了arc4random函數來生成隨機數然後轉換成Int類型,事實上應該要能夠生成負整數的:
extension Int: Arbitrary {
static func arbitrary() -> Int {
Int(arc4random())
}
static func random(from: Int, to: Int) -> Int {
from + arbitrary()%(to - from)
}
}
生成隨機字符,僅限大寫字母:
extension Character: Arbitrary {
static func arbitrary() -> Character {
Character(Unicode.Scalar(Int.random(from: 65, to: 90))!)
}
}
生成隨機字符串,在這裏只生成長度0~40的隨機大寫字母的字符串:
extension String: Arbitrary {
static func arbitrary() -> String {
let randomLength = Int.random(from: 0, to: 40)
let randomCharacters = (0..<randomLength).map { _ in Character.arbitrary() }
return String(randomCharacters)
}
}
實現check函數
實現第一個版本的檢驗函數:
func check1<A: Arbitrary>(message: String, property: (A) -> Bool) {
let numberOfIterations = 100
for _ in 0..<numberOfIterations {
let value = A.arbitrary()
guard property(value) else {
print("\(message) 沒有通過測試:\(value)")
return
}
}
print("\(message) 通過了\(numberOfIterations)次測試")
}
下面使用check1函數來檢查:
extension CGSize: Arbitrary {
var area: CGFloat { width*height }
static func arbitrary() -> CGSize {
CGSize(width: Int.arbitrary(), height: Int.arbitrary())
}
}
check1(message: "CGSize的面積最小是0") { (size: CGSize) in size.area >= 0 }
縮小範圍
通常反例所處的範圍越小越容易定位到失敗的原因,所以原則上對失敗的輸入進行不斷縮減,並重新測試。
因此需要定義一個Smaller協議用於縮減範圍:
protocol Smaller {
static func smaller() -> Self?
}
由於縮小數據範圍本身不是很明確,例如無法對空數組縮小隻能返回nil。
對所需數據類型實現Smaller協議:
extension Int: Smaller {
func smaller() -> Int? {
self == 0 ? nil : self/2
}
}
extension String: Smaller {
func smaller() -> String? {
isEmpty ? nil : String(dropFirst())
}
}
爲了使check函數中隨機生成的數據能夠縮小,可以將重新定義Arbitrary協議繼承Smaller協議:
protocol Arbitrary: Smaller {
static func arbitrary() -> Self
}
反覆縮小範圍
定義一個遞歸函數iterateWhile,只要條件成了就反覆調用自身:
func iterateWhile<A>(condition: (A) -> Bool, inital: A, next: (A) -> A?) -> A {
guard let vaule = next(inital) else {
return inital
}
return iterateWhile(condition: condition, inital: vaule, next: next)
}
通過iterateWhile函數反覆縮小檢查函數中反例數據的範圍:
func check2<A: Arbitrary & Smaller>(message: String, property: (A) -> Bool) {
let numberOfIterations = 100
for _ in 0..<numberOfIterations {
let value = A.arbitrary()
guard property(value) else {
let smaller = iterateWhile(condition: { !property($0) }, inital: value, next: { $0.smaller() })
print("\(message) 沒有通過測試:\(smaller)")
return
}
}
print("\(message) 通過了\(numberOfIterations)次測試")
}
隨機數組
寫一個快速排序的函數:
func qsort(array: [Int]) -> [Int] {
var data = array
if data.isEmpty {
return []
}
let pivot = data.removeFirst()
let lesser = data.filter { $0 >= pivot }
let gretter = data.filter { $0 < pivot }
return qsort(array: lesser) + [pivot] + qsort(array: gretter)
}
現在想要使用check2函數來判斷這個快速排序函數與內置的sort函數是否有區別,那麼需要讓[Int]遵守Arbitrary、Smaller這兩個協議:
extension Array: Arbitrary, Smaller where Element: Arbitrary {
static func arbitrary() -> Array<Element> {
let randomLength = Int.random(from: 0, to: 50)
return (0..<randomLength).map { _ in Element.arbitrary() }
}
func smaller() -> Array<Element>? {
if self.isEmpty { return nil }
return Array(self.dropFirst())
}
}
調用check2函數對前邊的快速排序函數進行驗證:
check2(message: "qsort函數和Array內置sort函數效果一致") { (array: [Int]) in qsort(array: array) == array.sorted(by: <) }
check2函數需要類型A遵守Arbitrary、Smaller兩個協議。換一種方式,可以將必要的smaller、arbitrary函數作爲參數傳入。
首先定義包含必要的smaller、arbitrary函數的輔助結構體:
struct ArbitraryInstance<T> {
let arbitrary: () -> T
let smaller: (T) -> T?
}
接着寫一個接受ArbitraryInstance作爲參數的輔助函數checkHelper,這個函數的定義參照了check2函數,不同點就是arbitrary和smaller函數定義的位置。check2中被泛型的協議所約束,而checkHelper中則通過ArbitraryInstance結構體顯式傳遞:
func checkHelper<A>(arbitraryInstance: ArbitraryInstance<A>, message: String, property: (A) -> Bool) {
let numberOfIterations = 100
for _ in 0..<numberOfIterations {
let value = arbitraryInstance.arbitrary()
guard property(value) else {
let smallerValue = iterateWhile(condition: property, inital: value, next: arbitraryInstance.smaller)
print("\(message) 沒有通過測試:\(smallerValue)")
return
}
}
print("\(message) 通過了\(numberOfIterations)次測試")
}
顯式將所需信息作爲參數傳遞,而不是定義於協議中。這樣做靈活性更高,不依賴Swift推斷所需信息,完全自己控制這一切。
可是使用checkHelper函數重新定義check2函數:
func check3<X: Arbitrary>(message: String, property: ([X]) -> Bool) {
let instance = ArbitraryInstance(arbitrary: Array<X>.arbitrary) { $0.smaller() }
checkHelper(arbitraryInstance: instance, message: message, property: property)
}
終於可以運行check3函數驗證快速排序了:
check3(message: "qsort函數和Array內置sort函數效果一致") { (x: [Int]) -> Bool in qsort(array: x) == x.sorted(by: <) }