Slow Swift Arrays和Strings性能

Question

这是两个非常相似的Levenshtein Distance algorithms.

Swift实施:https: //gist.github.com/bgreenlee/52d93a1d8fa1b8c1f38b

并Objective-C实施:https: //gist.github.com/boratlibre/1593632

在swift一个是慢得多然后ObjC执行我已经派几个小时,使其速度更快,但......好像Swift阵列和Strings不一样快的操作objC.

2000年的random Strings计算Swift实施速度大约慢了100(!!!)倍ObjC.

老实说,我不知道什么可能是错的,因为即使这部分也很快

func levenshtein(aStr: String, bStr: String) -> Int {
// create character arrays
let a = Array(aStr)
let b = Array(bStr)
...

比整个算法慢几倍 Objective C

有谁知道如何加速swift计算？

先感谢您!

附加

在所有建议的改进之后,swift代码看起来像这样.它在发布配置中比ObjC慢4倍.

import Foundation
class Array2D {
    var cols:Int, rows:Int
    var matrix:UnsafeMutablePointer<Int>


    init(cols:Int, rows:Int) {
        self.cols = cols
        self.rows = rows
        matrix = UnsafeMutablePointer<Int>(malloc(UInt(cols * rows) * UInt(sizeof(Int))))
        for i in 0...cols*rows {
            matrix[i] = 0
        }

    }

    subscript(col:Int, row:Int) -> Int {
        get {
            return matrix[cols * row + col] as Int
        }
        set {
            matrix[cols*row+col] = newValue
        }
    }

    func colCount() -> Int {
        return self.cols
    }

    func rowCount() -> Int {
        return self.rows
    }
}

extension String {
    func levenshteinDistanceFromStringSwift(comparingString: NSString) -> Int {
        let aStr = self
        let bStr = comparingString

//        let a = Array(aStr.unicodeScalars)
//        let b = Array(bStr.unicodeScalars)

        let a:NSString = aStr
        let b:NSString = bStr

        var dist = Array2D(cols: a.length + 1, rows: b.length + 1)



        for i in 1...a.length {
            dist[i, 0] = i
        }

        for j in 1...b.length {
            dist[0, j] = j
        }

        for i in 1...a.length {
            for j in 1...b.length {
                if a.characterAtIndex(i-1) == b.characterAtIndex(j-1) {
                    dist[i, j] = dist[i-1, j-1]  // noop
                } else {
                    dist[i, j] = min(
                        dist[i-1, j] + 1,  // deletion
                        dist[i, j-1] + 1,  // insertion
                        dist[i-1, j-1] + 1  // substitution
                    )
                }
            }
        }

        return dist[a.length, b.length]

    }
    func levenshteinDistanceFromStringObjC(comparingString: String) -> Int {
        let aStr = self
        let bStr = comparingString
        //It is really strange, but I should link Objective-C coz dramatic slow swift performance
        return aStr.compareWithWord(bStr, matchGain: 0, missingCost: 1)

    }

}

malloc的？的NSString？最后4倍减速？有人需要迅速吗？

Answer 1

Swift代码比Objective-C代码慢的原因有很多.我通过比较两个固定字符串100次来做一个非常简单的测试用例.

• Objective-C代码:0.026秒
• Swift代码:3.14秒

第一个原因是Swift Character代表一个"扩展字形集群",它可以包含几个Unicode代码点(例如"标志").这使得字符串的分解变得缓慢.另一方面,Objective-C NSString将字符串存储为UTF-16代码点序列.

如果你更换

let a = Array(aStr)
let b = Array(bStr)

通过

let a = Array(aStr.utf16)
let b = Array(bStr.utf16)

这样Swift代码也适用于UTF-16序列,然后时间下降到1.88秒.

二维数组的分配也很慢.分配单个一维数组更快.我在Array2D这里找到了一个简单的类:http: //blog.trolieb.com/trouble-multidimensional-arrays-swift/

class Array2D {
    var cols:Int, rows:Int
    var matrix: [Int]


    init(cols:Int, rows:Int) {
        self.cols = cols
        self.rows = rows
        matrix = Array(count:cols*rows, repeatedValue:0)
    }

    subscript(col:Int, row:Int) -> Int {
        get {
            return matrix[cols * row + col]
        }
        set {
            matrix[cols*row+col] = newValue
        }
    }

    func colCount() -> Int {
        return self.cols
    }

    func rowCount() -> Int {
        return self.rows
    }
}

在代码中使用该类

func levenshtein(aStr: String, bStr: String) -> Int {
    let a = Array(aStr.utf16)
    let b = Array(bStr.utf16)

    var dist = Array2D(cols: a.count + 1, rows: b.count + 1)

    for i in 1...a.count {
        dist[i, 0] = i
    }

    for j in 1...b.count {
        dist[0, j] = j
    }

    for i in 1...a.count {
        for j in 1...b.count {
            if a[i-1] == b[j-1] {
                dist[i, j] = dist[i-1, j-1]  // noop
            } else {
                dist[i, j] = min(
                    dist[i-1, j] + 1,  // deletion
                    dist[i, j-1] + 1,  // insertion
                    dist[i-1, j-1] + 1  // substitution
                )
            }
        }
    }

    return dist[a.count, b.count]
}

测试用例中的时间下降到0.84秒.

我在Swift代码中找到的最后一个瓶颈是min()函数.Swift库具有内置min()函数,速度更快.因此,只需从Swift代码中删除自定义函数,就可以将测试用例的时间缩短到0.04秒,这几乎与Objective-C版本一样好.

附录:使用Unicode标量似乎甚至更快:

let a = Array(aStr.unicodeScalars)
let b = Array(bStr.unicodeScalars)

并且具有与Emojis等代理对一起正常工作的优点.