Project Euler #8 - Largest product in a series I just finished Project Euler #8 in Swift, and since there is not any version yet on Code Review, I would like to have some comments on what I did to try to improve it.

The four adjacent digits in the 1000-digit number that have the
greatest product are 9 × 9 × 8 × 9 = 5832.
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450
Find the thirteen adjacent digits in the 1000-digit number that have the greatest
product. What is the value of this product?

import Foundation

extension String {
    subscript (r: Range&lt;Int&gt;) -&gt; String {
        get {
            let subStart = advance(self.startIndex, r.startIndex, self.endIndex)
            let subEnd = advance(subStart, r.endIndex - r.startIndex, self.endIndex)
            return self.substringWithRange(Range(start: subStart, end: subEnd))
        }
    }
    func substring(from: Int, length: Int) -&gt; String {
        let end = from + length
        return self[from..&lt;end]
    }
}

func largestProduct(digits:Int) -&gt; Int {
    
    let str = &quot;7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450&quot;
    
    var maxNumber = 0
    let maxLength = countElements(str) - digits
    for var i = 0; i &lt; maxLength; ++i {
        
        let strNumber = str.substring(i, length: digits)
        
        var number = 1
        for char in strNumber {
            let charNumber = String(char).toInt()!
            number *= charNumber
        }
        
        if number &gt; maxNumber {
            maxNumber = number
        }
    }
    
    return maxNumber
}

func printTimeElapsedWhenRunningCode(operation:(Int) -&gt; Int) {
    let startTime = CFAbsoluteTimeGetCurrent()
    let number = operation(4)
    println(number)
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    println(&quot;Time elapsed : \(timeElapsed) s&quot;)
}

printTimeElapsedWhenRunningCode(largestProduct)

The code executes in 0.0799770355224609 s
Let's start with a performance improvement: Your method repeatedly extracts substrings from the given digit string (which is a costly operation with Swift
strings), and then repeatedly converts the characters in a substring to an integer.
It is better to convert the entire string to an array of integers once with
a single string traversal:

func largestProduct(numDigits : Int) -&gt; Int {

    let str = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
    let digits = Array(str).map { String($0).toInt()! }

    var maxNumber = 0
    let maxLength = digits.count - numDigits
    for var i = 0; i &lt; maxLength; ++i {

        var number = 1
        for var j = i; j &lt; i + numDigits; j++ {
            number *= digits[j]
        }

        if number &gt; maxNumber {
            maxNumber = number
        }
    }

    return maxNumber
}


On my computer (with the program compiled in Release mode), this reduced the
time to compute largestProduct(13) from 0.065 seconds to 0.002 seconds.

(The rest of this review is about the code organisation and might be opinion based.)

I would pass the given digits as an argument to the largestProduct() function.
One advantage is that it is easier to add test cases.

for var i = 0; i &lt; maxLength; ++i {  }


can be written more swifty using a range:

for i in 0 ..&lt; maxLength { }


and the multiplication of the digits to a number can be done with reduce().

Then your function would look like this:

func largestProduct(digits : [Int], numDigits : Int) -&gt; Int {

    var maxNumber = 0
    let maxLength = digits.count - numDigits
    for i in 0 ..&lt; maxLength {
        let number = reduce(i ..&lt; i + numDigits, 1) { $0 * digits[$1] }
        if number &gt; maxNumber {
            maxNumber = number
        }
    }

    return maxNumber
}


Your printTimeElapsedWhenRunningCode() function is not optimal for reuse.
It is restricted to functions taking an Integer argument and returning
an integer, and the argument (4 or 13 in this case) is part of the function.

I would change that to take a ()-&gt;Void closure:

func printTimeElapsedWhenRunningCode(operation:()-&gt;Void) {
    let startTime = CFAbsoluteTimeGetCurrent()
    operation()
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    println("Time elapsed : \(timeElapsed) s")
}


and define a function doing all the work:

func euler8()  {
    let str = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"

    let digits = Array(str).map { String($0).toInt()! }

    let result = largestProduct(digits, 13)
    println(result)
}


Now you can measure the time with:

printTimeElapsedWhenRunningCode(euler8)


and the printTimeElapsedWhenRunningCode function is fully reusable.