String.hashValue not unique after reset app when build in Xcode 10

Question

I have a "get hash of string by String.hashValue" code, that I added it below. This code worked well in Xcode 9.4.1.

Worked well means that whenever I close app and re-open it, the result of hashValue is same (unique)

private func cacheName(of url: String) -> String {
    // The url is url of a png image, for example www.imageurl.com/image.png
    return "\(url.hashValue)"
}

When I build my Project in Xcode 10 the result changes everytime I restart the app (close and open app again). The version of iOS, device, Swift version is same. So I think the problem is Xcode 10 has change something that effect to the hashValue (maybe configure when build app ??)

If I use the String.hash instead of, it works well. But in the previous version, I saved the hashValue result, so I don't want to change it.

How can I keep the result of String.hashValue unique in every time. Or any suggestion would be appreciated

https://developer.apple.com/documentation/swift/hashable/1540917-hashvalue ALWAYS (since Swift 1.0) stated that “Hash values are not guaranteed to be equal across different executions of your program. Do not save hash values to use during a future execution." — Martin R, Sep 21 '18 at 09:26

ielyamani · Accepted Answer · 2019-03-11T21:56:00.710

23

Swift 4.2 has implemented SE-0206: Hashable Enhancements. This introduces a new Hasher struct that provides a randomly seeded hash function. That's why the hashing results differ everytime (since the seed is random). You can find the implementation of the Hasher struct, with the generation of a random seed, here.

If you want a stable hash value associated to a String, accross devices and app lauches, you could use this solution by Warren Stringer:

let str = "Hello"

func strHash(_ str: String) -> UInt64 {
    var result = UInt64 (5381)
    let buf = [UInt8](str.utf8)
    for b in buf {
        result = 127 * (result & 0x00ffffffffffffff) + UInt64(b)
    }
    return result
}

strHash(str)     //177798404835661

Or have these couple of extensions defined on String:

extension String {
    var djb2hash: Int {
        let unicodeScalars = self.unicodeScalars.map { $0.value }
        return unicodeScalars.reduce(5381) {
            ($0 << 5) &+ $0 &+ Int($1)
        }
    }

    var sdbmhash: Int {
        let unicodeScalars = self.unicodeScalars.map { $0.value }
        return unicodeScalars.reduce(0) {
            (Int($1) &+ ($0 << 6) &+ ($0 << 16)).addingReportingOverflow(-$0).partialValue
        }
    }
}

"Hello".djb2hash    //210676686969
"Hello".sdbmhash    //5142962386210502930

(This is executed on Xcode 10, Swift 4.2)

edited Mar 11 '19 at 21:56

answered Sep 21 '18 at 09:25

ielyamani

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Thanks for comment. But I tested it in Swift 4 and it only error in Xcode 10 – Quoc Nguyen Sep 21 '18 at 13:43
@QuocNguyen what is the error message you're getting? And what code have you tried? – ielyamani Sep 21 '18 at 13:49
the sdbmhash throws `thread 1 EXC_BREAKPOINT (code=1, subcode=0xXXXXXXXX)` in swift 4 – sudoExclaimationExclaimation Mar 11 '19 at 21:31
@PranoyC I can't reproduce the error on Swift `4.2` as mentioned in the answer – ielyamani Mar 11 '19 at 21:35
Strangely enough, when I test using strings like "hello" or "blah", it works fine. But if I test using "gamedev", it makes it crash with that error. I am relatively new to swift, so I don't understand the method well enough to determine why though. Can you test using "gamedev"? – sudoExclaimationExclaimation Mar 11 '19 at 21:41
@PranoyC: Well spotted! That was due to integer overflow. I've updated the answer – ielyamani Mar 11 '19 at 21:56
2

@ielyamani thank you! Works now! Such a coincidence that it failed on the first string I tested on lol – sudoExclaimationExclaimation Mar 11 '19 at 22:02
Slightly more condensed version of Warren's answer as a computed property on a String extension: `[UInt8](utf8).reduce(UInt64(5381)) { 127 * ($0 & 0x00ffffffffffffff) + UInt64($1) }` – Jacob Bartlett Feb 08 '22 at 11:18

String.hashValue not unique after reset app when build in Xcode 10

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