Implementing Java String's hashCode() method in Swift 3

Question

I am creating an iOS version of an existing Android app. On the Android side, hashCode() of a String (username) is sent to the server and based on that hash, a JSON object is returned.

On Swift, I tried hash and hashValue properties but both of them produces values that are different from their Android counterpart.

So I decided to write my own implementation based on Java's implementation:

int h = hash;
if (h == 0 && value.length > 0) {
  char val[] = value;
  for (int i = 0; i < value.length; i++) {
    h = 31 * h + val[i];
  }
  hash = h;
}
return h;

But when I write the above implementation in Swift, I get overflow crash. Can anybody help me here?

Thanks in advance.

Here is the Swift implementation:

I first had to write a Character extension that would return Ascii value of the character:

extension Character {
    var asciiValue: UInt32? {
        return String(self).unicodeScalars.filter{$0.isASCII}.first?.value
    }
}

Then I created a String extension with: 1. a property that returns Ascii values of each character in the String. 2. a hash method to return the hash (copying the Java code)

extension String {
    var asciiArray: [UInt32] {
        return unicodeScalars.filter{$0.isASCII}.map{$0.value}
    }
    func myHash() -> Int {
        var h = 0 as Int!
        for i in 0..<asciiArray.count {
            h = 31*h! + Int(array[i])
        }
        return h!
    }
}

Answer 1

As the post is still missing an answer, here is the working code:

1) Write an extension for Characters, to map their ascii representation to an int - we need UInt32, as Java uses for the hashing a 32 bit system, not 64 bit.

2) Make sure our represented string does ONLY contain characters, which are mappable to ASCII

3) Now perform the mentioned hashing function

int h = 0;
if (h == 0 && value.length > 0) {
  char val[] = value;
  for (int i = 0; i < value.length; i++) {
    h = 31 * h + val[i];
  }
  hash = h;
}
return h;

So, we will get the following code snippet, which could be used in your projects.

// 1) Here is our Character extension
extension Character {
    var asciiValue: UInt32? {
        return String(self).unicodeScalars.filter{$0.isASCII}.first?.value
    }
}

extension String {
    // 2) ascii array to map our string
    var asciiArray: [UInt32] {
        return unicodeScalars.filter{$0.isASCII}.map{$0.value}
    }

    // this is our hashCode function, which produces equal output to the Java or Android hash function
    func hashCode() -> Int32 {
        var h : Int32 = 0
        for i in self.asciiArray {
            h = 31 &* h &+ Int32(i) // Be aware of overflow operators, 
        }
        return h
    }
}

Answer 2

By default, swift throws an exception on integer overflow (unlike C or the standard Java integer operations). In order to preclude throwing the exception you have to use the special functions provided for that, like 'addWithOverflow'.

Alternatively you can build using the
-Ounchecked flag