How can we reinterpret a double or float as NSUInteger to create hash?

Question

This is my isEqual and hash custom operator

- (BOOL)isEqual:(id)object;
{
    BGSearchParameter * theOther = (BGSearchParameter *)object;

    BOOL isTheOtherEqual;
    isTheOtherEqual = isTheOtherEqual && [self.Location isEqual:theOther.Location];
    isTheOtherEqual = isTheOtherEqual && [self.keyword isEqual:theOther.keyword];
    isTheOtherEqual = isTheOtherEqual && (self.Distance == theOther.Distance);
    isTheOtherEqual = isTheOtherEqual && (self.SortByWhat == theOther.SortByWhat);
    isTheOtherEqual = isTheOtherEqual && (self.startFrom == theOther.startFrom);
    isTheOtherEqual = isTheOtherEqual && (self.numberOfIDstoGrab == theOther.numberOfIDstoGrab);

    return isTheOtherEqual;
}
- (NSUInteger)hash
{
    NSUInteger returnValue=0;
    returnValue ^= self.Location.hash;
    returnValue ^= self.keyword.hash;

    return returnValue;
}

That one does the job. However, say I want to incorporate the distance and the startfrom into the hash.

I guess I will simply add:

returnValue ^= self.Distance;

It's an error because it's not compatible.

So what should I do instead?

Answer 1

I ended up turning the number into NSNumber and got the hash:

   returnValue ^= @(self.Distance).hash;
   returnValue ^= @(self.SortByWhat).hash;
   returnValue ^= @(self.startFrom).hash;
   returnValue ^= @(self.numberOfIDstoGrab).hash;

Martin answer is good. However, the result should be the same anyway and I don't want to implement another complicated function.

Answer 2

This is what CFNumber/NSNumber use as hash value for float and double values, see for example ForFoundationOnly.h in Mac OS X 10.7.5 Source.

#define HASHFACTOR 2654435761U

CF_INLINE CFHashCode _CFHashDouble(double d) {
    double dInt;
    if (d < 0) d = -d;
    dInt = floor(d+0.5);
    CFHashCode integralHash = HASHFACTOR * (CFHashCode)fmod(dInt, (double)ULONG_MAX);
    return (CFHashCode)(integralHash + (CFHashCode)((d - dInt) * ULONG_MAX));
}

CFHashCode is defined as

typedef unsigned long CFHashCode;

Answer 3

Try this:

static NSUInteger doubleHash(float value) {
    return *(NSUInteger *)&value;//gets bits
}

Notice the conversion to float. It decreases precision, but sizeof(float)==sizeof(NSUInteger).