[swift-users] Range<UInt64>.count can abort

[davesweeris at mac.com]

Is there any reason that Distance has to be a simple Int? Since it’s defined per-type, UInt64 could use a custom struct without impacting the performance of other types:
struct UInt64Distance : Comparable { //I'm not sure what else it needs to conform to 
    var distance: UInt64
    var isPositive: Bool
}
func == (lhs: UInt64Distance, rhs: UInt64Distance) -> Bool {
    return lhs.distance == rhs.distance && lhs.isPositive == rhs.isPositive
}
func < (lhs: UInt64Distance, rhs: UInt64Distance) -> Bool {
    switch (lhs.isPositive, rhs.isPositive) {
    case (false, false): return lhs.distance > rhs.distance
    case (false, true ): return true
    case (true,  false): return false
    case (true,  true ): return lhs.distance < rhs.distance
    }
}
... //The rest of `Comparable`

(I’m assuming it’d need `Comparable`, but a quick glance around with cmd-click in Xcode didn’t tell me what, if any, protocols `Distance` needs to conform to.)

- Dave Sweeris

> On Apr 15, 2016, at 6:26 PM, Dmitri Gribenko via swift-users <swift-users at swift.org> wrote:
> 
> On Fri, Apr 15, 2016 at 4:23 PM, Jens Alfke via swift-users
> <swift-users at swift.org> wrote:
>> I’m writing some code that deals with ranges of “sequence numbers”, which
>> are consecutively-assigned positive 64-bit integers, thus Range<UInt64> in
>> Swift. I’m having trouble handling half-open ranges with no maximum, which
>> are very commonly used for representing all items created since a certain
>> time.
>> 
>> The trouble is, if I represent these with Ranges whose endIndex is
>> UInt64.max, those ranges tend to bomb:
>> 
>> let r: Range<UInt64> = 5..<UInt64.max
>> r.count   // FATAL ERROR
>> 
>> 
>> The problem is that Range.count’s type is Element.Distance, and
>> UInt64.Distance is a typealias of … Int. Huh? It’s pretty clear that the
>> distance between two UInt64s can’t be represented by a signed Int64. Why
>> isn’t Distance UInt64?
> 
> Hi Jens,
> 
> The distance is signed so that we can represent negative distances.
> r.distance(from:to:) should be able to measure distances backwards.
> 
> We are aware of this issue, but we don't know of a good solution.
> We'd appreciate your suggestions.
> 
> Dmitri
> 
> -- 
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
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