Value of `underestimatedCount` for lazy sequences

I'm using the underestimatedCount computed property of sequences to reserve capacity of a container before operations such as map. The underestimatedCount is used in the same way in the standard library, such as the Array initializer that use a sequence as input:

let sequence = [1, 2, 3]
let result = Array(sequence)  // Internally use `underestimatedCount`
                              // to avoid reallocatinons

Some sequences forward the underestimatedCount, such as LazySequence:

let seq = [1, 2, 3].lazy
print(seq.underestimatedCount)  // Prints '3'

Moreover, the Collection default implementation is the collection's count value, so a ReversedCollection forward the underestimatedCount automatically:

let seq = [1, 2, 3].reversed()
print(seq.underestimatedCount)  // Prints '3'

However, I found that some sequences does not forward the underestimatedCount such as:

let seq = [1, 2, 3].enumerated()
print(seq.underestimatedCount)  // Prints '0'

And some other combinations neither:

let seq = [[1, 2], [3]].lazy.joined()
print(seq.underestimatedCount)  // Prints '0', removing `lazy` prints `3`

Is this intended behavior? This could lead to poorly efficient code causing many unnecessary array reallocations.

There are cases where underestimatedCounts are improved in the past. If you think there are reasonable (and fast) implementations, you can file a bug report or just straight up pr.

That said, the lazy.joined case returning 0 sounds about right.

1 Like

thanks for the answer, could you expand on why returning underestimatedCount = 0 is correct for a LazySequence<FlattenSequence<Base>> but not for a FlattenSequence<Base>?

To me this is counterintuitive, LazySequence forwards the underlying base sequence underestimatedCount, but when the base is a FlattenSequence it appears that it is not the case.

I tried to fill bug reports in the past (here and here) but I never had an answer (excepted an automatic response of a bot). Since I don't know for sure that this is a bug, I feel that discussing it here in Swift Forums is more appropriated, I don't want to waste the time of Swift developers for a potential mistake on my side.

Oh right, I forgot FlattenSequence does have non-zero underestimated counts, albeit it doesn’t look like an O(1) implementation. I’d suspect some generic shenanigans since the following code exhibits similar behaviour.

func x<T : Collection>(_ t: T) -> Int {
    t.underestimatedCount
}

x([[1, 2]].joined()) // 0
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