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IntSet: O(1) maximum/minimum/issorted #22856

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Jul 20, 2017
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IntSet: O(1) maximum/minimum/issorted #22856

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11 changes: 11 additions & 0 deletions base/intset.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -178,6 +178,12 @@ done(s::IntSet, i) = i <= 0


@noinline _throw_intset_notempty_error() = throw(ArgumentError("collection must be non-empty"))

function first(s::IntSet)
idx = findfirst(s.bits)
idx == 0 ? _throw_intset_notempty_error() : idx
end

function last(s::IntSet)
idx = findprev(s.bits, length(s.bits))
idx == 0 ? _throw_intset_notempty_error() : idx
Expand Down Expand Up @@ -239,3 +245,8 @@ function hash(s::IntSet, h::UInt)
end
h
end

minimum(s::IntSet) = first(s)
maximum(s::IntSet) = last(s)
extrema(s::IntSet) = (first(s), last(s))
issorted(s::IntSet) = true
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What does this mean for a Set?

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Like for generic iterators: if the values yielded by iteration are sorted.

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Okay, the documentation specifically says that it checks if a vector is sorted. Should that be loosened?

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Good catch! yes I would say it should be loosened, as long as the method is defined for generic iterators (which makes sense for me).

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Is the order a set is iterated in even defined i. e does the order have to be the same every time even if the set is unchanged? Defining issorted for something unordered feels a bit iffy and perhaps mixes implementation with public API.

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Is the order a set is iterated in even defined

I guess yes for Set, but for IntSet it's always in sorted order, and I believe it will stay this way. I don't know whether it's documented though. This added method here is only an optimization.

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Same as the situation for Dict, SortedDict, and OrderedDict. Some sets might be sorted, others not.

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Yes but is the order that Sets and IntSets are iterated guaranteed to stay fixed? Otherwise being sorted is not a property of the object. For the other structures you mentioned, there is clearly a well defined iteration order so issorted makes sense.

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are any of these tested?

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For IntSet, the iteration order is guaranteed to stay fixed; if the implementation changes, then the issorted method will have to be updated accordingly.