add efficient findfirst method for StepRange

[marekkukan-tw]

Several functions like sum, reverse, etc. have efficient (O(1)) methods for ranges implemented in Base.
This PR adds such method for findfirst, where the predicate is equality, and the iterable is a StepRange.

before

julia> @btime findfirst(==(10^5), 1:3:10^6)
  27.386 μs (3 allocations: 64 bytes)
33334

after

julia> @btime findfirst(==(10^5), 1:3:10^6)
  15.881 ns (0 allocations: 0 bytes)
33334

[JeffBezanson]

Thanks, @marekkukan-tw , and welcome.

The issue here is that Fix2 is not defined yet when range.jl is loaded. For now, you could put the method in array.jl with other findfirst methods. Please also add a test in test/ranges.jl, or confirm that the new method is already exercised by existing tests.

[marekkukan-tw]

Thank you @JeffBezanson .
I used Revise.track(Base) and didn't realize there was this issue.
This time I ran make clean testall so hopefully it's OK now.

[JeffBezanson]

This should perhaps be restricted to Integer types. It's tricky to know whether this formula will hit a floating point value exactly; see for example the _in_range function in this file. You have to check that the value in the range actually equals what you're looking for. Restricting to Integers also makes it easy to allow different types; the signature can be

findfirst(p::Union{Fix2{typeof(isequal),<:Integer},Fix2{typeof(==),<:Integer}}, r::StepRange{<:Integer,S}) where {S}

[marekkukan-tw]

My actual use case was with DateTime objects, for example:

julia> @btime findfirst(==(DateTime(2019, 1, 23)), DateTime(2017):Second(1):DateTime(2020)) # before
  53.385 ms (3 allocations: 64 bytes)
64972801

julia> @btime findfirst(==(DateTime(2019, 1, 23)), DateTime(2017):Second(1):DateTime(2020)) # after
  64.659 ns (0 allocations: 0 bytes)
64972801

The fact that T occurs in StepRange{T,S} should restrict it to discrete types.

[JeffBezanson]

Good point. It's too bad that the method won't match for different types of integers, but I guess this is ok for now.

[mcabbott]

It would be nice if something like this method, or even simpler, was called for UnitRanges too:

julia> @btime findfirst(isequal(999), 1:1000);
  563.616 ns (0 allocations: 0 bytes)

julia> @btime findfirst(isequal(999), Base.OneTo(1000));
  563.892 ns (0 allocations: 0 bytes)

julia> @btime findfirst(isequal(999), 1:1:1000); # this method
  0.030 ns (0 allocations: 0 bytes)

[nalimilan]

This causes issues for ranges with a Year step, see #35203.