Approximate search now returns nothing when no match is found

[jakobnissen]

Approximate search functions now return nothing when no match is found. These are:

• approxsearch
• approxrsearch
• approxsearchindex
• approxrsearchindex

Behaviour is otherwise unaltered. Specifically, search for an empty sequence is valid, and returns an empty range, just like before. Weird, but it's consistent with how string search works in Base.

[codecov]

Codecov Report

Merging #176 (3200f9c) into v3 (a8e5b48) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               v3     #176      +/-   ##
==========================================
+ Coverage   82.16%   82.24%   +0.08%     
==========================================
  Files          31       31              
  Lines        2159     2169      +10     
==========================================
+ Hits         1774     1784      +10     
  Misses        385      385              

Flag	Coverage Δ
unittests	82.24% <100.00%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/search/approx.jl	95.72% <100.00%> (+0.39%)	⬆️

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[jakobnissen]

Okay, this is good to go. Leaving it up for some days or weeks if anyone wants to review it.

[CiaranOMara]

Is it intended that the isempty checks return a range?

I haven't thought this through, though; I wonder whether a range result could be useful for chaining/currying searches in that the result provides the start position for the next search, removing the need to track position?

findfirst(seq, vague_query) |> 
    (r) -> findfirst(seq, query, first(r), last(r))  |> 
    (r) -> findfirst(seq, specific_query, first(r), last(r))

[jakobnissen]

@CiaranOMara

Is it intended that the isempty checks return a range?

Yes. It's weird, but this is what Base does with strings (and it's also what BioSequences did until now).
A range results for a failure of a match can be useful in some cases. But it can also be a bit of a trap. If it returns nothing, then the user can implement whatever they want upon a non-match, including continuing to another search.

Merging this for now.