implement bandwidths for OneElement

[max-vassili3v]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.81%. Comparing base (47c15ab) to head (9332222).
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@@            Coverage Diff             @@
##           master     #447      +/-   ##
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+ Coverage   89.61%   89.81%   +0.20%     
==========================================
  Files          25       25              
  Lines        3571     3642      +71     
==========================================
+ Hits         3200     3271      +71     
  Misses        371      371              

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[dlfivefifty]

This test is a tautology since BandedMatrix calls bandwidths(o) to determine its bandwidths. I think the following will be better:

Suggested change

	@test bandwidths(o) == bandwidths(BandedMatrix(o))
	@test bandwidths(o) == bandwidths(sparse(o))

You'll need to add SparseArrays to the using statement

[max-vassili3v]

julia> o = OneElement(1,5,5)
5-element OneElement{Int64, 1, Tuple{Int64}, Tuple{Base.OneTo{Int64}}}:
 ⋅
 ⋅
 ⋅
 ⋅
 1

julia> bandwidths(sparse(o))
(4, 0)

Shouldn't it be (4, -4) or am I mistaken?

[dlfivefifty]

Ah probably it’s not overridden yet for SparseVector . You could add that too if you want

[max-vassili3v]

julia> o = OneElement(1,(5,1),(5,2))
5×2 OneElement{Int64, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}:
 ⋅  ⋅
 ⋅  ⋅
 ⋅  ⋅
 ⋅  ⋅
 1  ⋅

julia> bandwidths(sparse(o))
(4, -2)

julia> o = OneElement(1,(5,1),(5,3))
5×3 OneElement{Int64, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}:
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 1  ⋅  ⋅

julia> bandwidths(sparse(o))
(4, -3)

There seems to be a similar behaviour for matrices where the width bottlenecks the negative bandwidth.

[dlfivefifty]

OK. I don't think I wrote that code so it must be a bug. Do you think you can fix it?

It should be here:

https://github.com/JuliaLinearAlgebra/BandedMatrices.jl/blob/master/ext/BandedMatricesSparseArraysExt.jl