Add option to decompress into a triangle only

[gdalle]

• Add uplo = :U/:L/:UL parameter to decompress! and decompress_single_color! to allow picking the triangle for symmetric matrices
• Implement the corresponding methods
• Add tests for full decompression and for single color, structure into subtests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.86%. Comparing base (87508b9) to head (6c0f721).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   99.85%   99.86%           
=======================================
  Files          10       10           
  Lines         675      715   +40     
=======================================
+ Hits          674      714   +40     
  Misses          1        1           

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[gdalle]

@amontoison you asked why uplo is not a kwarg. The reason is that I want to be able to dispatch on whether it is present or not, and you cannot dispatch on kwargs (not even on their presence or absence).

My reasoning is that between the following methods, the first one is more efficient (leads to predictable memory accesses) and should thus be preferred when uplo is not specified. What do you think?

function decompress!(
    A::SparseMatrixCSC{R}, B::AbstractMatrix{R}, result::StarSetColoringResult
) where {R<:Real}
    @compat (; S, compressed_indices) = result
    check_same_pattern(A, S)
    nzA = nonzeros(A)
    for k in eachindex(nzA, compressed_indices)
        nzA[k] = B[compressed_indices[k]]
    end
    return A
end

function decompress!(
    A::SparseMatrixCSC{R}, B::AbstractMatrix{R}, result::StarSetColoringResult, uplo::Symbol
) where {R<:Real}
    @compat (; S, compressed_indices) = result
    check_same_pattern(A, S)
    rvA = rowvals(A)
    nzA = nonzeros(A)
    for j in axes(S, 2)
        for k in nzrange(S, j)
            i = rvA[k]
            if in_triangle(i, j, uplo)
                nzA[k] = B[compressed_indices[k]]
            end
        end
    end
    return A
end

[amontoison]

You will not gain a lot if you do that:

function decompress!(
    A::SparseMatrixCSC{R}, B::AbstractMatrix{R}, result::StarSetColoringResult; uplo::Symbol=:F
) where {R<:Real}
    @compat (; S, compressed_indices) = result
    check_same_pattern(A, S)
    nzA = nonzeros(A)
    if uplo == :F
      for k in eachindex(nzA, compressed_indices)
          nzA[k] = B[compressed_indices[k]]
      end
    else
      rvA = rowvals(A)
      for j in axes(S, 2)
        for k in nzrange(S, j)
          i = rvA[k]
          if in_triangle(i, j, uplo)
            nzA[k] = B[compressed_indices[k]]
          end
        end
      end
    end
    return A
end

[amontoison]

The issue is that sometimes uplo is a kwarg and sometimes not.
I prefer your approach but we should do that for all decompress!.

[gdalle]

You will not gain a lot if you do that:

True but with dispatch I avoid compiling the useless part of the code.

The issue is that sometimes uplo is a kwarg and sometimes not.

There was one spot where it was (erroneously) a kwarg, I fixed it in #85. I think now it should be a positional argument everywhere.

[amontoison]

You code too fast for me. It's perfect in that case.