Optimized implementation for structured matrices

[gdalle]

Fixes #64 and fixes #65 by optimizing structured coloring & decompression for these matrix types:

Package	Type	Optimal coloring	Optimal decompression
LinearAlgebra	Diagonal	✅	✅
LinearAlgebra	Bidiagonal	✅	✅
LinearAlgebra	Tridiagonal	✅	✅
BandedMatrices	BandedMatrix	✅	no need
BlockBandedMatrices	BlockBandedMatrix	❓	no need
BlockBandedMatrices	BandedBlockBandedMatrix	✅

For BandedMatrix and BlockBandedMatrix, the standard decompression based on SparseMatrixCSC will work fine because their storage is column-major (ref1, ref2). We only need a specific overload for BandedBlockBandedMatrix, which is a BlockMatrix of BandedMatrix-like blocks (so we need to decompress block by block).

General philosophy

Separate coloring from decompression:

• For coloring, define a new StructuredColoringAlgorithm that uses optimal coloring formulas (note that for block-banded the coloring given here is not actually optimal if the blocks themselves are sparse).
• For decompression, leverage the usual result type (ColumnColoringResult / RowColoringResult), which means that even greedy colorings on structured matrices benefit from optimized decompression.

Remaning issues

For decompression of BlockBandedMatrices, I still need to figure out JuliaArrays/BlockArrays.jl#418. But it already works because the fallback algorithm relies on the nonzeros of the sparse encoding, so it does not try to access illegal indices outside of the bands.

Implementation details

This is a clean rewrite of some code that was already present in ArrayInterface.jl and FiniteDiff.jl:

https://github.com/JuliaArrays/ArrayInterface.jl/blob/2e2b81bfb53e9e9fc41f26b927fb980afcbfa9f5/src/ArrayInterface.jl#L372-L384

https://github.com/JuliaArrays/ArrayInterface.jl/blob/2e2b81bfb53e9e9fc41f26b927fb980afcbfa9f5/ext/ArrayInterfaceBandedMatricesExt.jl#L68-L72

https://github.com/JuliaArrays/ArrayInterface.jl/blob/2e2b81bfb53e9e9fc41f26b927fb980afcbfa9f5/ext/ArrayInterfaceBlockBandedMatricesExt.jl#L207-L244

https://github.com/JuliaDiff/FiniteDiff.jl/blob/b574440c5d54708aa23a90c48f4a000dd1c6d915/ext/FiniteDiffBandedMatricesExt.jl#L13-L27

I still need to implement optimal decompression for block-banded matrices.

https://github.com/JuliaDiff/FiniteDiff.jl/blob/b574440c5d54708aa23a90c48f4a000dd1c6d915/ext/FiniteDiffBlockBandedMatricesExt.jl#L16-L68

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d6b9e65) to head (b24c181).

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #139    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           17        20     +3     
  Lines         1939      2070   +131     
==========================================
+ Hits          1939      2070   +131     

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