[dlfivefifty]

sparse(A) / SparseMatrixCSC(A) for A a RectDiagonal (with any band, not only fills), an OffDiagonal, or a Diagonal of a fill now builds colptr/rowval/nzval directly from the band. It runs in O(n + length(band)) instead of using SparseArrays' generic AbstractMatrix fallback, which visits all m×n entries.

julia> A = FillArrays.OffDiagonal(rand(5000), 1);

julia> @time sparse(A);   # master: 0.0198 s, 30 allocations
  0.000016 seconds (10 allocations: 144.234 KiB)

• This replaces the RectOrDiagonalFill method, which built the matrix from getindex_value(v)*I and so only covered constant bands. It now uses one method for Union{Diagonal{<:Any,<:AbstractFillVector}, RectDiagonal, OffDiagonal}, built on the _band/_bandindex/_bandlength helpers from src/offdiagonal.jl.
• As for SparseMatrixCSC(::Diagonal) in SparseArrays, zeros in the band are not stored (uses SparseArrays._isnotzero). The old method also dropped them for a Zeros band, so this keeps the existing behaviour.
• SparseMatrixCSC{Tv}(A) keeps using eltype(axes(A,1)) as the index type, as before.
• A RectDiagonal whose diag is longer than min(m, n) (e.g. RectDiagonal([1,2,3], 3, 1)) only stores the visible entries.

Tests run testsparsediag on a range of RectDiagonal/OffDiagonals: super- and sub-diagonals, rectangular, empty bands, bands containing zeros, Fill/Zeros bands, and slices of Eye. They also check that the CSC structure is identical to sparse(Matrix(A)). The full test suite passes on Julia 1.10 and 1.13.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3f8b7e9) to head (fd8331b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           11        11           
  Lines         1316      1362   +46     
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+ Hits          1316      1362   +46     

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