Fix bandwidths of blocks of banded matrices

Project.toml

@@ -1,6 +1,6 @@
 name = "BlockBandedMatrices"
 uuid = "ffab5731-97b5-5995-9138-79e8c1846df0"
-version = "0.13.7"
+version = "0.13.8"
 
 [deps]
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"

src/BandedBlockBandedMatrix.jl

@@ -514,10 +514,16 @@ function getindex(A::BandedBlockBandedMatrix{T}, KR::BlockRange{1}, JR::BlockRan
     _BandedBlockBandedMatrix(A.data[:,JR], axes(axes(A,1)[KR],1), (l-sh, u+sh), subblockbandwidths(A))
 end
 
-@inline function bandwidths(V::SubArray{T,2,<:AbstractMatrix,<:Tuple{BlockSlice1,BlockSlice1}}) where T
+@inline bandwidths(V::SubArray{T,2,<:AbstractMatrix,<:Tuple{BlockSlice1,BlockSlice1}}) where T = _block_bandwidths(MemoryLayout(parent(V)), V)
+
+@inline function _blockbanded_block_bandwidths(V)
     inblockbands(V) && return subblockbandwidths(parent(V))
     (-720,-720)
 end
+@inline _block_bandwidths(_, V) = _blockbanded_block_bandwidths(V)
+@inline _block_bandwidths(::DiagonalLayout, V) = _blockbanded_block_bandwidths(V)
+# a block of a banded matrix is banded with the bandwidths shifted by the offset of the block from the diagonal
+@inline _block_bandwidths(::AbstractBandedLayout, V) = min.(size(V) .- 1, bandwidths(parent(V)) .+ (-1,1) .* bandshift(V))
 
 function bandwidths(V::SubArray{T,2,<:AbstractMatrix,<:Tuple{BlockSlice{<:BlockIndexRange1},BlockSlice{<:BlockIndexRange1}}}) where T
    B = parentblock(V)

test/test_misc.jl

@@ -204,6 +204,14 @@ Base.size(F::FiniteDifference) = (F.n,F.n)
         @test Q isa BandedMatrix
         @test blockrowsupport(Q,Block(1)) == Block.(2:2)
 
+        @test bandwidths(view(B, Block(2), Block(1))) == (1,0)
+        @test all(B[Block(K,J)] == Matrix(B)[a[Block(K)], a[Block(J)]] for K=1:5, J=1:5)
+
+        # blocks of a banded matrix with blocked axes, including blocks where the bandwidths are shifted
+        B = _BandedMatrix(BlockedArray(randn(5,6), (Base.OneTo(5), blockedrange(1:3))), blockedrange(1:2), 2, 2)
+        @test bandwidths(view(B, Block(2), Block(3))) == (1,0)
+        @test all(B[Block(K,J)] == Matrix(B)[axes(B,1)[Block(K)], axes(B,2)[Block(J)]] for K=1:2, J=1:3)
+
         @testset "constant blocks" begin
             a = blockedrange(Fill(2,5))
             Q = Eye((a,))[:,Block(2)]