Unifying and generalizing hcat, vcat, hvcat, blockdiag etc.

[JeffFessler]

This is a rough idea that I want to sketch out now, even though I don't have time to work on it.
Suppose A is a MxN linear operator and we think of it being embedded with zeros around it to make a bigger linear operator B = [0 0 0; 0 A 0; 0 0 0] where the zeros around it are matrices some of which might be of size 0x0. All one needs to specify B is the size of the upper left 0 and the lower right 0. Call this a OffsetMap or such, where the operation y = B * x is simply

y = zeros(T,size(B,1))
rowrange = (1:M) .+ nrow_upper_left_zero
colrange = (1:N) .+ ncol_upper_left_zero
y[rowrange] .= A * x[colrange]

We cache the ranges at construction time, very much in the spirit of how hvcat is done.
All of the hcat and vcat and hvcat objects, I mean BlockMap objects, are simply sums of these OffsetBlock objects with appropriate offsets for each block.
A BlockDiagonalMap is also just a sum of such OffsetMap objects.
One could imagine other block sparse matrices made by summing several such objects.
Instead of having to define separate types for each pattern, all of these are just sums (LinearCombinationMap) of OffsetMap objects.
Wish I had thought of this earlier. The only question I have is whether the summation in a LinearCombinationMap will be as fast as the loops for a BlockMap. Or am I overlooking any other drawback?

[dkarrasch]

This is a great idea! I think the summation loop won't be a problem. A mild challenge is multiply-and-add, because we will rely on LinearCombination multiplication. But we will need to work on that anyway, to make use of 5-arg mul! for matrices, which can do multiplication and addition all in one go, allocation-free. Our current generic 5-arg mul! does allocate an intermediate vector since that is unavoidable for FunctionMaps, for instance. So we should introduce some clever multiple dispatch to do that only when really necessary. We already do have a test that requests that there is at most one allocation in LinearCombination multiplication, and we should get that down to zero when all involved linear maps are matrix-based.

I may give it a try when I'm done with the current PRs.

[JeffFessler]

I've got it 90% done. Famous saying about the last 10%... Will send something soon.

[dkarrasch]

🤣 just like me: I also keep telling I won't have time, and then I'm too curious wether it works and start working on it immediately.

[JeffFessler]

Exactly.