# ACAFact.jl

A simple and dependency-free adaptive cross approximation (ACA) factorization in Julia.

# Usage

Let's create a matrix with low numerical rank:

using LinearAlgebra
pts1 = range(0.0, 1.0, length=100)
pts2 = range(0.0, 1.0, length=110)
K    = [exp(-abs2(pj - pk)) for pj in pts1, pk in pts2]
@show rank(K) # 10


One easy way to use ACAFact.jl is to just provide your matrix and a maximum allowed rank for the approximation:

for rk in (5, 10, 15, 20, 25)
(U, V) = aca(K, rk)
@show (rk, opnorm(K - U*V'))
end


Another thing you can do is provide a desired opnorm error for ||K - U*V'||...but I wouldn't bet the farm that the error control is all that guaranteed. Greedy deterministic pivoting schemes can be tricked, or at least made very inefficient! It does work pretty well in toy applications at least though, so it isn't useless.

for tol in (1e-4, 1e-6, 1e-8, 1e-10, 1e-12, 1e-14)
(U, V) = aca(K, 100, tol=tol)
@show tol, size(U, 2), opnorm(K - U*V')
end


If you want something that is completely non-allocating, you can pre-allocate a cache. Note that if you also provide a tol here, you will terminate early but the function won't remove the unused columns in U and V. The rnk return parameter gives you the rank of the approximation, so you should work with view(U, :, 1:rnk), for example.

cache    = ACAFact.ACACache(Float64, length(pts1), length(pts2), 50) # max rank 50
(U, V)   = (zeros(length(pts1), 50), zeros(length(pts2), 50))
(rnk, _) = ACAFact.aca!(K, U, V, cache=cache) # zero allocations


One note here though: this ACACache does carry state that gets used in the factorizations, so if you want to factorize a new matrix that is the same size as K with the same cache, be sure to ACAFact.resetcache!(cache).

# Working with arbitrary matrix-like objects

To me, the best reason to use a greedy partially-pivoted factorization like the ACA is to build low-rank approximations for matrices where you cannot afford to pass over every entry even once. To accommodate this use case, ACAFact.jl has an interface

ACAFact.col!(buf, K, j)
ACAFact.row!(buf, K, j)


that expects buf to be filled with the corresponding column/row of K. So if you have some cool operator that is defined implicitly or whatever, all you need to do add special methods

ACAFact.col!(buf, K::MyCoolObject, j) = # ...
ACAFact.row!(buf, K::MyCoolObject, j) = # ...


and you can create low-rank approximations that never touch rows/columns of your matrix that aren't selected as pivots. It would be nice if you could also implement Base.size(K::MyCoolObject), but if for some reason you don't want to you can compute the ACA of K with aca(K, [...], sz=(size_K_1, size_K_2)).