Periodic input iterators

[mforets]

In #15 we implement either constant or varying input iterators. An abstraction of an infinitely periodic input could be useful, and maybe it can be implemented by combining Base.IsInfinite() with an AbstractVector field.

[ueliwechsler]

Not sure, if I understand the concept and the usage of Input in MathematicalSystems, but I implemented a periodic input in my MPC package something like this:

# TODO:  make immutable (with Ref)
using TreeViews
using Crayons

mutable struct FixedInput{MT<:AbstractMatrix} <: AbstractController
    U::MT
    n::Integer
    k::Integer
end

function control_input(ctrl::FixedInput)
    u = ctrl.U[:, mod(ctrl.k, ctrl.n) + 1]
    ctrl.k += 1
    return u
end

This would be used as

U = [1 2;
     1 2]
input = FixedInput(U, 10, 0)
u1 = control_input(input) # [1, 1]
u2 = control_input(input) # [2, 2]
u3 = control_input(input) # [1, 1]

Maybe this idea can be of use.

[mforets]

Not sure, if I understand the concept and the usage of Input in MathematicalSystems,

We can probably improve the docs, I've commented here. The idea is that this type models an input to a system and it can be iterated over; the concrete interpretation is left for downstream packages. Very much like in your example.