ReCo.jl/src/PreVector.jl

import Base: push!, iterate

mutable struct PreVector{T}
    last_ind::UInt64
    v::Vector{T}

    PreVector{T}(n_particles) where {T} = new{T}(UInt64(0), Vector{T}(undef, n_particles))
end

function push!(pv::PreVector{T}, x::T) where {T}
    pv.last_ind += 1
    pv.v[pv.last_ind] = x

    return nothing
end

function reset!(pv::PreVector{T}) where {T}
    pv.last_ind = 0

    return nothing
end

function iterate(pv::PreVector{T}, state=1) where {T}
    if state > pv.last_ind
        return nothing
    else
        return (pv.v[state], state + 1)
    end
end