structure_factor.jl

function calculate_sfs(cf::T) where {R <: Real, T <: Matrix{Measurement{R}}}
    # cf is an nxn matrix of correlation function values indexed by site offset
    # so first make a matrix with the same shape...
    n = size(cf, 1)
    sf = similar(cf)
    coeff = 2. * pi / n
    for iqx in 1:n, iqy in 1:n
        qy = iqy * coeff
        qx = iqx * coeff
        sf[iqy, iqx] = calculate_single_sf(cf, qx, qy, n)
    end
    return sf
end

function calculate_single_sf(cf::T, qx::R, qy::R, n::N) where {R <: Real, N <: Integer, T <: Matrix{Measurement{R}}}
    sf = measurement(0., 0.)

    for ix in 1:n, iy in 1:n,
        jx in 1:n, jy in 1:n

        dx = ix - jx
        dy = iy - jy
        absdx = Int(abs(dx))
        absdy = Int(abs(dy))

        sf += cos(qx * dx + qy * dy) * cf[absdy + 1, absdx + 1]
    end
    return sf / n^2
end