In general, wavelet packet crystals can be arranged in the so-called natural order
ala \(W_{0,0} , W_{1,0}, W_{1,1}, W_{2,0}, W_{2,1}, W_{2,2}, W_{2,3}, \ldots , W_{J,0}, \ldots, W_{J, N_J}\)
where \(J\) is the number of decomposition levels and \(N_J=2^J-1\).
By definition, \(W_{0,0}\) is the original time series.
A given crystal is identified in the \(W_{j,n}\) form above or by a flattened index.
For example, the DWPT crystal in level 2 at oscillation index 1 is identified either by j,n=2,1 or
by its flattened index 4 (with zero based indexing, 4 represents the fifth crystal of the wavelet packet
transform in natural order). This function converts such flattened wavelet packet indices to
the \(W_{j,n}\) form.