I'm not as familiar with how XArray translates its groupby operations to dask.array operations under the hood.

No worries, this is indeed, pretty confusing!

For time.dayofyear in my groupby_transform pseudocode above (https://github.com/pydata/xarray/issues/2237#issuecomment-398580421): ```python

suppose N is the number of years of data

list_of_group_indices = [ [0, 365, 730, ..., (N-1)365], # day 1, ordered by year [1, 366, 731, ..., (N-1)365 + 1], # day 2, ordered by year ... ] indices_to_restore_orig_order = [ 0, N, 2N, 3N, ..., # year 1, ordered by day 1, N+1, 2N+1, 3N+1, ..., # year 2, ordered by day ... ] ``` As you can see, if you concatenate together the first set of indices and index by the second set of indices, it would arrange them into sequential integers.