Use blockwise to extract final result (#182)

* Use blockwise to extract final result for method="blockwise"

* FOr all methods

* bugfix

* Try return_array from _finalize_results

* Revert "Try return_array from _finalize_results"

This reverts commit cb25e38.

* Fixes.

Author: dcherian

flox/core.py

@@ -1195,7 +1195,6 @@ def dask_groupby_agg(
 
     import dask.array
     from dask.array.core import slices_from_chunks
-    from dask.highlevelgraph import HighLevelGraph
 
     # I think _tree_reduce expects this
     assert isinstance(axis, Sequence)
@@ -1268,6 +1267,9 @@ def dask_groupby_agg(
             engine=engine,
             sort=sort,
         ),
+        # output indices are the same as input indices
+        # Unlike xhistogram, we don't always know what the size of the group
+        # dimension will be unless reindex=True
         inds,
         array,
         inds,
@@ -1277,7 +1279,7 @@ def dask_groupby_agg(
         dtype=array.dtype,  # this is purely for show
         meta=array._meta,
         align_arrays=False,
-        token=f"{name}-chunk-{token}",
+        name=f"{name}-chunk-{token}",
     )
 
     if expected_groups is None:
@@ -1364,35 +1366,63 @@ def dask_groupby_agg(
         groups = (np.concatenate(groups_in_block),)
         ngroups_per_block = tuple(len(grp) for grp in groups_in_block)
         group_chunks = (ngroups_per_block,)
-
     else:
         raise ValueError(f"Unknown method={method}.")
 
-    # extract results from the dict
+    out_inds = inds[: -len(axis)] + (inds[-1],)
     output_chunks = reduced.chunks[: -len(axis)] + group_chunks
+    if method == "blockwise" and len(axis) > 1:
+        # The final results are available but the blocks along axes
+        # need to be reshaped to axis=-1
+        # I don't know that this is possible with blockwise
+        # All other code paths benefit from an unmaterialized Blockwise layer
+        reduced = _collapse_blocks_along_axes(reduced, axis, group_chunks)
+
+    # Can't use map_blocks because it forces concatenate=True along drop_axes,
+    result = dask.array.blockwise(
+        _extract_result,
+        out_inds,
+        reduced,
+        inds,
+        adjust_chunks=dict(zip(out_inds, output_chunks)),
+        dtype=agg.dtype[agg.name],
+        key=agg.name,
+        name=f"{name}-{token}",
+        concatenate=False,
+    )
+
+    return (result, groups)
+
+
+def _collapse_blocks_along_axes(reduced, axis, group_chunks):
+    import dask.array
+    from dask.highlevelgraph import HighLevelGraph
+
+    nblocks = tuple(reduced.numblocks[ax] for ax in axis)
+    output_chunks = reduced.chunks[: -len(axis)] + ((1,) * (len(axis) - 1),) + group_chunks
+
+    # extract results from the dict
     ochunks = tuple(range(len(chunks_v)) for chunks_v in output_chunks)
     layer2: dict[tuple, tuple] = {}
-    agg_name = f"{name}-{token}"
-    for ochunk in itertools.product(*ochunks):
-        if method == "blockwise":
-            if len(axis) == 1:
-                inchunk = ochunk
-            else:
-                nblocks = tuple(len(array.chunks[ax]) for ax in axis)
-                inchunk = ochunk[:-1] + np.unravel_index(ochunk[-1], nblocks)
-        else:
-            inchunk = ochunk[:-1] + (0,) * (len(axis) - 1) + (ochunk[-1],)
+    name = f"reshape-{reduced.name}"
 
-        layer2[(agg_name, *ochunk)] = (operator.getitem, (reduced.name, *inchunk), agg.name)
+    for ochunk in itertools.product(*ochunks):
+        inchunk = ochunk[: -len(axis)] + np.unravel_index(ochunk[-1], nblocks)
+        layer2[(name, *ochunk)] = (reduced.name, *inchunk)
 
-    result = dask.array.Array(
-        HighLevelGraph.from_collections(agg_name, layer2, dependencies=[reduced]),
-        agg_name,
+    return dask.array.Array(
+        HighLevelGraph.from_collections(name, layer2, dependencies=[reduced]),
+        name,
         chunks=output_chunks,
-        dtype=agg.dtype[agg.name],
+        dtype=reduced.dtype,
     )
 
-    return (result, groups)
+
+def _extract_result(result_dict: FinalResultsDict, key) -> np.ndarray:
+    from dask.array.core import deepfirst
+
+    # deepfirst should be not be needed here but sometimes we receive a list of dict?
+    return deepfirst(result_dict)[key]
 
 
 def _validate_reindex(