[BREAKING CHANGE] Adapt nonzero and where APIs to match NumPy

heat/core/dndarray.py

@@ -920,7 +920,7 @@ def __getitem__(self, key: Union[int, Tuple[int, ...], List[int, ...]]) -> DNDar
             # TODO: remove this resplit!!
             key = manipulations.resplit(key)
             if key.larray.dtype in [torch.bool, torch.uint8]:
-                key = indexing.nonzero(key)
+                key = indexing.nonzero(key, as_tuple=False)
 
             if key.ndim > 1:
                 key = list(key.larray.split(1, dim=1))
@@ -1626,7 +1626,7 @@ def __setitem__(
             to be used."""
             key = manipulations.resplit(key)
             if key.larray.dtype in [torch.bool, torch.uint8]:
-                key = indexing.nonzero(key)
+                key = indexing.nonzero(key, as_tuple=False)
 
             if key.ndim > 1:
                 key = list(key.larray.split(1, dim=1))

heat/core/indexing.py

@@ -3,39 +3,40 @@
 """
 
 import torch
-from typing import List, Dict, Any, TypeVar, Union, Tuple, Sequence
 
 from .communication import MPI
 from .dndarray import DNDarray
-from . import sanitation
+from . import factories
+from .sanitation import sanitize_in
 from . import types
+from . import manipulations
 
 __all__ = ["nonzero", "where"]
 
 
-def nonzero(x: DNDarray) -> DNDarray:
+def nonzero(x: DNDarray, as_tuple: bool = True) -> tuple[DNDarray, ...] | DNDarray:
     """
-    Return a :class:`~heat.core.dndarray.DNDarray` containing the indices of the elements that are non-zero (using ``torch.nonzero``).
-    If ``x`` is split then the result is split in the first dimension. However, this :class:`~heat.core.dndarray.DNDarray`
+    Return a tuple of :class:`~heat.core.dndarray.DNDarray`s, one for each dimension of ``x``,
+    containing the indices of the non-zero elements in that dimension. If ``x`` is split then
+    the result is split in the first dimension. However, this :class:`~heat.core.dndarray.DNDarray`
     can be UNBALANCED as it contains the indices of the non-zero elements on each node.
-    Returns an array with one entry for each dimension of ``x``, containing the indices of the non-zero elements in that dimension.
     The values in ``x`` are always tested and returned in row-major, C-style order.
     The corresponding non-zero values can be obtained with: ``x[nonzero(x)]``.
 
     Parameters
     ----------
     x: DNDarray
         Input array
+    as_tuple: bool, optional
+        Default is True for numpy-style nonzero output. If False, the output is a torch-style single 2D ``DNDarray`` of shape `(num_nonzero, ndim)` containing the indices of the non-zero elements.
 
     Examples
     --------
     >>> import heat as ht
     >>> x = ht.array([[3, 0, 0], [0, 4, 1], [0, 6, 0]], split=0)
     >>> ht.nonzero(x)
-    DNDarray([[0, 0],
-              [1, 1],
-              [1, 2],
-              [2, 1]], dtype=ht.int64, device=cpu:0, split=0)
+    (DNDarray([0, 1, 1, 2], dtype=ht.int64, device=cpu:0, split=None),
+        DNDarray([0, 1, 2, 1], dtype=ht.int64, device=cpu:0, split=None))
     >>> y = ht.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]], split=0)
     >>> y > 3
     DNDarray([[False, False, False],
@@ -48,48 +49,104 @@ def nonzero(x: DNDarray) -> DNDarray:
               [2, 0],
               [2, 1],
               [2, 2]], dtype=ht.int64, device=cpu:0, split=0)
+    (DNDarray([1, 1, 1, 2, 2, 2], dtype=ht.int64, device=cpu:0, split=None),
+        DNDarray([0, 1, 2, 0, 1, 2], dtype=ht.int64, device=cpu:0, split=None))
     >>> y[ht.nonzero(y > 3)]
     DNDarray([4, 5, 6, 7, 8, 9], dtype=ht.int64, device=cpu:0, split=0)
     """
-    sanitation.sanitize_in(x)
-
+    sanitize_in(x)
+
+    if not x.is_distributed():
+        # nonzero indices as tuple
+        nonzero = torch.nonzero(input=x.larray, as_tuple=as_tuple)
+        # bookkeeping for final DNDarray construct
+        if as_tuple:
+            nonzero = list(nonzero)
+            for i, nz_tensor in enumerate(nonzero):
+                nonzero[i] = factories.array(nz_tensor, device=x.device, comm=x.comm)
+            return tuple(nonzero)
+        else:
+            # nonzero indices as single 2D DNDarray
+            return factories.array(nonzero, device=x.device, comm=x.comm)
+
+    # distributed case
     lcl_nonzero = torch.nonzero(input=x.larray, as_tuple=False)
-
-    # add offsets mapping from local indices to global indices if x is split
-    if x.split is not None:
-        _, _, slices = x.comm.chunk(x.shape, x.split)
-        lcl_nonzero[..., x.split] += slices[x.split].start
-
-    if x.ndim == 1:
-        lcl_nonzero = lcl_nonzero.squeeze(dim=1)
-
-    # compute global shape of the index array
-    gout = list(lcl_nonzero.shape)
-    if x.split is None:
-        is_split = None
+    nonzero_size = torch.tensor(lcl_nonzero.shape[0], dtype=torch.int64, device="cpu")
+    nonzero_dtype = types.canonical_heat_type(lcl_nonzero.dtype)
+
+    # global nonzero_size
+    x.comm.Allreduce(MPI.IN_PLACE, nonzero_size, MPI.SUM)
+    # correct indices along split axis
+    _, displs = x.counts_displs()
+    lcl_nonzero[:, x.split] += displs[x.comm.rank]
+
+    if x.split == 0:
+        # for split=0, the local nonzero indices are already globally ordered along the split axis
+        if as_tuple:  # return indices as tuple of 1D DNDarrays
+            lcl_nonzero = lcl_nonzero.unbind(dim=1)
+            return tuple(
+                DNDarray(
+                    nz_tensor,
+                    gshape=(nonzero_size.item(),),
+                    dtype=nonzero_dtype,
+                    split=0,
+                    device=x.device,
+                    comm=x.comm,
+                    balanced=False,
+                )
+                for nz_tensor in lcl_nonzero
+            )
+        else:  # return indices as single 2D DNDarray
+            return DNDarray(
+                lcl_nonzero,
+                gshape=(nonzero_size.item(), x.ndim),
+                dtype=nonzero_dtype,
+                split=0,
+                device=x.device,
+                comm=x.comm,
+                balanced=False,
+            )
     else:
-        gout[0] = x.comm.allreduce(gout[0], MPI.SUM)
-        is_split = 0
-
-    return DNDarray(
-        lcl_nonzero,
-        gshape=tuple(gout),
-        dtype=types.canonical_heat_type(lcl_nonzero.dtype),
-        split=is_split,
-        device=x.device,
-        comm=x.comm,
-        balanced=False,
-    )
-
-
-DNDarray.nonzero = lambda self: nonzero(self)
+        # construct global 2D DNDarray of nz indices:
+        shape_2d = (nonzero_size.item(), x.ndim)
+        global_nonzero = DNDarray(
+            lcl_nonzero,
+            gshape=shape_2d,
+            dtype=nonzero_dtype,
+            split=0,
+            device=x.device,
+            comm=x.comm,
+            balanced=False,
+        )
+        # vectorized sorting of nz indices along axis 0
+        global_nonzero.balance_()
+        global_nonzero = manipulations.unique(global_nonzero, axis=0)
+        if as_tuple:  # return indices as tuple of 1D DNDarrays
+            lcl_nonzero = global_nonzero.larray.unbind(dim=1)
+            return tuple(
+                DNDarray(
+                    nz_tensor,
+                    gshape=(nonzero_size.item(),),
+                    dtype=nonzero_dtype,
+                    split=0,
+                    device=x.device,
+                    comm=x.comm,
+                    balanced=True,
+                )
+                for nz_tensor in lcl_nonzero
+            )
+        else:  # return indices as single 2D DNDarray
+            return global_nonzero
+
+
+DNDarray.nonzero = lambda self: nonzero(self, as_tuple=True)
 DNDarray.nonzero.__doc__ = nonzero.__doc__
 
 
 def where(
     cond: DNDarray,
-    x: Union[None, int, float, DNDarray] = None,
-    y: Union[None, int, float, DNDarray] = None,
+    x: None | int | float | DNDarray = None,
+    y: None | int | float | DNDarray = None,
 ) -> DNDarray:
     """
     Return a :class:`~heat.core.dndarray.DNDarray` containing elements chosen from ``x`` or ``y`` depending on condition.
@@ -114,34 +171,42 @@ def where(
 
     Notes
     -----
-    When only condition is provided, this function is a shorthand for :func:`nonzero`.
+    When only condition is provided, this function is a shorthand for :func:`nonzero` and the function returns a tuple
+    of :class:`~heat.core.dndarray.DNDarray`, analogously to ``numpy.where``.
 
     Examples
     --------
     >>> import heat as ht
     >>> x = ht.arange(10, split=0)
     >>> ht.where(x < 5, x, 10 * x)
-    DNDarray([ 0,  1,  2,  3,  4, 50, 60, 70, 80, 90], dtype=ht.int64, device=cpu:0, split=0)
+    DNDarray(MPI-rank: 0, Shape: (10,), Split: 0, Local Shape: (10,), Device: cpu:0, Dtype: int32, Data:
+         [ 0,  1,  2,  3,  4, 50, 60, 70, 80, 90])
     >>> y = ht.array([[0, 1, 2], [0, 2, 4], [0, 3, 6]])
     >>> ht.where(y < 4, y, -1)
-    DNDarray([[ 0,  1,  2],
-              [ 0,  2, -1],
-              [ 0,  3, -1]], dtype=ht.int64, device=cpu:0, split=None)
+    DNDarray(MPI-rank: 0, Shape: (3, 3), Split: None, Local Shape: (3, 3), Device: cpu:0, Dtype: int64, Data:
+         [[ 0,  1,  2],
+          [ 0,  2, -1],
+          [ 0,  3, -1]])
     """
-    if cond.split is not None and (isinstance(x, DNDarray) or isinstance(y, DNDarray)):
-        if (isinstance(x, DNDarray) and cond.split != x.split) or (
-            isinstance(y, DNDarray) and cond.split != y.split
-        ):
-            if len(y.shape) >= 1 and y.shape[0] > 1:
-                raise NotImplementedError("binary op not implemented for different split axes")
+    # binary where(cond, x, y) branch
+    if cond.split is not None and isinstance(y, DNDarray) and len(y.shape) >= 1 and y.shape[0] > 1:
+        if (isinstance(x, DNDarray) and cond.split != x.split) or cond.split != y.split:
+            raise NotImplementedError("binary op not implemented for different split axes")
+
     if isinstance(x, (DNDarray, int, float)) and isinstance(y, (DNDarray, int, float)):
+        # Simple elementwise selection using arithmetic:
+        # cond == 0 -> take y, cond == 1 -> take x
         for var in [x, y]:
             if isinstance(var, int):
                 var = float(var)
         return cond.dtype(cond == 0) * y + cond * x
-    elif x is None and y is None:
-        return nonzero(cond)
+
+    # where(cond) "indices only" branch
+    elif x is None and y is None:  # delegate to nonzero(cond)
+        return nonzero(cond)  # tuple of DNDarrays, one per dimension
+
     else:
         raise TypeError(
-            f"either both or neither x and y must be given and both must be DNDarrays or numerical scalars({type(x)}, {type(y)})"
+            "either both or neither x and y must be given and both must be "
+            f"DNDarrays or numerical scalars (got {type(x)}, {type(y)})"
         )

heat/core/linalg/eigh.py

@@ -75,7 +75,7 @@ def _subspaceiteration(
             device=columnnorms.device,
         )
         * statistics.percentile(columnnorms, 100.0 * (1 - (k + safetyparam) / columnnorms.shape[0]))
-    )
+    )[0]
     X = C[:, idx].balance()
 
     # actual subspace iteration