Fix chunks in uneven step sizes

Fixes dask#188

dask/array/slicing.py

@@ -1,6 +1,7 @@
 from itertools import count, product
 from toolz import merge, first, accumulate
 from operator import getitem, add
+from math import ceil
 from ..compatibility import long
 import numpy as np
 
@@ -560,7 +561,7 @@ def new_blockdim(dim_shape, lengths, index):
     [4]
 
     >>> new_blockdim(100, [20, 10, 20, 10, 40], slice(90, 10, -2))
-    [15, 5, 10, 5, 4]
+    [16, 5, 10, 5, 4]
     """
     if isinstance(index, list):
         return [len(index)]
@@ -570,7 +571,7 @@ def new_blockdim(dim_shape, lengths, index):
                 for i, slc in pairs]
     if isinstance(index, slice) and index.step and index.step < 0:
         slices = slices[::-1]
-    return [(slc.stop - slc.start) // slc.step for slc in slices]
+    return [int(ceil((1. * slc.stop - slc.start) / slc.step)) for slc in slices]
 
 
 def replace_ellipsis(n, index):

dask/array/tests/test_slicing.py

@@ -2,7 +2,7 @@
 from dask.compatibility import skip
 import dask.array as da
 from dask.array import Array
-from dask.array.slicing import slice_array, _slice_1d, take
+from dask.array.slicing import slice_array, _slice_1d, take, new_blockdim
 from operator import getitem
 import numpy as np
 from toolz import merge
@@ -437,3 +437,11 @@ def test_multiple_list_slicing():
     x = np.random.rand(6, 7, 8)
     a = da.from_array(x, chunks=(3, 3, 3))
     assert eq(x[:, [0, 1, 2]][[0, 1]], a[:, [0, 1, 2]][[0, 1]])
+
+
+def test_uneven_chunks():
+    assert da.ones(20, chunks=5)[::2].chunks == ((3, 2, 3, 2),)
+
+
+def test_new_blockdim():
+    assert new_blockdim(20, [5, 5, 5, 5], slice(0, None, 2)) == [3, 2, 3, 2]