Optimize out first where in label (#102)

* Store index and slice iterator before `for`-loop

* Handle `input` block selection in iterator

* Optimize out first `where` in `label`

* Move input selection within `zip`

As `starmap` is less familiar to others, move the selection operation
within `zip` using `map`. This should also make it clearer what the
selection is being used for and what is being selected from.

* Use Python 2/3 compatibility functions

Make use of iterator friendly Python 2/3 `imap` and `izip` functions
when generating the `block_iter`. This should ensure things like `next`
behave correctly (particularly on Python 2).

* Replace `lambda` with `operator.getitem`

Uses `operator.getitem` to make it a bit clearer to the reader as to
what is happening in this map operation (as opposed to the `lambda`).

dask_image/ndmeasure/__init__.py

@@ -6,6 +6,7 @@
 
 import collections
 import functools
+import operator
 
 import numpy
 
@@ -210,14 +211,19 @@ def label(input, structure=None):
     input = dask.array.asarray(input)
 
     labeled_blocks = numpy.empty(input.numblocks, dtype=object)
-    total = _label.LABEL_DTYPE.type(0)
 
     # First, label each block independently, incrementing the labels in that
     # block by the total number of labels from previous blocks. This way, each
     # block's labels are globally unique.
-    for index, cslice in zip(numpy.ndindex(*input.numblocks),
-                             dask.array.core.slices_from_chunks(input.chunks)):
-        input_block = input[cslice]
+    block_iter = _pycompat.izip(
+        numpy.ndindex(*input.numblocks),
+        _pycompat.imap(functools.partial(operator.getitem, input),
+                       dask.array.core.slices_from_chunks(input.chunks))
+    )
+    index, input_block = next(block_iter)
+    labeled_blocks[index], total = _label.block_ndi_label_delayed(input_block,
+                                                                  structure)
+    for index, input_block in block_iter:
         labeled_block, n = _label.block_ndi_label_delayed(input_block,
                                                           structure)
         block_label_offset = dask.array.where(labeled_block > 0,