Help!

[H. S. Teoh]

On Fri, Jun 16, 2023 at 08:41:48AM +0000, FeepingCreature via Digitalmars-d wrote:
> On Thursday, 15 June 2023 at 19:18:03 UTC, H. S. Teoh wrote:
> > https://issues.dlang.org/show_bug.cgi?id=23976
[...]
> I actually need some help with this.
> 
> The behavior of `withPartial` seems to be random! For instance, since
> the default of `slide` is stated to be `Yes.withPartial`, shouldn't
> `[["1", "2"], ["2"]]` be the correct answer, since the last element
> has less than two entries?
[...]

OK, sorry for the really slow response, but I finally took a look at the
current Phobos code and the implementation of withPartial.  I think I've
figured out how the original design works.  Looks like the original
design was that the window (of size windowSize) is advanced by stepSize
every iteration, stopping *once the last element of the original range
has been reached*.  The withPartial parameter appears to be meaningful
only for the case where stepSize > 1, because that's the only time
you'll get a window of size < windowSize.

I.e., the way I understand the original, based on my reading of the
code, is this: let's say we have a range like this:

	0 1 2 3 4 5 6

then for windowSize=2, stepSize=1, we have the following windows:

	0 1 2 3 4 5 6
	[ ]
	  [ ]
	    [ ]
	      [ ]
	        [ ]
		  [ ]

At this point, the window is [5, 6], which means the last element 6 of
the original range has been reached, so the range stops.

But with stepSize=2:

	0 1 2 3 4 5 6
	[ ]
	    [ ]
	        [ ]
		    []

Here, after [4, 5] we haven't reached the last element 6 yet, so we have
to advance the window once more.  But there are no more elements after
6, so the next window contains only [6].  This is where withPartial
comes into effect: if it's Yes, which is the default (sorry, I was wrong
about this in my OP), then [6], which is smaller than windowSize, will
be included in the output.  Otherwise, it will be discarded.

In other words, the stopping criteria is whether the last element has
been reached yet (and not necessarily at the start of a window).  When
stepSize=1, this will always produce a full-sized window as the last
element, because once that last full-sized window is generated, the last
element of the original range has been seen, and the range will end.
The only time you'll get a window of less the full size is when
stepSize > 1.

Another example: if windowSize=2, stepSize=3:

	0 1 2 3 4 5 6 7 8
	[ ]
	      [ ]
	            [ ]

After the window [6,7], advancing the window by stepSize=3 passes the
end of the original range, so no more windows are produced.

Hope this helps clear up the intended semantics of slide().  We should
probably also document the above behaviour, because the current docs are
not clear at all about exactly how this works.


T

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