Stephen Colebourne wrote:
>
> We have the following operations to define:
> - contains
> - abuts
> - overlaps
> - overlap (the amount of overlap of two intervals)
> - gap (the gap between two intervals)
>
> a = [08:00,09:00)
> b = [09:00,09:00)
> c = [09:00,10:00)
>
> One option is to define an empty interval as returning false to the
> first three and null to the last two:
> - a abuts c
> - neither a or c abut or contain b
> But what does the interval start/end datetime represent in this case?
>
maybe there's a need for an explicit constant value
EMPTY_INTERVAL that represents an empty zero-length interval? if it
doesn't have specific start/end times (returns null for boundary
instants?) there's no question of interpreting it as the limit of a
series of finite length intervals. then all of Barend's (and Claudio and
JBs) comments apply. so whenever an interval of the form [x,x) is about
to be returned, EMPTY_INTERVAL would be returned instead. does this seem
like sophistry?
if the other interval types were added to joda later,
EMPTY_INTERVAL would have to be used in place of [x, x) (x, x] and (x,
x). but [x, x] would be a non-empty zero-length interval and need a
different representation.
> A second non-maths option is to make the empty interval lie between two
> real intervals:
> - a, b and c all abut each other at 09:00
> - neither a nor c contains b
>
this seems too ad-hoc.
> A third option is to define the empty interval as a point on the
> timeline at 09:00 that is therefore contained in c:
> - a, b and c all abut each other at 09:00
> - c contains b, thus b and c overlap
>
as i said in another email, this is like defining a closed interval for
the zero-length case. having thought some more about it, i think it's
not such a great idea to special-case a closed zero length interval as
an instant. it would probably need to be deprecated if other interval
types were introduced at a later time.
overall, option a) seems to make the most sense.
my 2c FWIW.
al
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