Add spec helper for generating (non-)overlapping spans

[izzergh]

e.g.

acts_as_span/spec/lib/span_instance/overlap_spec.rb

Line 35 in 64dbb8e

context "span_overlap_count == 0" do

The above spec is skipped because it is not fully implemented and its functionality is essentially tested in the no overlap validator spec. BUT in AAS proper and in the systems that use it, it is common to require an overlapping span and non-overlapping span to compare with. It is done slightly differently just about everywhere.

AAS could add a helper method for specs (can't think of a non-spec use case right now) that would look like:

overlapping_span_for(span, type = :right, amount = 1)

span is an array of two dates, type is :left, :right, :inner, or :outer, and amount is any number of days (0 for "inclusive" overlap) up to the number of days the input span spans. For example,

span = [Date.new(1, 1, 2000), Date.new(2, 1, 2000)]

overlapping_span_for(span)
# => [<1/31/2000>, <2/2/2000>]

overlapping_span_for(span, type: :outer, amount: 30)
# => [<12/2/1999>, <3/2/2000>]

        |--------| <- span
    |------| <- left overlap
          |----| <- inner overlap
              |---------| <- right overlap
    |----------------| <- outer overlap

It may also be useful to add an option for a negative amount, i.e. not overlapping. Or perhaps another method entirely, to avoid confusion. I suppose it would be equivalent to...

def non_overlapping_span_for(span, type = :right, amount = 1)
  fail ArgumentError(whatever) unless [:right, :left].include? type

  fail ArgumentError(whatever) unless amount.positive? # I think "positive" comes from ActiveSupport

  overlapping_span_for(span, type, amount: -amount)
end