Faceted search and availability

I've set up a availability event type with hourly granularity. When using bat state facet only on available state with start/end dates including fraction of an hour i get invalid results.

An example:

- i've setup only a single unit, say A, for unit type Car
- i have already an event in "booked" (blocking) state from 2018-04-15 0:00 to 2018-04-16 12:30 for in the availability calendar of unit A
- i search for availability from 2018-04-16 12:00 to 2018-04-17 13:30

expected result:

An empty set

actual result:

1 unit of type Car (Unit A)

After some debugging i found that this behaviour come from the call to getMatchingUnits in bat_facet.module at line 77

$response = $calendar->getMatchingUnits($start_date, $end_date, $valid_states, $constraints, TRUE);

the last parameter (TRUE) makes the search using "intersection" of states of events found in date range with the "available" state instead of difference.

In the date range from 2018-04-16 12:00 to 2018-04-17 13:30 the states are two ["booked", "available"]. This is right since the resource is booked until 2018-04-16 12:30 and then available.

But intersection (AbstractCalendar, line 148)

array_intersect($current_states, $valid_states); 

returns "available" element since $current_states = ["booked", "available"] and $valid_states = ["available"].

This puts the unit in the "included" set but this resource is clearly not bookable in the requested range.

The solution, at least for this specific case, is to use difference instead of intersection by changing:

$response = $calendar->getMatchingUnits($start_date, $end_date, $valid_states, $constraints);

Probably could be useful to choose the search behaviour from Bat State facet setting page.

Attached patch only changes the line above.