The Infrastructure

The CoordGauge implementation schedules five groups:

CoordGauge
LapseSelect IN CoordGauge BEFORE PickCoordGauge
LapseApply  IN CoordGauge AFTER  PickCoordGauge
ShiftSelect IN CoordGauge BEFORE PickCoordGauge
ShiftApply  IN CoordGauge AFTER  PickCoordGauge

and one function

PickCoordGauge IN CoordGauge

and has two public grid scalars

selected_lapse
selected_shift

and two string parameters

lapse_list
shift_list

It also provides four aliased functions

int CoordGauge_RegisterLapse("lapse-name")
int CoordGauge_RegisterShift("shift-name")
CoordGauge_Lapse("lapse-name")
CoordGauge_Shift("shift-name")

(If someone can think of better names, please say so 8-)

Then each thorn which wants to apply a coordinate gauge condition registers itself, receiving a unique integer as an id, and schedules a selection routine and an application routine in the appropriate schedule groups.

The selection routine decides if this gauge condition should be applied at this time, and calls the CoordGauge_Lapse/Shift aliased function. (It should check that it is actually in the appropriate parameter as a minimum.)

The PickCoordGauge function traverses the list of lapses/shifts and selects the first one in the list which has called the CoordGauge_Lapse/Shift aliased function and sets the appropriate grid scalar to the id of this one.

The application routine checks to see if the grid scalar is set to its id, and if so, applies the gauge condition.

Evolution thorns could schedule CoordGauge at the appropriate point or points in their schedule.

An advantage of this scheme over the current one is that it provides the selection routines with a full set of variables from which to decide whether they should apply a guage or not. So it becomes very easy to choose to switch off maximal if the lapse has collapsed within a certain volume, etc.

This is simpler than the previous scheme as there is no arbitrary 'bid' floating around. It also allows us to keep the logic of the final selection in one place, thus allowing people to override this logic if they need to.