The event queue in TOSSIM is unstable (in terms of sorting order)

[MBradbury]

Currently running a simulation with a seed produces a deterministic output. Running the same code but inserting simulation events that do nothing will produce a different deterministic output. I was expecting the output from the two simulations to be the same.

It turns out that because the queue is implemented in terms of a heap it is not stable (in terms of sorting order). For example, if events A, B and C are scheduled to run at time t they may be executed in the order BCA. If an empty event E is also scheduled at t then that order may change to CAEB, essentially CAB.

This was unexpected behaviour, as I didn't expect asking for an event that does nothing to be called to change the output of the simulation.

The solution to this is to add a counter that records the number of events scheduled and assign each event a unique counter. The event counter would be used in the heap comparison to ensure events added first would be called first.

However, I'm not sure if this is a bug that needs fixing, or if it is just something worth documenting.