Draft of interlocking system

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+-- interlocking/database.lua
+-- saving the location of TCB's, their neighbors and their state
+--[[
+The interlocking system is based on track circuits.
+Track circuit breaks must be manually set by the user. Signals must be assigned to track circuit breaks and to a direction(connid).
+To simplify the whole system, there is no overlap.
+== Trains ==
+A train always reserves the space between two signals facing it. All track circuits in this space
+have a reservation entry TRAIN with the train's ID
+== Database storage ==
+The things that are actually saved are the Track Circuit Breaks. Each TCB holds a list of the TCBs that are adjacent in each direction.
+TC occupation/state is then saved inside each (TCB,Direction) and held in sync across all TCBs adjacent to this one. If something should not be in sync,
+all entries are merged to perform the most restrictive setup.
+== Traverser function ==
+To determine and update the list of neighboring TCBs, we need a traverser function.
+It will start at one TCB in a specified direction (connid) and use get_adjacent_rail to crawl along the track. When encountering a turnout or a crossing,
+it needs to branch(call itself recursively) to find all required TCBs. Those found TCBs are then saved in a list as tuples (TCB,Dir)
+In the last step, they exchange their neighbors.
+== TC states ==
+A track circuit does not have a state as such, but has more or less a list of "reservations"
+type can be one of these:
+TRAIN See Trains obove
+ROUTE Route set from a signal, but no train has yet passed that signal.
+Certain TCs can be marked as "allow call-on".
+== Route setting: ==
+Routes are set from a signal (the entry signal) to another signal facing the same direction (the exit signal)
+Remember that signals are assigned to a TCB and a connid.
+Whenever this is done, the following track circuits are set "reserved" by the train by saving the entry signal's ID:
+- all TCs on the direct way of the route - set as ROUTE
+Route setting fails whenever any TC that we want to set ROUTE to is already set ROUTE or TRAIN from another signal (except call-on, see below)
+Apart from this, we need to set turnouts
+- Turnouts on the track are set held as ROUTE
+- Turnouts that purpose as flank protection are set held as FLANK (NOTE: left as an idea for later, because it's not clear how to do this properly without an engineer)
+== Call-On/Multiple Trains ==
+It will be necessary to join and split trains using call-on routes. A call-on route may be set when:
+- there are no ROUTE reservations
+- there are TRAIN reservations only inside TCs that have "allow call-on" set
+
+
+== More notes ==
+- It may not be possible to switch turnouts when their TC has any state entry
+
+]]--
+
+
+