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-- Signal API implementation

local F = advtrains.formspec

local signal = {}

signal.MASP_HALT = {
	name = "_halt",
	halt = true,
}

signal.MASP_DEFAULT = {
	name = "_default",
	default = true,
}

signal.ASPI_HALT = {
	main = 0,
	shunt = false,
}

signal.ASPI_FREE = {
	main = -1,
	shunt = false,
	proceed_as_main = true,
}

--[[
Implementation plan orwell 2024-01-28:
Most parts of ywang's implementation are fine, especially I like the formspecs. But I would like to change a few aspects (no pun intended) of this.
- Signal gets distant assigned via field in signal aspect table (instead of explicitly)
- Signal speed/shunt are no longer free-text but rather they need to be predefined in the node definition
To do this: Differentiation between:
== Main Aspect ==
This is what a signal is assigned by either the route system or the user.
It is a string key which has an appropriate entry in the node definition (where it has a description assigned)
The signal mod defines a function to set a signal to the most appropriate aspect. This function gets
a) the main aspect table (straight from node def)
b) the distant signal's aspect group name & aspect table

== Aspect ==
One concrete combination of lights/shapes that a signal signal shows. Handling these is at the discretion of
the signal mod defining the signal, and they are typically combinations of main aspect and distant aspect
Example:
- A Ks signal has the main_aspect="proceed_12" set for a route
- The signal at the end of the route shows main_aspect="proceed_8", advtrains also passes on that this means {main=8, shunt=false}
- The ndef.afunction(pos, node, main_aspect, rem_aspect, rem_aspinfo) determines that the signal should now show
		blinking green with main indicator 12 and dst indicator 8, and sets the nodes accordingly.
		This function can now return the Aspect Info table, which will be cached by advtrains until the aspect changes again
		and will be used when a train approaches the signal. If nil is returned, then the aspect will be queried next time
		by calling ndef.advtrains.get_aspect_info(pos)

Note that once apply_aspect returns, there is no need for advtrains anymore to query the aspect info.
When the signal, for any reason, wants to change its aspect by itself *without* going through the signal API then
it should update the aspect info cache by calling advtrains.interlocking.signal.update_aspect_info(pos)

Apply_aspect may also receive the special main aspect { name = "_halt", halt = true }. It usually means that the signal is not assigned to anything particular,
and it should cause the signal to show its most restrictive aspect. Typically it is a halt aspect, but e.g. for distant-only
signals this would be "expect stop".

A special case occurs for pure distant signals: Such signals must set apply_aspect, but must not set main_aspects. Behavior is as follows:
- Signal is uninitialized, distant signal is not assigned to a main signal, or no route is set: main_aspect == { name = "_halt", halt = true } and rem_aspect == nil
- A remote main signal is assigned (either by user or by route): main_aspect is always { name = "_default" } and rem_aspect / rem_aspinfo give the correct information

Main aspect names starting with underscore (e.g. "_default") are reserved and must not be used!

== Aspect Info ==
The actual signal aspect in the already-known format. This is what the trains use to determine halt/proceed and speed.
asp = {
	main = 0 (halt) / -1 (max speed) / false (no info) / <number> (speed limit)
	shunt = true (shunt free) / false (shunt not free)
	proceed_as_main = true (shunt move can proceed and become train move when main!=0) / false (no)
	dst = speed of the remote signal (like main, informative character, not actually used)
}

Node definition of signals:
- The signal needs some logic to figure out, for each combination of its own aspect group and the distant signal's aspect, what aspect info it can/will show.
ndef.advtrains = {
	main_aspects = {
		{ name = "proceed" description = "Proceed at full speed", <more data at discretion of signal>}
		{ name = "reduced" description = "Proceed at reduced speed", <more data at discretion of signal>}
	}
		-- This list is mainly for the selection dialog. Order of entries determines list order in the dropdown.
		-- Some fields have special meaning:
		-- name: A unique key to identify the main aspect. Might be required by some code.
		-- description: Text shown in UI dropdown
		-- Node can set any other fields at its discretion. They are not touched.
		-- Note: Pure distant signals (that cannot show halt) should NOT have a main_aspects table.
		--       For these signals no main aspect selection UI is shown and they cannot be startpoint of a route
	apply_aspect = function(pos, node, main_aspect, rem_aspect, rem_aspinfo)
		-- set the node to show the desired aspect
		-- called by advtrains when this signal's aspect group or the remote signal's aspect changes
		-- main_aspect is never nil, but can be one of the special aspects { halt = true } or { default = true }
		-- MAY return the aspect_info. If it returns nil then get_aspect_info will be queried at a later point.
	get_aspect_info(pos, main_aspect)
		-- Returns the aspect info table (main, shunt, dst etc.)
	distant_support = true or false
		-- If true, signal is considered in distant signalling. If false or nil, rem_aspect and rem_aspinfo are never set.
	route_role = one of "main", "main_distant", "shunt", "distant", "distant_repeater", "end"
		-- Determines how the signal behaves when routes are set. Only in effect when signal is assigned to a TCB.
		-- main: The signal is a possible endpoint for a train move route. Distant signals before it refer to it.
		-- shunt: The signal is a possible endpoint for a shunt move route. Ignored for distant signals.
		-- distant, distant_repeater: The next signal with role="main" is set as the remote signal. main_aspects may be undefined, the main aspect passed to apply_aspect is a dummy one in this case.
		-- distant: if more than one distant signal is before a main signal, only the last one is assigned (but any number of distant_repeater signals are allowed)