--trainlogic.lua
--controls train entities stuff about connecting/disconnecting/colliding trains and other things

local setting_overrun_mode = minetest.settings:get("advtrains_overrun_mode")

local benchmark=false
local bm={}
local bmlt=0
local bmsteps=0
local bmstepint=200
atprintbm=function(action, ta)
	if not benchmark then return end
	local t=(os.clock()-ta)*1000
	if not bm[action] then
		bm[action]=t
	else
		bm[action]=bm[action]+t
	end
	bmlt=bmlt+t
end
function endstep()
	if not benchmark then return end
	bmsteps=bmsteps-1
	if bmsteps<=0 then
		bmsteps=bmstepint
		for key, value in pairs(bm) do
			minetest.chat_send_all(key.." "..(value/bmstepint).." ms avg.")
		end
		minetest.chat_send_all("Total time consumed by all advtrains actions per step: "..(bmlt/bmstepint).." ms avg.")
		bm={}
		bmlt=0
	end
end

--acceleration for lever modes (trainhud.lua), per wagon
local t_accel_all={
	[0] = -10,
	[1] = -3,
	[11] = -2, -- calculation base for LZB
	[2] = -0.5,
	[4] = 0.5,
}
--acceleration per engine
local t_accel_eng={
	[0] = 0,
	[1] = 0,
	[11] = 0,
	[2] = 0,
	[4] = 1.5,
}

local VLEVER_EMERG = 0
local VLEVER_BRAKE = 1
local VLEVER_LZBCALC = 11
local VLEVER_ROLL = 2
local VLEVER_HOLD = 3
local VLEVER_ACCEL = 4

-- How far in front of a whole index with LZB 0 restriction the train should come to a halt
-- value must be between 0 and 0.5, exclusively
local LZB_ZERO_APPROACH_DIST = 0.1
-- Speed the train temporarily approaches the stop point with
local LZB_ZERO_APPROACH_SPEED = 0.2



tp_player_tmr = 0

advtrains.mainloop_trainlogic=function(dtime, stepno)
	--build a table of all players indexed by pts. used by damage and door system.
	advtrains.playersbypts={}
	for _, player in pairs(minetest.get_connected_players()) do
		if not advtrains.player_to_train_mapping[player:get_player_name()] then
			--players in train are not subject to damage
			local ptspos=minetest.pos_to_string(vector.round(player:get_pos()))
			advtrains.playersbypts[ptspos]=player
		end
	end
	
	if tp_player_tmr<=0 then
		-- teleport players to their train every 2 seconds
		for _, player in pairs(minetest.get_connected_players()) do
			advtrains.tp_player_to_train(player)
		end
		tp_player_tmr = 2
	else
		tp_player_tmr = tp_player_tmr - dtime
	end
	--regular train step
	--[[ structure:
	1. make trains calculate their occupation windows when needed (a)
	2. when occupation tells us so, restore the occupation tables (a)
	4. make trains move and update their new occupation windows and write changes
	   to occupation tables (b)
	5. make trains do other stuff (c)
	]]--
	local t=os.clock()
	
	for k,v in pairs(advtrains.trains) do
		advtrains.atprint_context_tid=k
		--atprint("=== Step",stepno,"===")
		advtrains.train_ensure_init(k, v)
	end
	
	advtrains.lock_path_inval = true
	
	for k,v in pairs(advtrains.trains) do
		advtrains.atprint_context_tid=k
		advtrains.train_step_b(k, v, dtime)
	end
	
	for k,v in pairs(advtrains.trains) do
		advtrains.atprint_context_tid=k
		advtrains.train_step_c(k, v, dtime)
	end
	
	advtrains.lock_path_inval = false
	
	advtrains.atprint_context_tid=nil
	
	atprintbm("trainsteps", t)
	endstep()
end

function advtrains.tp_player_to_train(player)
	local pname = player:get_player_name()
	local id=advtrains.player_to_train_mapping[pname]
	if id then
		local train=advtrains.trains[id]
		if not train then advtrains.player_to_train_mapping[pname]=nil return end
		--set the player to the train position.
		--minetest will emerge the area and load the objects, which then will call reattach_all().
		--because player is in mapping, it will not be subject to dying.
		player:set_pos(train.last_pos)
	end
end
minetest.register_on_joinplayer(function(player)
		advtrains.hud[player:get_player_name()] = nil
		advtrains.hhud[player:get_player_name()] = nil
		--independent of this, cause all wagons of the train which are loaded to reattach their players
		--needed because already loaded wagons won't call reattach_all()
		local pname = player:get_player_name()
		local id=advtrains.player_to_train_mapping[pname]
		if id then
			for _, wagon in advtrains.wagon_entity_pairs_in_train(id) do
				wagon:reattach_all()
			end
		end
end)


minetest.register_on_dieplayer(function(player)
		local pname=player:get_player_name()
		local id=advtrains.player_to_train_mapping[pname]
		if id then
			local train=advtrains.trains[id]
			if not train then advtrains.player_to_train_mapping[pname]=nil return end
			for _, wagon in advtrains.wagon_entity_pairs_in_train(id) do
				--when player dies, detach him from the train
				--call get_off_plr on every wagon since we don't know which one he's on.
				wagon:get_off_plr(pname)
			end
			-- just in case no wagon felt responsible for this player: clear train mapping
			advtrains.player_to_train_mapping[pname] = nil
		end
end)

--[[

Zone diagram of a train (copy from occupation.lua!):
              |___| |___| --> Direction of travel
              oo oo+oo oo
=|=======|===|===========|===|=======|===================|========|===
 |SafetyB|CpB|   Train   |CpF|SafetyF|        Brake      |Aware   |
[1]     [2] [3]         [4] [5]     [6]                 [7]      [8]
This mapping from indices in occwindows to zone ids is contained in WINDOW_ZONE_IDS


The occupation system has been abandoned. The constants will still be used
to determine the couple distance
(because of the reverse lookup, the couple system simplifies a lot...)

]]--
-- unless otherwise stated, in meters.
local SAFETY_ZONE = 10
local COUPLE_ZONE = 2 --value in index positions!
local BRAKE_SPACE = 10
local AWARE_ZONE = 10
local WINDOW_ZONE_IDS = {
	2, -- 1 - SafetyB
	4, -- 2 - CpB
	1, -- 3 - Train
	5, -- 4 - CpF
	3, -- 5 - SafetyF
	6, -- 6 - Brake
	7, -- 7 - Aware
}


-- If a variable does not exist in the table, it is assigned the default value
local function assertdef(tbl, var, def)
	if not tbl[var] then
		tbl[var] = def
	end
end

function advtrains.get_acceleration(train, lever)
	local acc_all = t_accel_all[lever]
	if not acc_all then return 0 end
	
	local acc_eng = t_accel_eng[lever]
	local nwagons = #train.trainparts
	if nwagons == 0 then
		-- empty train! avoid division through zero
		return -1
	end
	local acc = acc_all + (acc_eng*train.locomotives_in_train)/nwagons
	return acc
end

-- Small local util function to recalculate train's end index
local function recalc_end_index(train)
	train.end_index = advtrains.path_get_index_by_offset(train, train.index, -train.trainlen)
end

-- Occupation Callback system
-- see occupation.lua
-- signature is advtrains.te_register_on_<?>(function(id, train) ... end)

local function mkcallback(name)
	local callt = {}
	advtrains["te_register_on_"..name] = function(func)
		assertt(func, "function")
		table.insert(callt, func)
	end
	return callt, function(id, train, param1, param2, param3)
		for _,f in ipairs(callt) do
			f(id, train, param1, param2, param3)
		end
	end
end

local callbacks_new_path, run_callbacks_new_path = mkcallback("new_path")
local callbacks_invahead
callbacks_invahead, advtrains.run_callbacks_invahead = mkcallback("invalidate_ahead") -- (id, train, start_idx)
local callbacks_update, run_callbacks_update = mkcallback("update")
local callbacks_create, run_callbacks_create = mkcallback("create")
local callbacks_remove, run_callbacks_remove = mkcallback("remove")

-- required to call from couple.lua
function advtrains.update_train_start_and_end(train)
	recalc_end_index(train)
	run_callbacks_update(train.id, train)
end

-- train_ensure_init: responsible for creating a state that we can work on, after one of the following events has happened:
-- - the train's path got cleared
-- - save files were loaded
-- Additionally, this gets called outside the step cycle to initialize and/or remove a train, then occ_write_mode is set.
function advtrains.train_ensure_init(id, train)
	if not train then
		atwarn("train_ensure_init: Called with id =",id,"but a nil train!")
		atwarn(debug.traceback())
		return nil
	end
	
	train.dirty = true
	if train.no_step then
		--atprint("in ensure_init: no_step set, train step ignored!")
		return nil
	end

	assertdef(train, "velocity", 0)
	--assertdef(train, "tarvelocity", 0)
	assertdef(train, "acceleration", 0)
	assertdef(train, "id", id)
	
	
	if not train.drives_on or not train.max_speed then
		--atprint("in ensure_init: missing properties, updating!")
		advtrains.update_trainpart_properties(id)
	end
	
	--restore path
	if not train.path then
		--atprint("in ensure_init: Needs restoring path...")
		if not train.last_pos then
			atlog("Train",id,": Restoring path failed, no last_pos set! Train will be disabled. You can try to fix the issue in the save file.")
			train.no_step = true
			return nil
		end
		if not train.last_connid then
			atwarn("Train",id,": Restoring path: no last_connid set! Will assume 1")
			train.last_connid = 1
			--[[
			Why this fix was necessary:
			Issue: Migration problems on Grand Theft Auto Minetest
			1. Run of this code, warning printed.
			2. path_create failed with result==nil (there was an unloaded node, wait_for_path set)
			3. in consequence, the supposed call to path_setrestore does not happen
			4. train.last_connid is still unset
			5. next step, warning is printed again
			Result: log flood.
			]]
		end
		
		local result = advtrains.path_create(train, train.last_pos, train.last_connid or 1, train.last_frac or 0)
		
		--atprint("in ensure_init: path_create result ",result)
		
		if result==false then
			atlog("Train",id,": Restoring path failed, node at",train.last_pos,"is gone! Train will be disabled. You can try to place a rail at this position and restart the server.")
			train.no_step = true
			return nil
		elseif result==nil then
			if not train.wait_for_path then
				atlog("Train",id,": Can't initialize: Waiting for the (yet unloaded) node at",train.last_pos," to be loaded.")
			end
			train.wait_for_path = true
			return false
		end
		-- by now, we should have a working initial path
		train.wait_for_path = false
		
		advtrains.update_trainpart_properties(id)
		recalc_end_index(train)
		
		--atdebug("Train",id,": Successfully restored path at",train.last_pos," connid",train.last_connid," frac",train.last_frac)
		
		-- run on_new_path callbacks
		run_callbacks_new_path(id, train)
	end
	
	train.dirty = false -- TODO einbauen!
	return true
end

local function v_target_apply(v_targets, lever, vel)
	v_targets[lever] = v_targets[lever] and math.min(v_targets[lever], vel) or vel
end

function advtrains.train_step_b(id, train, dtime)
	if train.no_step or train.wait_for_path or not train.path then return end
	
	-- in this code, we check variables such as path_trk_? and path_dist. We need to ensure that the path is known for the whole 'Train' zone
	advtrains.path_get(train, atfloor(train.index + 2))
	advtrains.path_get(train, atfloor(train.end_index - 1))
	
	-- run pre-move hooks
	-- TODO: if more pre-move hooks are added, make a separate callback hook
	advtrains.lzb_look_ahead(id, train)
	
	--[[ again, new velocity control:
	There are two heterogenous means of control:
	-> set a fixed acceleration and ignore speed (user)
	-> steer towards a target speed, distance doesn't matter
		-> needs to specify the maximum acceleration/deceleration values they are willing to accelerate/brake with
	-> Reach a target speed after a certain distance (LZB, handled specially)
	
	]]--
	
	--- 3. handle velocity influences ---
	
	local v0 = train.velocity
	local sit_v_cap = train.max_speed -- Maximum speed in current situation (multiple limit factors)
	-- The desired speed change issued by the active control (user or atc)
	local ctrl_v_tar -- desired speed which should not be crossed by braking or accelerating
	local ctrl_accelerating = false -- whether the train should accelerate
	local ctrl_braking = false -- whether the train should brake
	local ctrl_lever -- the lever value to use to calculate the acceleration
	-- the final speed change after applying LZB
	local v_cap -- absolute maximum speed
	local v_tar -- desired speed which should not be crossed by braking or accelerating
	local accelerating = false-- whether the train should accelerate
	local braking = false -- whether the train should brake
	local lever -- the lever value to use to calculate the acceleration
	local train_moves = (v0 > 0)
	
	if train.recently_collided_with_env then
		if not train_moves then
			train.recently_collided_with_env=nil--reset status when stopped
		end
		--atprint("in train_step_b: applying collided_with_env")
		sit_v_cap = 0
	elseif train.locomotives_in_train==0 then
		--atprint("in train_step_b: applying no_locomotives")
		sit_v_cap = 0
	-- interlocking speed restriction
	elseif train.speed_restriction then
		--atprint("in train_step_b: applying interlocking speed restriction",train.speed_restriction)
		sit_v_cap = math.min(sit_v_cap or math.huge, train.speed_restriction)
	end
	
	--apply off-track handling:
	local front_off_track = train.index>train.path_trk_f
	local back_off_track=train.end_index<train.path_trk_b
	train.off_track = front_off_track or back_off_track
	
	if back_off_track and (not sit_v_cap or sit_v_cap > 1) then
		--atprint("in train_step_b: applying back_off_track")
		sit_v_cap = 1
	elseif front_off_track then
		--atprint("in train_step_b: applying front_off_track")
		sit_v_cap = 0
	end
	
	
	--interpret ATC command and apply auto-lever control when not actively controlled
	local userc = train.ctrl_user
	if userc then
		--atprint("in train_step_b: ctrl_user active",userc)
		advtrains.atc.train_reset_command(train)
		
		if userc >= VLEVER_ACCEL then
			ctrl_accelerating = true
		else
			ctrl_braking = true
		end
		ctrl_lever = userc
	else
		if train.atc_command then
			if (not train.atc_delay or train.atc_delay<=0)
					and not train.atc_wait_finish
					and not train.atc_wait_autocouple then
				advtrains.atc.execute_atc_command(id, train)
			elseif train.atc_delay and train.atc_delay > 0 then
				train.atc_delay=train.atc_delay-dtime
			end
		elseif train.atc_delay then
			train.atc_delay = nil
		end
	
		local braketar = train.atc_brake_target
		local emerg = false -- atc_brake_target==-1 means emergency brake (BB command)
		if braketar == -1 then
			braketar = 0
			emerg = true
		end
		--atprint("in train_step_b: ATC: brake state braketar=",braketar,"emerg=",emerg)
		if braketar and braketar>=v0 then
			--atprint("in train_step_b: ATC: brake target cleared")
			train.atc_brake_target=nil
			braketar = nil
		end
		--if train.tarvelocity and train.velocity==train.tarvelocity then
		--	train.tarvelocity = nil
		--end
		if train.atc_wait_finish then
			if not train.atc_brake_target and (not train.tarvelocity or train.velocity==train.tarvelocity) then
				train.atc_wait_finish=nil
			end
		end
		
		if train.tarvelocity and train.tarvelocity>v0 then
			--atprint("in train_step_b: applying ATC ACCEL", train.tarvelocity)
			ctrl_accelerating = true
			ctrl_lever = VLEVER_ACCEL
		elseif train.tarvelocity and train.tarvelocity<v0 then
			ctrl_braking = true
			
			if (braketar and braketar<v0) then
				if emerg then
					--atprint("in train_step_b: applying ATC EMERG", train.tarvelocity)
					ctrl_lever = VLEVER_EMERG
				else
					--atprint("in train_step_b: applying ATC BRAKE", train.tarvelocity)
					ctrl_v_tar = braketar
					ctrl_lever = VLEVER_BRAKE
				end
			else
				--atprint("in train_step_b: applying ATC ROLL", train.tarvelocity)
				ctrl_v_tar = train.tarvelocity
				ctrl_lever = VLEVER_ROLL
			end
		end
	end
	
	--- 2b. look at v_target, determine the effective v_target and desired acceleration ---
	--atprint("in train_step_b: Resulting control before LZB: accelerating",ctrl_accelerating,"braking",ctrl_braking,"lever", ctrl_lever, "target", ctrl_v_tar)
	--train.debug = dump({tv_target,tv_lever})
	
	--atprint("in train_step_b: Current index",train.index,"end",train.end_index,"vel",v0)
	--- 3a. calculate the acceleration required to reach the speed restriction in path_speed (LZB) ---
	-- Iterates over the path nodes we WOULD pass if we were continuing with the current speed
	-- and determines the MINIMUM of path_speed in this range.
	-- Then, determines acceleration so that we can reach this 'overridden' target speed in this step (but short-circuited)
	local lzb_next_zero_barrier -- if defined, train should not pass this point as it's a 0-LZB
	local new_index_curr_tv -- pre-calculated new train index in lzb check
	local lzb_v_cap -- the maximum speed that LZB dictates
	
	local dst_curr_v = v0 * dtime
	new_index_curr_tv = advtrains.path_get_index_by_offset(train, train.index, dst_curr_v)
	local i = atfloor(train.index)
	local psp
	while true do
		psp = train.path_speed[i]
		if psp then
			lzb_v_cap = lzb_v_cap and math.min(lzb_v_cap, psp) or psp
			if psp == 0 and not lzb_next_zero_barrier then
				--atprint("in train_step_b: Found zero barrier: ",i)
				lzb_next_zero_barrier = i - LZB_ZERO_APPROACH_DIST
			end
		end
		if i > new_index_curr_tv then
			break
		end
		i = i + 1
	end
	
	if lzb_next_zero_barrier and train.index < lzb_next_zero_barrier then
		lzb_v_cap = LZB_ZERO_APPROACH_SPEED
	end
	
	--atprint("in train_step_b: LZB calculation yields newindex=",new_index_curr_tv,"lzbtarget=",lzb_v_cap,"zero_barr=",lzb_next_zero_barrier,"")
	
	-- LZB HUD: decrement timer and delete when 0
	if train.hud_lzb_effect_tmr then
		if train.hud_lzb_effect_tmr <=0 then
			train.hud_lzb_effect_tmr = nil
		else
			train.hud_lzb_effect_tmr = train.hud_lzb_effect_tmr - 1
		end
	end
	
	-- We now need to bring ctrl_*, sit_v_cap and lzb_v_cap together to determine the final controls.
	local v_cap = sit_v_cap -- always defined, by default train.max_speed
	if lzb_v_cap and lzb_v_cap < v_cap then
		v_cap = lzb_v_cap
		lever = VLEVER_BRAKE -- actually irrelevant, acceleration is not considered anyway unless v_tar is also set.
		-- display LZB control override in the HUD
		if lzb_v_cap <= v0 then
			train.hud_lzb_effect_tmr = 1
			-- This is to signal the HUD that LZB is active. This works as a timer to avoid HUD blinking
		end
	end
	
	v_tar = ctrl_v_tar
	-- if v_cap is smaller than the current speed, we need to brake in all cases.
	if v_cap < v0 then
		braking = true
		lever = VLEVER_BRAKE
		-- set v_tar to v_cap to not slow down any further than required.
		-- unless control wants us to brake too, then we use control's v_tar.
		if not ctrl_v_tar or ctrl_v_tar > v_cap then
			v_tar = v_cap
		end
	else -- else, use what the ctrl says
		braking = ctrl_braking
		accelerating = ctrl_accelerating and not braking
		lever = ctrl_lever
	end
	train.lever = lever
	
	--atprint("in train_step_b: final control: accelerating",accelerating,"braking",braking,"lever", lever, "target", v_tar)
	
	-- reset train acceleration when holding speed
	if not braking and not accelerating then
		train.acceleration = 0
	end
	
	--- 3b. if braking, modify the velocity BEFORE the movement
	if braking then
		local dv = advtrains.get_acceleration(train, lever) * dtime
		local v1 = v0 + dv
		if v_tar and v1 < v_tar then
			--atprint("in train_step_b: Braking: Hit v_tar!")
			v1 = v_tar
		end
		if v1 > v_cap then
			--atprint("in train_step_b: Braking: Hit v_cap!")
			v1 = v_cap
		end
		if v1 < 0 then
			--atprint("in train_step_b: Braking: Hit 0!")
			v1 = 0
		end
		
		train.acceleration = (v1 - v0) / dtime
		train.velocity = v1
		--atprint("in train_step_b: Braking: New velocity",v1," (yields acceleration",train.acceleration,")")
		-- make saved new_index_curr_tv invalid because speed has changed
		new_index_curr_tv = nil
	end
	
	--- 4. move train ---
	-- if we have calculated the new end index before, don't do that again
	if not new_index_curr_tv then
		local dst_curr_v = train.velocity * dtime
		new_index_curr_tv = advtrains.path_get_index_by_offset(train, train.index, dst_curr_v)
		--atprint("in train_step_b: movement calculation (re)done, yields newindex=",new_index_curr_tv)
	else
		--atprint("in train_step_b: movement calculation reusing from LZB newindex=",new_index_curr_tv)
	end

	-- if the zeroappr mechanism has hit, go no further than zeroappr index
	if lzb_next_zero_barrier and new_index_curr_tv > lzb_next_zero_barrier then
		--atprint("in train_step_b: Zero barrier hit, clipping to newidx_tv=",new_index_curr_tv, "zb_idx=",lzb_next_zero_barrier)
		new_index_curr_tv = lzb_next_zero_barrier
	end

	-- New same-track collision system - check for any other trains within the range we're going to move
	-- do the checks if we either are moving or about to start moving
	if new_index_curr_tv > train.index or accelerating then -- only if train is actually advancing
		-- Note: duplicate code from path_project() because of subtle differences: no frac processing and scanning all occupations
		--[[train.debug = ""
		local atdebug = function(t, ...)
			local text=advtrains.print_concat_table({t, ...})
			train.debug = train.debug..text.."\n"
		end]]
		local base_idx = atfloor(new_index_curr_tv + 1)
		local base_pos = advtrains.path_get(train, base_idx)
		local base_cn =  train.path_cn[base_idx]
		--atdebug(id,"Begin Checking for on-track collisions new_idx=",new_index_curr_tv,"base_idx=",base_idx,"base_pos=",base_pos,"base_cn=",base_cn)
		-- query occupation
		local occ = advtrains.occ.reverse_lookup_sel(base_pos, "close_proximity")
		-- iterate other trains
		for otid, ob_idx in pairs(occ) do
			if otid ~= id then
				--atdebug(id,"Found other train",otid," with matching index ",ob_idx)
				-- Phase 1 - determine if trains are facing and which is the relefant stpo index
				local otrn = advtrains.trains[otid]

				-- retrieve other train's cn and cp
				local ocn = otrn.path_cn[ob_idx]
				local ocp = otrn.path_cp[ob_idx]

				local target_is_inside, ref_index, facing

				if base_cn == ocn then
					-- same direction
					ref_index = otrn.end_index
					same_dir = true
					target_is_inside = (ob_idx >= ref_index)
					--atdebug("Same direction: ref_index",ref_index,"inside=",target_is_inside)
				elseif base_cn == ocp then
					-- facing trains - subtract index frac
					ref_index = otrn.index
					same_dir = false
					target_is_inside = (ob_idx <= ref_index)
					--atdebug("Facing direction: ref_index",ref_index,"inside=",target_is_inside)
				end

				-- Phase 2 - project ref_index back onto our path and check again (necessary because there might be a turnout on the way and we are driving into the flank
				if target_is_inside then
					local our_index = advtrains.path_project(otrn, ref_index, id, "before_end")
					--atdebug("Backprojected our_index",our_index)
					if our_index and our_index <= new_index_curr_tv
							and our_index >= train.index then --FIX: If train was already past the collision point in the previous step, there is no collision! Fixes bug with split_at_index
						-- ON_TRACK COLLISION IS HAPPENING
						-- the actual collision is handled in train_step_c, so set appropriate signal variables
						train.ontrack_collision_info = {
							otid = otid,
							same_dir = same_dir,
						}
						-- clip newindex
						--atdebug("-- Collision detected!")
						new_index_curr_tv = our_index
					end
				end
			end
		end
	end

	-- ## Movement happens here ##
	train.index = new_index_curr_tv
	
	recalc_end_index(train)
	--atprint("in train_step_b: New index",train.index,"end",train.end_index,"vel",train.velocity)
	
	--- 4a. if accelerating, modify the velocity AFTER the movement
	if accelerating then
		local dv = advtrains.get_acceleration(train, lever) * dtime
		local v1 = v0 + dv
		if v_tar and v1 > v_tar then
			--atprint("in train_step_b: Accelerating: Hit v_tar!")
			v1 = v_tar
		end
		if v1 > v_cap then
			--atprint("in train_step_b: Accelerating: Hit v_cap!")
			v1 = v_cap
		end
		
		train.acceleration = (v1 - v0) / dtime
		train.velocity = v1
		--atprint("in train_step_b: Accelerating: New velocity",v1," (yields acceleration",train.acceleration,")")
	end
end

function advtrains.train_step_c(id, train, dtime)
	if train.no_step or train.wait_for_path or not train.path then return end
	
	-- all location/extent-critical actions have been done.
	-- calculate the new occupation window
	run_callbacks_update(id, train)
	
	-- Return if something(TM) damaged the path
	if train.no_step or train.wait_for_path or not train.path then return end
	
	advtrains.path_clear_unused(train)
	
	advtrains.path_setrestore(train)
	
	-- less important stuff
	
	train.check_trainpartload=(train.check_trainpartload or 0)-dtime
	if train.check_trainpartload<=0 then
		advtrains.spawn_wagons(id)
		train.check_trainpartload=2
	end

	local train_moves=(train.velocity~=0)
	local very_short_train = train.trainlen < 3

	--- On-track collision handling - detected in train_step_b, but handled here so all other train movements have already happened.
	if train.ontrack_collision_info then
		train.velocity = 0
		train.acceleration = 0
		--advtrains.atc.train_reset_command(train) will occur in couple_initiate_with if required

		local otrn = advtrains.trains[train.ontrack_collision_info.otid]

		if otrn.velocity == 0 then -- other train must be standing, else don't initiate coupling
			advtrains.couple_initiate_with(train, otrn, not train.ontrack_collision_info.same_dir)
		else
			-- other collision - stop any ATC control
			advtrains.atc.train_reset_command(train)
		end

		train.ontrack_collision_info = nil
		train.couples_up_to_date = true
	end

	-- handle couples if on_track collision handling did not fire
	if train_moves then
		train.couples_up_to_date = nil
	elseif not train.couples_up_to_date then
		if not very_short_train then -- old coupling system is buggy for short trains
			advtrains.train_check_couples(train) -- no guarantee for train order here
		end
		train.couples_up_to_date = true
	end

	--- 8. check for collisions with other trains and damage players ---
	if train_moves then
		-- Note: this code handles collisions with trains that are not on the same path as the current train
		-- The same-track collisions and coupling handling is found in couple.lua and handled from train_step_b() and code 2 blocks above.
		local collided = false
		local coll_grace=2
		local collindex = advtrains.path_get_index_by_offset(train, train.index, -coll_grace)
		local collpos = advtrains.path_get(train, atround(collindex))
		if collpos then
			local rcollpos=advtrains.round_vector_floor_y(collpos)
			local is_loaded_area = advtrains.is_node_loaded(rcollpos)
			for x=-train.extent_h,train.extent_h do
				for z=-train.extent_h,train.extent_h do
					local testpos=vector.add(rcollpos, {x=x, y=0, z=z})
					--- 8a Check collision ---
					if not collided then
						if not very_short_train then -- position collision system is buggy for short trains
							local col_tr = advtrains.occ.check_collision(testpos, id)
							if col_tr then
								train.velocity = 0
								train.acceleration = 0
								advtrains.atc.train_reset_command(train)
								collided = true
							end
						end

						--- 8b damage players ---
						if is_loaded_area and train.velocity > 3 and (setting_overrun_mode=="drop" or setting_overrun_mode=="normal") then
							local testpts = minetest.pos_to_string(testpos)
							local player=advtrains.playersbypts[testpts]
							if player and player:get_hp()>0 and advtrains.is_damage_enabled(player:get_player_name()) then
								--atdebug("damage found",player:get_player_name())
								if setting_overrun_mode=="drop" then
									--instantly kill player
									--drop inventory contents first, to not to spawn bones
									local player_inv=player:get_inventory()
									for i=1,player_inv:get_size("main") do
										minetest.add_item(testpos, player_inv:get_stack("main", i))
									end
									for i=1,player_inv:get_size("craft") do
										minetest.add_item(testpos, player_inv:get_stack("craft", i))
									end
									-- empty lists main and craft
									player_inv:set_list("main", {})
									player_inv:set_list("craft", {})
								end
								player:set_hp(0)
							end
						end
					end
				end
			end
			--- 8c damage other objects ---
			if is_loaded_area then
				local objs = minetest.get_objects_inside_radius(rcollpos, 2)
				for _,obj in ipairs(objs) do
					if not obj:is_player() then
						local armor = obj:get_armor_groups()
						local luaentity = obj:get_luaentity()
						if armor.fleshy and armor.fleshy > 0 and luaentity and luaentity.name ~= "signs_lib:text" then
							obj:punch(obj, 1, { full_punch_interval = 1.0, damage_groups = {fleshy = 1000}, }, nil)
						end
					end
				end
			end
		end
	end
end

-- Default occupation callbacks for node callbacks
-- (remember, train.end_index is set separately because callbacks are
--  asserted to rely on this)

local function mknodecallback(name)
	local callt = {}
	advtrains["tnc_register_on_"..name] = function(func, prio)
		assertt(func, "function")
		if prio then
			table.insert(callt, 1, func)
		else
			table.insert(callt, func)
		end
	end
	return callt, function(pos, id, train, index, paramx1, paramx2, paramx3)
		for _,f in ipairs(callt) do
			f(pos, id, train, index, paramx1, paramx2, paramx3)
		end
	end
end

-- enter/leave-node callbacks
-- signature is advtrains.tnc_register_on_enter/leave(function(pos, id, train, index) ... end)
local callbacks_enter_node, run_callbacks_enter_node = mknodecallback("enter")
local callbacks_leave_node, run_callbacks_leave_node = mknodecallback("leave")

-- Node callback for approaching
-- Might be called multiple times, whenever path is recalculated. Also called for the first node the train is standing on, then has_entered is true.
-- signature is function(pos, id, train, index, has_entered, lzbdata)
-- has_entered: true if the "enter" callback has already been executed for this train in this location
-- lzbdata: arbitrary data (shared between all callbacks), deleted when LZB is restarted.
-- These callbacks are called in order of distance as train progresses along tracks, so lzbdata can be used to
-- keep track of a train's state once it passes this point
local callbacks_approach_node, run_callbacks_approach_node = mknodecallback("approach")


local function tnc_call_enter_callback(pos, train_id, train, index)
	--atdebug("tnc enter",pos,train_id)
	local node = advtrains.ndb.get_node(pos) --this spares the check if node is nil, it has a name in any case
	local mregnode=minetest.registered_nodes[node.name]
	if mregnode and mregnode.advtrains and mregnode.advtrains.on_train_enter then
		mregnode.advtrains.on_train_enter(pos, train_id, train, index)
	end

	-- call other registered callbacks
	run_callbacks_enter_node(pos, train_id, train, index)
	
	-- check for split points
	if mregnode and mregnode.at_conns and #mregnode.at_conns == 3 and train.path_cp[index] == 3 then
		-- train came from connection 3 of a switch, so it split points.
		if not train.points_split then
			train.points_split = {}
		end
		train.points_split[advtrains.encode_pos(pos)] = true
		--atdebug(train_id,"split points at",pos)
	end
end
local function tnc_call_leave_callback(pos, train_id, train, index)
	--atdebug("tnc leave",pos,train_id)
	local node = advtrains.ndb.get_node(pos) --this spares the check if node is nil, it has a name in any case
	local mregnode=minetest.registered_nodes[node.name]
	if mregnode and mregnode.advtrains and mregnode.advtrains.on_train_leave then
		mregnode.advtrains.on_train_leave(pos, train_id, train, index)
	end
	
	-- call other registered callbacks
	run_callbacks_leave_node(pos, train_id, train, index)
	
	-- split points do not matter anymore. clear them
	if train.points_split then
		if train.points_split[advtrains.encode_pos(pos)] then
			train.points_split[advtrains.encode_pos(pos)] = nil
			--atdebug(train_id,"has passed split points at",pos)
		end
		-- any entries left?
		for _,_ in pairs(train.points_split) do
			return
		end
		train.points_split = nil
	end
	-- WARNING possibly unreachable place!
end

function advtrains.tnc_call_approach_callback(pos, train_id, train, index, lzbdata)
	--atdebug("tnc approach",pos,train_id, lzbdata)
	local has_entered = atround(train.index) == index
	
	local node = advtrains.ndb.get_node(pos) --this spares the check if node is nil, it has a name in any case
	local mregnode=minetest.registered_nodes[node.name]
	if mregnode and mregnode.advtrains and mregnode.advtrains.on_train_approach then
		mregnode.advtrains.on_train_approach(pos, train_id, train, index, has_entered, lzbdata)
	end
	
	-- call other registered callbacks
	run_callbacks_approach_node(pos, train_id, train, index, has_entered, lzbdata)
end

-- === te callback definition for tnc node callbacks ===

advtrains.te_register_on_new_path(function(id, train)
	train.tnc = {
		old_index = atround(train.index),
		old_end_index = atround(train.end_index),
	}
	--atdebug(id,"tnc init",train.index,train.end_index)
end)

advtrains.te_register_on_update(function(id, train)
	local new_index = atround(train.index)
	local new_end_index = atround(train.end_index)
	local old_index = train.tnc.old_index
	local old_end_index = train.tnc.old_end_index
	while old_index < new_index do
		old_index = old_index + 1
		local pos = advtrains.round_vector_floor_y(advtrains.path_get(train,old_index))
		tnc_call_enter_callback(pos, id, train, old_index)
	end
	while old_end_index < new_end_index do
		local pos = advtrains.round_vector_floor_y(advtrains.path_get(train,old_end_index))
		tnc_call_leave_callback(pos, id, train, old_end_index)
		old_end_index = old_end_index + 1
	end
	train.tnc.old_index = new_index
	train.tnc.old_end_index = new_end_index
end)

advtrains.te_register_on_create(function(id, train)
	local index = atround(train.index)
	local end_index = atround(train.end_index)
	while end_index <= index do
		local pos = advtrains.round_vector_floor_y(advtrains.path_get(train,end_index))
		tnc_call_enter_callback(pos, id, train, end_index)
		end_index = end_index + 1
	end
	--atdebug(id,"tnc create",train.index,train.end_index)
end)

advtrains.te_register_on_remove(function(id, train)
	local index = atround(train.index)
	local end_index = atround(train.end_index)
	while end_index <= index do
		local pos = advtrains.round_vector_floor_y(advtrains.path_get(train,end_index))
		tnc_call_leave_callback(pos, id, train, end_index)
		end_index = end_index + 1
	end
	--atdebug(id,"tnc remove",train.index,train.end_index)
end)

--returns new id
function advtrains.create_new_train_at(pos, connid, ioff, trainparts)
	local new_id=advtrains.random_id()
	while advtrains.trains[new_id] do new_id=advtrains.random_id() end--ensure uniqueness
	
	local t={}
	t.id = new_id
	
	t.last_pos=pos
	t.last_connid=connid
	t.last_frac=ioff
	
	--t.tarvelocity=0
	t.velocity=0
	t.trainparts=trainparts
	
	advtrains.trains[new_id] = t
	--atdebug("Created new train:",t)
	
	if not advtrains.train_ensure_init(new_id, advtrains.trains[new_id]) then
		atwarn("create_new_train_at",pos,connid,"failed! This might lead to temporary bugs.")
		return
	end
	
	run_callbacks_create(new_id, advtrains.trains[new_id])
	
	return new_id
end

function advtrains.remove_train(id)
	local train = advtrains.trains[id]
	
	if not advtrains.train_ensure_init(id, train) then
		atwarn("remove_train",id,"failed! This might lead to temporary bugs.")
		return
	end
	
	run_callbacks_remove(id, train)
	
	advtrains.path_invalidate(train, true)
	advtrains.couple_invalidate(train)
	
	local tp = train.trainparts
	--atdebug("Removing train",id,"leftover trainparts:",tp)
	
	advtrains.trains[id] = nil
	
	return tp
	
end


function advtrains.add_wagon_to_train(wagon_id, train_id, index)
	local train=advtrains.trains[train_id]
	
	if not advtrains.train_ensure_init(train_id, train) then
		atwarn("Train",train_id,"is not initialized! Operation aborted!")
		return
	end
	
	if index then
		table.insert(train.trainparts, index, wagon_id)
	else
		table.insert(train.trainparts, wagon_id)
	end
	
	advtrains.update_trainpart_properties(train_id)
	recalc_end_index(train)
	run_callbacks_update(train_id, train)
end

-- Note: safe_decouple_wagon() has been moved to wagons.lua

-- this function sets wagon's pos_in_train(parts) properties and train's max_speed and drives_on (and more)
function advtrains.update_trainpart_properties(train_id, invert_flipstate)
	local train=advtrains.trains[train_id]
	train.drives_on=advtrains.merge_tables(advtrains.all_tracktypes)
	--FIX: deep-copy the table!!!
	train.max_speed=20
	train.extent_h = 0;
	
	local rel_pos=0
	local count_l=0
	local shift_dcpl_lock=false
	for i, w_id in ipairs(train.trainparts) do
		
		local data = advtrains.wagons[w_id]
		
		-- 1st: update wagon data (pos_in_train a.s.o)
		if data then
			local wagon = advtrains.wagon_prototypes[data.type or data.entity_name]
			if not wagon then
				local ent = advtrains.wagon_objects[w_id]
				local pdesc
				if ent then
					pdesc = "at " .. minetest.pos_to_string(ent:get_pos())
				elseif train.last_pos then
					pdesc = "near " .. minetest.pos_to_string(train.last_pos)
				else
					pdesc = "at an unknown location"
				end
				atwarn(string.format("Wagon %q %s could not be found. Please check that all required modules are loaded!", data.type, pdesc))
				wagon = advtrains.wagon_prototypes["advtrains:wagon_placeholder"]

			end
			rel_pos=rel_pos+wagon.wagon_span
			data.train_id=train_id
			data.pos_in_train=rel_pos
			data.pos_in_trainparts=i
			if wagon.is_locomotive then
				count_l=count_l+1
			end
			if invert_flipstate then
				data.wagon_flipped = not data.wagon_flipped
				shift_dcpl_lock, data.dcpl_lock = data.dcpl_lock, shift_dcpl_lock
			end
			rel_pos=rel_pos+wagon.wagon_span
			
			if wagon.drives_on then
				for k,_ in pairs(train.drives_on) do
					if not wagon.drives_on[k] then
						train.drives_on[k]=nil
					end
				end
			end
			train.max_speed=math.min(train.max_speed, wagon.max_speed)
			train.extent_h = math.max(train.extent_h, wagon.extent_h or 1);
		end
	end
	train.trainlen = rel_pos
	train.locomotives_in_train = count_l
end


local ablkrng = advtrains.wagon_load_range
-- This function checks whether entities need to be spawned for certain wagons, and spawns them.
-- Called from train_step_*(), not required to check init.
function advtrains.spawn_wagons(train_id)
	local train = advtrains.trains[train_id]
	
	for i = 1, #train.trainparts do
		local w_id = train.trainparts[i]
		local data = advtrains.wagons[w_id]
		if data then
			if data.train_id ~= train_id then
				atwarn("Train",train_id,"Wagon #",i,": Saved train ID",data.train_id,"did not match!")
				data.train_id = train_id
			end
			if not advtrains.wagon_objects[w_id] or not advtrains.wagon_objects[w_id]:get_yaw() then
				-- eventually need to spawn new object. check if position is loaded.
				local index = advtrains.path_get_index_by_offset(train, train.index, -data.pos_in_train)
				local pos   = advtrains.path_get(train, atfloor(index))
				
				if advtrains.position_in_range(pos, ablkrng) then
					--atdebug("wagon",w_id,"spawning")
					local wt = advtrains.get_wagon_prototype(data)
					local wobj = minetest.add_entity(pos, wt)
					if not wobj then
						atwarn("Failed to spawn wagon", w_id, "of type", wt)
					else
						local wagon = wobj:get_luaentity()
						if not wagon then
							atwarn("Wagon", w_id, "of type", wt, "spawned with nil luaentity")
						else
							wagon:set_id(w_id)
						end
					end
				end
			end
		else
			atwarn("Train",train_id,"Wagon #",1,": A wagon with id",w_id,"does not exist! Wagon will be removed from train.")
			table.remove(train.trainparts, i)
			i = i - 1
		end
	end
end

function advtrains.split_train_at_index(train, index)
	-- this function splits a train at index, creating a new train from the back part of the train.
	--atdebug("split_train_at_index invoked on",train.id,"index",index)

	local train_id=train.id
	if index > #train.trainparts then
		-- index specified too long
		return
	end
	local w_id = train.trainparts[index]
	local data = advtrains.wagons[w_id]
	local _, wagon = advtrains.get_wagon_prototype(data)
	if not advtrains.train_ensure_init(train_id, train) then
		atwarn("Train",train_id,"is not initialized! Operation aborted!")
		return
	end
	
	-- make sure that the train is fully on track before splitting. May cause problems otherwise
	if train.index > train.path_trk_f or train.end_index < train.path_trk_b then
		atwarn("Train",train_id,": cannot split train because it is off track!")
		return
	end

	local p_index=advtrains.path_get_index_by_offset(train, train.index, - data.pos_in_train + wagon.wagon_span)
	local pos, connid, frac = advtrains.path_getrestore(train, p_index)
	--atdebug("new train position p_index",p_index,"pos",pos,"connid",connid,"frac",frac)
	local tp = {}
	for k,v in ipairs(train.trainparts) do
		if k >= index then
			table.insert(tp, v)
			train.trainparts[k] = nil
		end
	end
	advtrains.update_trainpart_properties(train_id)
	recalc_end_index(train)
	--atdebug("old train index",train.index,"end_index",train.end_index)
	run_callbacks_update(train_id, train)
	
	--create subtrain
	local newtrain_id=advtrains.create_new_train_at(pos, connid, frac, tp)
	local newtrain=advtrains.trains[newtrain_id]
	--atdebug("new train created with ID",newtrain_id,"index",newtrain.index,"end_index",newtrain.end_index)

	newtrain.velocity=train.velocity
	-- copy various properties from the old to the new train
	newtrain.door_open = train.door_open
	newtrain.text_outside = train.text_outside
	newtrain.text_inside = train.text_inside
	newtrain.line = train.line
	newtrain.routingcode = train.routingcode
	newtrain.speed_restriction = train.speed_restriction
	newtrain.speed_restrictions_t = table.copy(train.speed_restrictions_t or {main=train.speed_restriction})
	newtrain.is_shunt = train.is_shunt
	newtrain.points_split = advtrains.merge_tables(train.points_split)
	newtrain.autocouple = train.autocouple

	return newtrain_id -- return new train ID, so new train can be manipulated

end

function advtrains.invert_train(train_id)
	local train=advtrains.trains[train_id]
	
	if not advtrains.train_ensure_init(train_id, train) then
		atwarn("Train",train_id,"is not initialized! Operation aborted!")
		return
	end
	
	advtrains.path_setrestore(train, true)
	
	-- rotate some other stuff
	if train.door_open then
		train.door_open = - train.door_open
	end
	if train.atc_command then
		train.atc_arrow = not train.atc_arrow
	end
	
	advtrains.path_invalidate(train, true)
	advtrains.couple_invalidate(train)
	
	local old_trainparts=train.trainparts
	train.trainparts={}
	for k,v in ipairs(old_trainparts) do
		table.insert(train.trainparts, 1, v)--notice insertion at first place
	end
	advtrains.update_trainpart_properties(train_id, true)
	
	-- recalculate path
	
	-- If interlocking present, check whether this train is in a section and then set as shunt move after reversion
	if advtrains.interlocking and train.il_sections and #train.il_sections > 0 then
		train.is_shunt = true
		advtrains.speed.set_restriction(train, "main", advtrains.SHUNT_SPEED_MAX)
	else
		train.is_shunt = false
		advtrains.speed.set_restriction(train, "main", -1)
	end
end

-- returns: train id, index of one of the trains that stand at this position.
function advtrains.get_train_at_pos(pos)
	local t = advtrains.occ.get_trains_at(pos)
	for tid,idx in pairs(t) do
		return tid, idx
	end
end


-- ehm... I never adapted this function to the new path system ?!
function advtrains.invalidate_all_paths(pos)
	local tab
	if pos then
		-- if position given, check occupation system
		tab = advtrains.occ.reverse_lookup_quick(pos)
	else
		tab = advtrains.trains
	end
	
	for id, _ in pairs(tab) do
		advtrains.invalidate_path(id)
	end
end

-- Calls invalidate_path_ahead on all trains occupying (having paths over) this node
-- Can be called during train step.
function advtrains.invalidate_all_paths_ahead(pos)
	local tab = advtrains.occ.reverse_lookup_sel(pos, "first_ahead")
	
	for id,index in pairs(tab) do
		local train = advtrains.trains[id]
		advtrains.path_invalidate_ahead(train, index, true)
	end
end

function advtrains.invalidate_path(id)
	--atdebug("Path invalidate:",id)
	local v=advtrains.trains[id]
	if not v then return end
	advtrains.path_invalidate(v)
	advtrains.couple_invalidate(v)
	v.dirty = true
end

--not blocking trains group

if minetest.settings:get_bool("advtrains_forgiving_collision") then
	function advtrains.train_collides(node)
		if node and minetest.registered_nodes[node.name] then
			local ndef = minetest.registered_nodes[node.name]
			-- if the node is drawtype normal (that is a full cube) then it does collide
			if ndef.drawtype == "normal" then
				-- except if it is not_blocking_trains
				if ndef.groups.not_blocking_trains and ndef.groups.not_blocking_trains ~= 0 then
					return false
				end
				return true
			end
		end
		return false
	end
else
	function advtrains.train_collides(node)
		if node and minetest.registered_nodes[node.name] and minetest.registered_nodes[node.name].walkable then
		if not minetest.registered_nodes[node.name].groups.not_blocking_trains then
			return true
		end
		end
		return false
	end
	
	local nonblocknodes={
		"default:fence_wood",
		"default:fence_acacia_wood",
		"default:fence_aspen_wood",
		"default:fence_pine_wood",
		"default:fence_junglewood",
		"default:torch",
		"bones:bones",
		
		"default:sign_wall",
		"signs:sign_wall",
		"signs:sign_wall_blue",
		"signs:sign_wall_brown",
		"signs:sign_wall_orange",
		"signs:sign_wall_green",
		"signs:sign_yard",
		"signs:sign_wall_white_black",
		"signs:sign_wall_red",
		"signs:sign_wall_white_red",
		"signs:sign_wall_yellow",
		"signs:sign_post",
		"signs:sign_hanging",
		
	}
	minetest.after(0, function()
							for _,name in ipairs(nonblocknodes) do
								if minetest.registered_nodes[name] then
									minetest.registered_nodes[name].groups.not_blocking_trains=1
								end
							end
	end)
end