local dig_dust = function(pos, facing)
	local direction = minetest.facedir_to_dir(facing)
	return {
		amount = 10,
		time = 1.0,
		minpos = vector.subtract(pos, vector.new(0.5,0.5,0.5)),
		maxpos = vector.add(pos, vector.new(0.5,0.5,0.5)),
		minvel = vector.multiply(direction, -10),
		maxvel = vector.multiply(direction, -20),
		minacc = {x=0, y=-40, z=0},
		maxacc = {x=0, y=-40, z=0},
		minexptime = 0.25,
		maxexptime = 0.5,
		minsize = 2,
		maxsize = 5,
		collisiondetection = false,
		vertical = false,
		texture = "default_item_smoke.png^[colorize:#9F817080",
	}
end

local burn_smoke = function(pos, amount)
	return {
		amount = math.min(amount, 40),
		time = 1.0,
		minpos = vector.subtract(pos, vector.new(0.5,0.5,0.5)),
		maxpos = vector.add(pos, vector.new(0.5,0.5,0.5)),
		minvel = {x=0, y=2, z=0},
		maxvel = {x=0, y=5, z=0},
		minacc = {x=0, y=0, z=0},
		maxacc = {x=0, y=0, z=0},
		minexptime = 0.5,
		maxexptime = 1.5,
		minsize = 8,
		maxsize = 12,
		collisiondetection = false,
		vertical = false,
		texture = "default_item_smoke.png^[colorize:#000000DD",
	}
end

-- returns newpos, status string, and a return code indicating why the method returned (so the auto-controller can keep trying if it's due to unloaded nodes)
-- 0 - success
-- 1 - failed due to unloaded nodes
-- 2 - failed due to insufficient traction
-- 3 - obstructed by undiggable node
-- 4 - insufficient fuel
-- 5 - unknown builder error during testing
-- 6 - builder with unset output
-- 7 - insufficient builder materials in inventory
digtron.execute_cycle = function(pos, clicker)
	local meta = minetest.get_meta(pos)
	local fuel_burning = meta:get_float("fuel_burning") -- get amount of burned fuel left over from last cycle
	local status_text = string.format("Heat remaining in controller furnace: %d", fuel_burning)
	
	local layout = digtron.get_all_digtron_neighbours(pos, clicker)
	if layout.all == nil then
		-- get_all_digtron_neighbours returns nil if the digtron array touches unloaded nodes, too dangerous to do anything in that situation. Abort.
		minetest.sound_play("buzzer", {gain=0.25, pos=pos})
		return pos, "Digtron is adjacent to unloaded nodes.\n" .. status_text, 1
	end
	
	if layout.water_touching == true then
		minetest.sound_play("sploosh", {gain=1.0, pos=pos})
	end
	if layout.lava_touching == true then
		minetest.sound_play("woopwoopwoop", {gain=1.0, pos=pos})
	end	
	if layout.traction * digtron.traction_factor < table.getn(layout.all) then
		-- digtrons can't fly
		minetest.sound_play("squeal", {gain=1.0, pos=pos})
		return pos, string.format("Digtron has %d nodes but only enough traction to move %d nodes.\n", table.getn(layout.all), layout.traction * digtron.traction_factor)
			 .. status_text, 2
	end

	local facing = minetest.get_node(pos).param2
	local move_dir = minetest.facedir_to_dir(facing)
	local controlling_coordinate = digtron.get_controlling_coordinate(pos, facing)
	
	----------------------------------------------------------------------------------------------------------------------
	
	local nodes_dug = Pointset.create()
	local items_dropped = {}
	local digging_fuel_cost = 0
	local particle_systems = {}
	
	-- execute the execute_dig method on all digtron components that have one
	-- This builds a set of nodes that will be dug and returns a list of products that will be generated
	-- but doesn't actually dig the nodes yet. That comes later.
	-- If we dug them now, sand would fall and some digtron nodes would die.
	for k, location in pairs(layout.diggers) do
		local target = minetest.get_node(location)
		local targetdef = minetest.registered_nodes[target.name]
		if targetdef.execute_dig ~= nil then
			local fuel_cost, dropped = targetdef.execute_dig(location, layout.protected, nodes_dug, controlling_coordinate)
			if dropped ~= nil then
				for _, itemname in pairs(dropped) do
					table.insert(items_dropped, itemname)
				end
				if digtron.particle_effects then
					table.insert(particle_systems, dig_dust(vector.add(location, move_dir), target.param2))
				end
			end
			digging_fuel_cost = digging_fuel_cost + fuel_cost
		else
			minetest.log(string.format("%s has digger group but is missing execute_dig method! This is an error in mod programming, file a bug.", targetdef.name))
		end
	end
	
	----------------------------------------------------------------------------------------------------------------------
	
	-- test if any digtrons are obstructed by non-digtron nodes that haven't been marked
	-- as having been dug.
	local can_move = true
	for _, location in pairs(layout.all) do
		local newpos = digtron.find_new_pos(location, facing)
		if not digtron.can_move_to(newpos, layout.protected, nodes_dug) then
			can_move = false
		end
	end
	
	if not can_move then
		-- mark this node as waiting, will clear this flag in digtron.cycle_time seconds
		minetest.get_meta(pos):set_string("waiting", "true")
		minetest.after(digtron.cycle_time,
			function (pos)
				minetest.get_meta(pos):set_string("waiting", nil)
			end, pos
		)
		minetest.sound_play("squeal", {gain=1.0, pos=pos})
		minetest.sound_play("buzzer", {gain=0.5, pos=pos})
		return pos, "Digtron is obstructed.\n" .. status_text, 3 --Abort, don't dig and don't build.
	end

	----------------------------------------------------------------------------------------------------------------------
	
	-- ask each builder node if it can get what it needs from inventory to build this cycle.
	-- This is a complicated test because each builder needs to actually *take* the item it'll
	-- need from inventory, and then we put it all back afterward.
	-- Note that this test may overestimate the amount of work that will actually need to be done so don't treat its fuel cost as authoritative.
	local can_build = true
	local test_build_return_code = nil
	local test_build_return_item = nil
	local test_items = {}
	local test_fuel_items = {}
	local test_build_fuel_cost = 0		
	for k, location in pairs(layout.builders) do
		local target = minetest.get_node(location)
		local targetdef = minetest.registered_nodes[target.name]
		local test_location = digtron.find_new_pos(location, facing)
		if targetdef.test_build ~= nil then
			test_build_return_code, test_build_return_item = targetdef.test_build(location, test_location, layout.inventories, layout.protected, nodes_dug, controlling_coordinate, layout.controller)
			if test_build_return_code > 1 then
				can_build = false
				break
			end
			if test_build_return_code == 1 then
				table.insert(test_items, test_build_return_item)
				test_build_fuel_cost = test_build_fuel_cost + digtron.build_cost
			end
		else
			minetest.log(string.format("%s has builder group but is missing test_build method! This is an error in mod programming, file a bug.", targetdef.name))
		end
	end
	
	local test_fuel_needed = test_build_fuel_cost + digging_fuel_cost - fuel_burning
	local test_fuel_burned = 0
	if test_fuel_needed > 0 then
		test_fuel_burned = digtron.burn(layout.fuelstores, test_fuel_needed, true)
	end
	
	--Put everything back where it came from
	for k, item_return in pairs(test_items) do
		digtron.place_in_specific_inventory(item_return.item, item_return.location, layout.inventories, layout.controller)
	end
	
	if test_fuel_needed > fuel_burning + test_fuel_burned then
		minetest.sound_play("buzzer", {gain=0.5, pos=pos})
		return pos, "Digtron needs more fuel.", 4 -- abort, don't dig and don't build.
	end
	
	if not can_build then
		minetest.get_meta(pos):set_string("waiting", "true")
		minetest.after(digtron.cycle_time,
			function (pos)
				minetest.get_meta(pos):set_string("waiting", nil)
			end, pos
		)
		local return_string = nil
		local return_code = 5
		if test_build_return_code == 3 then
			minetest.sound_play("honk", {gain=0.5, pos=pos}) -- A builder is not configured
			return_string = "Digtron connected to at least one builder with no output material assigned.\n"
			return_code = 6
		elseif test_build_return_code == 2 then
			minetest.sound_play("dingding", {gain=1.0, pos=pos}) -- Insufficient inventory
			return_string = string.format("Digtron has insufficient building materials. Needed: %s\n",
				test_build_return_item:get_name())
			return_code = 7
		end
		return pos, return_string .. status_text, return_code --Abort, don't dig and don't build.
	end	

	----------------------------------------------------------------------------------------------------------------------
	
	-- All tests passed, ready to go for real!
	minetest.sound_play("construction", {gain=1.0, pos=pos})

	-- store or drop the products of the digger heads
	for _, itemname in pairs(items_dropped) do
		digtron.place_in_inventory(itemname, layout.inventories, pos)
	end

	-- if the player is standing within the array or next to it, move him too.
	local player_pos = clicker:getpos()
	local move_player = false
	if player_pos.x >= layout.extents.min_x - 1 and player_pos.x <= layout.extents.max_x + 1 and
	   player_pos.y >= layout.extents.min_y - 1 and player_pos.y <= layout.extents.max_y + 1 and
	   player_pos.z >= layout.extents.min_z - 1 and player_pos.z <= layout.extents.max_z + 1 then
		move_player = true
	end
	
	-- damage the weak flesh
	if digtron.diggers_damage_creatures then
		for k, location in pairs(layout.diggers) do
			local target = minetest.get_node(location)
			local targetdef = minetest.registered_nodes[target.name]
			if targetdef.damage_creatures ~= nil then
				targetdef.damage_creatures(clicker, location, digtron.find_new_pos(location, target.param2), controlling_coordinate)
			end
		end
	end
	
	--move the array
	digtron.move_digtron(facing, layout.all, layout.extents, nodes_dug, clicker:get_player_name())
	local oldpos = {x=pos.x, y=pos.y, z=pos.z}
	pos = digtron.find_new_pos(pos, facing)
	meta = minetest.get_meta(pos)
	if move_player then
		clicker:moveto(digtron.find_new_pos(player_pos, facing), true)
	end
	
	local building_fuel_cost = 0
	local strange_failure = false
	-- execute_build on all digtron components that have one
	for k, location in pairs(layout.builders) do
		local target = minetest.get_node(location)
		local targetdef = minetest.registered_nodes[target.name]
		if targetdef.execute_build ~= nil then
			--using the old location of the controller as fallback so that any leftovers land with the rest of the digger output. Not that there should be any.
			local build_return = targetdef.execute_build(location, clicker, layout.inventories, layout.protected, nodes_dug, controlling_coordinate, oldpos)
			if build_return == false then
				-- This happens if there's insufficient inventory, but we should have confirmed there was sufficient inventory during test phase.
				-- So this should never happen. However, "should never happens" happen sometimes. So
				-- don't interrupt the build cycle as a whole, we've already moved so might as well try to complete as much as possible.
				strange_failure = true
			elseif build_return == true and not digtron.creative_mode == true then
				building_fuel_cost = building_fuel_cost + digtron.build_cost
			end
		else
			minetest.log(string.format("%s has builder group but is missing execute_build method! This is an error in mod programming, file a bug.", targetdef.name))
		end
	end

	local status_text = ""
	if strange_failure then
		-- We weren't able to detect this build failure ahead of time, so make a big noise now. This is strange, shouldn't happen.
		minetest.sound_play("dingding", {gain=1.0, pos=pos})
		minetest.sound_play("buzzer", {gain=0.5, pos=pos})
		status_text = "Digtron unexpectedly failed to execute one or more build operations, likely due to an inventory error.\n"
	end
	
	-- acutally burn the fuel needed
	local fuel_cost = digging_fuel_cost + building_fuel_cost
	fuel_burning = fuel_burning - fuel_cost
	if digtron.particle_effects then
		table.insert(particle_systems, burn_smoke(pos, fuel_cost))
	end
	if fuel_burning < 0 then
		fuel_burning = fuel_burning + digtron.burn(layout.fuelstores, -fuel_burning, false)
	end
	meta:set_float("fuel_burning", fuel_burning)
	status_text = status_text .. string.format("Heat remaining in controller furnace: %d", fuel_burning)

	-- Eyecandy
	for _, particles in pairs(particle_systems) do
		minetest.add_particlespawner(particles)
	end
	
	-- finally, dig out any nodes remaining to be dug. Some of these will have had their flag revoked because
	-- a builder put something there or because they're another digtron node.
	local node_to_dig, whether_to_dig = nodes_dug:pop()
	while node_to_dig ~= nil do
		if whether_to_dig == true then
			minetest.log("action", string.format("%s uses Digtron to dig %s at (%d, %d, %d)", clicker:get_player_name(), minetest.get_node(node_to_dig).name, node_to_dig.x, node_to_dig.y, node_to_dig.z))
			minetest.remove_node(node_to_dig)
		end
		-- all of the digtron's nodes wind up in nodes_dug, so this is an ideal place to stick
		-- a check to make sand fall after the digtron has passed.
		minetest.check_for_falling({x=node_to_dig.x, y=node_to_dig.y+1, z=node_to_dig.z})
		node_to_dig, whether_to_dig = nodes_dug:pop()
	end
	return pos, status_text, 0
end