-- TODO: check inscription of a sign?
-- TODO: check mesecons related things?

-- Which diffrent types of preconditions are available?
-- -> The following fields are part of a precondition:
-- 	p_id		the ID/key of the precondition/prerequirement
-- 	p_type		selected from values_what
-- 	p_value		used to store the subtype of p_type
--
-- a state/variable ("state"):
--	p_variable	name of a variable the player has read access to;
--	                dropdown list with allowed options
--	p_operator	selected from values_operator
--	p_var_cmp_value can be set freely by the player
--
-- a block in the world ("block"):
--	p_pos		a position in the world; determined by asking the player
--			to punch the block
--	p_node		(follows from p_pos)
--	p_param2	(follows from p_pos)
--
-- a trade defined as an action ("trade"): no variables needed (buy and pay stack
-- 			follow from the trade set as action)
--
-- an inventory: ("player_inv", "npc_inv" or "block_inv")
-- 	p_itemstack	an itemstack; needs to be a minetest.registered_item[..];
-- 			size/count is also checked
--
-- the inventory of a block on the map: ("block_inv", in addition to the ones above)
--	p_pos		a position in the world; determined by asking the player
--			to punch the block
--	p_inv_list_name name of the inventory list of the block
--
-- the player offered/gave the NPC an item: ("player_offered_item"):
-- 	p_value    	an itemstack; needs to be a minetest.registered_item[..];
-- 	                size/count is checked for some subtypes
--      p_match_stack_size   does the NPC expect exactly one stack size - or is
--                           more or less etc. also ok?
--      p_item_group    are items of this group instead of the exact item name
--                      also acceptable?
--      p_item_desc     the description of the itemstack (set by another quest NPC
--                      so that the player can distinguish it from other itemstacks
--                      with the same item name; see action "npc_gives")
--      p_item_quest_id Special ID to make sure that it is really the *right*
--                      item and not just something the player faked with an
--                      engraving table or something similar
--
-- a function ("function"): requires npc_master to create and edit
--      p_value         the lua code to execute and evaulate
--
-- a custom function ("custon"):
--      p_value         parameter for the custom function
--
-- depends on another option:
--      p_value         name of the other option of this dialog that is considered
--      p_fulfilled     shall option p_value be true or false?


-- some helper lists for creating the formspecs and evaulating
-- the player's answers:

-- general direction of what a prerequirement may be about
local check_what = {
	"- please select -",
	"an internal state (i.e. of a quest)", -- 2
	"a block somewhere", -- 3
	"a trade", -- 4
	"the inventory of the player", -- 5
	"the inventory of the NPC", -- 6
	"the inventory of a block somewhere", -- 7
	"an item the player offered/gave to the NPC", -- 8
	"execute Lua code (requires npc_master priv)", -- 7 -> 9
	"Call custom functions that are supposed to be overridden by the server.", -- 8 -> 10
	"The preconditions of another dialog option are fulfilled/not fulfilled.", -- 9 -> 11
}

-- how to store these as p_type in the precondition:
local values_what = {"", "state", "block", "trade",
	"player_inv", "npc_inv", "block_inv",
	"player_offered_item",
	-- "function" requires npc_master priv:
	"function",
	-- custom function (does not require npc_master priv)
	"custom",
	-- depends on the preconditions of another option
	"other"}

-- options for "a trade"
local check_trade = {
	"- please select -",
	"The NPC has the item(s) he wants to sell in his inventory.", -- 2
	"The player has the item(s) needed to pay the price.", -- 3
	"The NPC ran out of stock.", -- 4
	"The player cannot afford the price.", -- 5
}

-- how to store these as p_value:
local values_trade = {"", "npc_can_sell", "player_can_buy", "npc_is_out_of_stock", "player_has_not_enough"}

-- options for "the inventory of " (either player or NPC; perhaps blocks later on)
local check_inv = {
	"- please select -",
	"The inventory contains the following item:",
	"The inventory *does not* contain the following item:",
	"There is room for the following item in the inventory:",
	"The inventory is empty.",
}

-- how to store these as p_value (the actual itemstack gets stored as p_itemstack):
local values_inv = {"", "inv_contains", "inv_does_not_contain", "has_room_for", "inv_is_empty"}

local check_block = {
	"- please select -",
	"The block is as it is now.",
	"There shall be air instead of this block.",
	"The block is diffrent from how it is now.",
	"I can't punch it. The block is as the block *above* the one I punched.",
}

-- how to store these as p_value (the actual node data gets stored as p_node, p_param2 and p_pos):
-- Note: "node_is_like" occours twice because it is used to cover blocks that
--       cannot be punched as well as normal blocks.
local values_block = {"", "node_is_like", "node_is_air", "node_is_diffrent_from", "node_is_like"}

-- comparison operators for variables
local check_operator = {
	"- please select -", -- 1
	"== (is equal)", -- 2
	"~= (is not equal)", -- 3
	">= (is greater or equal)", -- 4
	">  (is greater)", -- 5
	"<= (is smaller or equal)", -- 6
	"<  (is smaller)", -- 7
	"not (logically invert)", -- 8
	"is_set (has a value)", -- 9
	"is_unset (has no value)", -- 10
	"more than x seconds ago", -- 11
	"less than x seconds ago", -- 12
	"has completed quest step", -- 13
	"quest step *not* completed", -- 14
}

-- how to store these as p_value (the actual variable is stored in p_variable, and the value in p_cmp_value):
local values_operator = {"", "==", "~=", ">=", ">", "<=", "<", "not", "is_set", "is_unset",
	"more_than_x_seconds_ago","less_than_x_seconds_ago",
	"quest_step_done", "quest_step_not_done"}

-- some internal ones...
local check_variable = {
--	"- please select -", -- this is automaticly added to the var list
	"(internal) hour of ingame day", -- 2
	"(internal) player's health points", -- 3
}


-- get the list of variables the player has read access to
yl_speak_up.get_sorted_player_var_list_read_access = function(pname)
	local var_list = {}
	-- copy the values from check_variable
	for i, v in ipairs(check_variable) do
		table.insert(var_list, v)
	end
	-- get the list of variables the player can read
	local tmp = yl_speak_up.get_quest_variables_with_read_access(pname)
	-- sort that list (the dropdown formspec element returns just an index)
	table.sort(tmp)
	for i, v in ipairs(tmp) do
		table.insert(var_list, v)
	end
	return var_list
end


-- returns a human-readable text as description of the precondition
-- (as shown in the edit options dialog and in the edit precondition formspec)
yl_speak_up.show_precondition = function(p, pname)
	if(not(p.p_type) or p.p_type == "") then
		return "(nothing): Always true."
	elseif(p.p_type == "item") then
		return "item: The player has \""..tostring(p.p_value).."\" in his inventory."
	elseif(p.p_type == "quest") then
		return "quest: Always false."
	elseif(p.p_type == "auto") then
		return "auto: Always true."
	elseif(p.p_type == "function") then
		return "function: evaluate "..tostring(p.p_value)
	elseif(p.p_type == "state") then
		local var_name = "VALUE_OF[ - ? - ]"
		if(p.p_variable) then
			var_name = "VALUE_OF[ "..tostring(
					yl_speak_up.strip_pname_from_var(p.p_variable, pname)).." ]"
		end
		if(not(p.p_operator)) then
			return "Error: Operator not defined."
		elseif(p.p_operator == "not") then
			return "not( "..var_name.." )"
		elseif(p.p_operator == "is_set") then
			return var_name.." ~= nil (is_set)"
		elseif(p.p_operator == "is_unset") then
			return var_name.." == nil (is_unset)"
		elseif(p.p_operator == "more_than_x_seconds_ago") then
			return var_name.." was set to current time "..
				"*more* than "..tostring(p.p_var_cmp_value).." seconds ago"
		elseif(p.p_operator == "less_than_x_seconds_ago") then
			return var_name.." was set to current time "..
				"*less* than "..tostring(p.p_var_cmp_value).." seconds ago"
		elseif(p.p_operator == "quest_step_done") then
			return var_name.." shows: player completed quest step \""..
				tostring(p.p_var_cmp_value).."\" successfully"
		elseif(p.p_operator == "quest_step_not_done") then
			return var_name.." shows: player has not yet completed quest step \""..
				tostring(p.p_var_cmp_value).."\""
		end
		if(p.p_var_cmp_value == "") then
			return var_name.." "..tostring(p.p_operator).." \"\""
		end
		return var_name.." "..tostring(p.p_operator).." "..
			tostring(p.p_var_cmp_value)
	elseif(p.p_type == "block") then
		if(not(p.p_pos) or type(p.p_pos) ~= "table"
		  or not(p.p_pos.x) or not(p.p_pos.y) or not(p.p_pos.z)) then
			return "ERROR: p.p_pos is "..minetest.serialize(p.p_pos)
		elseif(p.p_value == "node_is_like") then
			return "The block at "..minetest.pos_to_string(p.p_pos).." is \""..
				tostring(p.p_node).."\" with param2: "..tostring(p.p_param2).."."
		elseif(p.p_value == "node_is_air") then
			return "There is no block at "..minetest.pos_to_string(p.p_pos).."."
		elseif(p.p_value == "node_is_diffrent_from") then
			return "There is another block than \""..tostring(p.p_node).."\" at "..
				minetest.pos_to_string(p.p_pos)..", or it is at least "..
				"rotated diffrently (param2 is not "..tostring(p.p_param2)..")."
		end
	elseif(p.p_type == "trade") then
		local nr = table.indexof(values_trade, p.p_value)
		if(nr and check_trade[ nr ]) then
			return check_trade[ nr ]
		end
	elseif(p.p_type == "player_inv" or p.p_type == "npc_inv" or p.p_type == "block_inv") then
		local who = "The player"
		local what = "\""..tostring(p.p_itemstack).."\" in his inventory."
		if(p.p_type == "npc_inv") then
			who = "The NPC"
		elseif(p.p_type == "block_inv") then
			if(not(p.p_pos) or type(p.p_pos) ~= "table"
			  or not(p.p_pos.x) or not(p.p_pos.y) or not(p.p_pos.z)) then
				return "ERROR: p.p_pos is "..minetest.serialize(p.p_pos)
			end
			who = "The block at "..minetest.pos_to_string(p.p_pos)
			what = "\""..tostring(p.p_itemstack).."\" in inventory list \""..
				tostring(p.p_inv_list_name).."\"."
		end
		if(p.p_value == "inv_contains") then
			return who.." has "..what
		elseif(p.p_value == "inv_does_not_contain") then
			return who.." does not have "..what
		elseif(p.p_value == "has_room_for") then
			return who.." has room for "..what
		elseif(p.p_value == "inv_is_empty") then
			if(p.p_type == "block_inv") then
				return who.." has an empty inventory list \""..
					tostring(p.p_inv_list_name).."\"."
			end
			return who.." has an empty inventory."
		end
	elseif(p.p_type == "player_offered_item") then
		local item = tostring(p.p_value:split(" ")[1])
		local amount = tostring(p.p_value:split(" ")[2])
		local match = "any amount"
		if(p.p_match_stack_size == "any") then
			match = "any amount"
		elseif(p.p_match_stack_size == "exactly") then
			match = "exactly "..tostring(amount)
		elseif(p.p_match_stack_size == "less"
		    or p.p_match_stack_size == "more") then
			match = p.p_match_stack_size.." than "..tostring(amount)
		elseif(p.p_match_stack_size == "another") then
			match = "another amount than " ..tostring(amount)
		end
		if(p.p_item_group and p.p_item_group ~= "") then
			return "The player offered "..tostring(match).." item(s) of the group \""..
				tostring(item).."\"."
		elseif((p.p_item_quest_id and p.p_item_quest_id ~= "")
		    or (p.p_item_desc and p.p_item_desc ~= "")) then
			return "The player offered "..tostring(match).." of \""..
				tostring(p.p_item_desc or "- default description -")..
	                        "\" (\""..tostring(item or "- ? -").."\") "..
	                        "with ID \""..tostring(p.p_item_quest_id or "- no special ID -").."\"."
		else
			return "The player offered "..tostring(match).." of \""..tostring(item).."\"."
		end
	elseif(p.p_type == "custom") then
		return "Call custom function with param: \""..tostring(p.p_value).."\"."
	elseif(p.p_type == "other") then
		local fulfilled = "fulfilled"
		if(not(p.p_fulfilled) or p.p_fulfilled ~= "true") then
			fulfilled = "*not* fulfilled"
		end
		return "The preconditions for dialog option \""..tostring(p.p_value).."\" are "..
			fulfilled.."."
	end
	-- fallback
	return tostring(p.p_value)
end


-- this is called directly in yl_speak_up.get_fs_talkdialog
-- it returns a list of options whose preconditions are fulfilled
-- allow_recursion may be false - we need to avoid infinite loops
yl_speak_up.calculate_displayable_options = function(pname, d_options, in_edit_mode, allow_recursion)
    -- Let's go through all the options and see if we need to display them to the user

    local retval = {}

    local player = minetest.get_player_by_name(pname)

    if d_options == nil then
        return {}
    end

    -- sort the entries by o_sort so that preconditions referencing options earlier in the
    -- list can work without causing loops or the like
    local sorted_list = yl_speak_up.get_sorted_options(d_options, "o_sort")
    for i, o_k in ipairs(sorted_list) do
	if(not(in_edit_mode)) then
		local o_v = d_options[ o_k ]
		-- Can we display this option?
		retval[o_k] = yl_speak_up.eval_all_preconditions(player, o_v.o_prerequisites, o_k, retval)
		-- do we need to take care of an automatic autoanswer?
		if(retval[o_k] and retval[o_k] == true and o_v.o_autoanswer and o_v.o_autoanswer == 1
		   and allow_recursion) then
			-- abort here - because we already know which option needs to be selected next
			retval["autoanswer"] = o_k
			return retval
		end
	-- if in edit mode: no need to evaluate preconditions
	else
		retval[o_k ] = true
	end
    end
    return retval
end


-- called by calculate_displayable_options(..);
-- returns false if a single precondition is false
-- Important: If something cannot be determined (i.e. the node is nil),
--            *both* the condition and its inverse condition may be
--            true (or false).
yl_speak_up.eval_all_preconditions = function(player, prereq, o_id, other_options_true_or_false)
	local pname = player:get_player_name()
	local n_id = yl_speak_up.speak_to[pname].n_id
	if(not(prereq)) then
		yl_speak_up.debug_msg(player, n_id, o_id, "No preconditions given.")
		-- no prerequirements? then they are automaticly fulfilled
		return true
	end
	yl_speak_up.debug_msg(player, n_id, o_id, "Checking preconditions..")
	for k, p in pairs(prereq) do
		yl_speak_up.debug_msg(player, n_id, o_id, "..checking "..
			tostring(p.p_id)..": "..yl_speak_up.show_precondition(p, pname))
		if(not(yl_speak_up.eval_precondition(player, n_id, p, other_options_true_or_false))) then
			yl_speak_up.debug_msg(player, n_id, o_id, tostring(p.p_id)..
				" -> is false. Aborting.")
			-- no need to look any further - once we hit a false, it'll stay false
			return false
		end
	end
	-- all preconditions are true
	yl_speak_up.debug_msg(player, n_id, o_id, "OK. All preconditions true.")
	return true
end


-- checks if precondition p is true for the player and npc n_id
yl_speak_up.eval_precondition = function(player, n_id, p, other_options_true_or_false)
	if(not(p.p_type) or p.p_type == "") then
		-- empty prerequirement: automaticly true (fallback)
		return true
	elseif(p.p_type == "item") then
		-- a precondition set by using the staff;
		-- aequivalent to p.p_type == "player_inv" and p.p_itemstack == "inv_contains"
		return player:get_inventory():contains_item("main", p.p_value)
	elseif(p.p_type == "quest") then
		-- a precondition set by using the staff; intended as future quest interface?
		return false
	elseif(p.p_type == "auto") then
		-- a precondition set by using the staff; kept for compatibility
		return true
	elseif(p.p_type == "function") then
		-- a precondition set by using the staff;
		-- extremly powerful (executes any lua code)
		return yl_speak_up.eval_and_execute_function(player, p, "p_")
	elseif(p.p_type == "state") then
		local var_val = false
		if(not(p.p_variable) or p.p_variable == "") then
			-- broken precondition
			return false
		-- "(internal) hour of ingame day", -- 2
		elseif(p.p_variable == check_variable[2]) then
			-- timeofday is between 0..1; translate to 24 hours
			var_val = math.floor((minetest.get_timeofday() * 24)+0.5)
		-- "(internal) player's health points", -- 3
		elseif(p.p_variable == check_variable[3]) then
			var_val = player:get_hp()
		else
			local pname = player:get_player_name()
			local owner = yl_speak_up.npc_owner[ n_id ]
			-- get the value of the variable
			-- the owner is alrady encoded in the variable name
			var_val = yl_speak_up.get_quest_variable_value(pname, p.p_variable)
		end

		if(p.p_operator == "not") then
			return not(var_val)
		elseif(p.p_operator == "is_set") then
			return var_val ~= nil
		elseif(p.p_operator == "is_unset") then
			return var_val == nil
		-- for security reasons: do this manually instead of just evaluating a term
		elseif(p.p_operator == "==") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			-- best do these comparisons in string form to make sure both are of same type
			return tostring(var_val) == tostring(p.p_var_cmp_value)
		elseif(p.p_operator == "~=") then
			return tostring(var_val) ~= tostring(p.p_var_cmp_value)
		elseif(p.p_operator == ">=") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			-- compare numeric if possible
			if(tonumber(var_val) and tonumber(p.p_var_cmp_value)) then
				return tonumber(var_val) >= tonumber(p.p_var_cmp_value)
			-- fallback: compare as strings
			else
				return tostring(var_val) >= tostring(p.p_var_cmp_value)
			end
		elseif(p.p_operator == ">") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			if(tonumber(var_val) and tonumber(p.p_var_cmp_value)) then
				return tonumber(var_val) >  tonumber(p.p_var_cmp_value)
			else
				return tostring(var_val) >  tostring(p.p_var_cmp_value)
			end
		elseif(p.p_operator == "<=") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			if(tonumber(var_val) and tonumber(p.p_var_cmp_value)) then
				return tonumber(var_val) <= tonumber(p.p_var_cmp_value)
			else
				return tostring(var_val) <= tostring(p.p_var_cmp_value)
			end
		elseif(p.p_operator == "<") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			if(tonumber(var_val) and tonumber(p.p_var_cmp_value)) then
				return tonumber(var_val) <  tonumber(p.p_var_cmp_value)
			else
				return tostring(var_val) <  tostring(p.p_var_cmp_value)
			end
		elseif(p.p_operator == "more_than_x_seconds_ago") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			if(not(tonumber(var_val)) or not(tonumber(p.p_var_cmp_value))) then
				return true
			end
			return (tonumber(var_val) + tonumber(p.p_var_cmp_value)) <
				math.floor(minetest.get_us_time()/1000000)
		elseif(p.p_operator == "less_than_x_seconds_ago") then
			if(p.p_var_cmp_value == nil) then
				return false
			end
			if(not(tonumber(var_val)) or not(tonumber(p.p_var_cmp_value))) then
				return false
			end
			return (tonumber(var_val) + tonumber(p.p_var_cmp_value)) >
				minetest.get_us_time()/1000000
		-- this is currently equivalent to >= but may change in the future
		-- TODO: quest steps may be strings in the future
		elseif(p.p_operator == "quest_step_done") then
			-- if the variable is not set at all, then the quest step definitely
			-- has not been reached yet
			if((p.p_var_cmp_value == nil) or (var_val == nil)) then
				return false
			end
			-- compare numeric if possible
			if(tonumber(var_val) and tonumber(p.p_var_cmp_value)) then
				return tonumber(var_val) >= tonumber(p.p_var_cmp_value)
			-- fallback: compare as strings
			else
				return tostring(var_val) >= tostring(p.p_var_cmp_value)
			end
		-- this is currently equivalent to < but may change in the future
		-- TODO: quest steps may be strings in the future
		elseif(p.p_operator == "quest_step_not_done") then
			-- if the variable is not set at all, then the quest step definitely
			-- has not been reached yet
			if((p.p_var_cmp_value == nil) or (var_val == nil)) then
				return true
			end
			if(tonumber(var_val) and tonumber(p.p_var_cmp_value)) then
				return tonumber(var_val) <  tonumber(p.p_var_cmp_value)
			else
				return tostring(var_val) <  tostring(p.p_var_cmp_value)
			end
		end
		-- unsupported operator
		return false
	elseif(p.p_type == "block") then
		if(not(p.p_pos) or type(p.p_pos) ~= "table"
		  or not(p.p_pos.x) or not(p.p_pos.y) or not(p.p_pos.z)) then
			return false
		elseif(p.p_value == "node_is_like") then
			local node = minetest.get_node_or_nil(p.p_pos)
			return (node and node.name and node.name == p.p_node and node.param2 == p.p_param2)
		elseif(p.p_value == "node_is_air") then
			local node = minetest.get_node_or_nil(p.p_pos)
			return (node and node.name and node.name == "air")
		elseif(p.p_value == "node_is_diffrent_from") then
			local node = minetest.get_node_or_nil(p.p_pos)
			return (node and node.name and (node.name ~= p.p_node or node.param2 ~= p.p_param2))
		end
		-- fallback - unsupported option
		return false
	elseif(p.p_type == "trade") then
		local pname = player:get_player_name()
		local dialog = yl_speak_up.speak_to[pname].dialog
		local n_id = yl_speak_up.speak_to[pname].n_id
		local d_id = yl_speak_up.speak_to[pname].d_id
		local o_id = yl_speak_up.speak_to[pname].o_id
		-- if there is no trade, then this condition is true
		if(not(dialog) or not(dialog.trades) or not(d_id) or not(o_id)) then
			return true
		end
		local trade = dialog.trades[ tostring(d_id).." "..tostring(o_id) ]
		-- something is wrong with the trade
		if(not(trade)
		  or not(trade.pay) or not(trade.pay[1]) or not(trade.buy) or not(trade.buy[1])) then
			return false
		end
		if(    p.p_value == "npc_can_sell") then
			local npc_inv = minetest.get_inventory({type="detached",
				name="yl_speak_up_npc_"..tostring(n_id)})
			return npc_inv:contains_item("npc_main", trade.buy[1])
		elseif(p.p_value == "npc_is_out_of_stock") then
			local npc_inv = minetest.get_inventory({type="detached",
				name="yl_speak_up_npc_"..tostring(n_id)})
			return not(npc_inv:contains_item("npc_main", trade.buy[1]))
		elseif(p.p_value == "player_can_buy") then
			local player_inv = player:get_inventory()
			return player_inv:contains_item("main", trade.pay[1])
		elseif(p.p_value == "player_has_not_enough") then
			local player_inv = player:get_inventory()
			return not(player_inv:contains_item("main", trade.pay[1]))
		end
		return false
	elseif(p.p_type == "player_inv" or p.p_type == "npc_inv" or p.p_type == "block_inv") then
		local inv = nil
		local inv_name = "main"
		-- determine the right inventory
		if(p.p_type == "player_inv") then
			inv = player:get_inventory()
		elseif(p.p_type == "npc_inv") then
			inv = minetest.get_inventory({type="detached",
                                name="yl_speak_up_npc_"..tostring(n_id)})
			inv_name = "npc_main"
		elseif(p.p_type == "block_inv") then
			if(not(p.p_pos) or type(p.p_pos) ~= "table"
			  or not(p.p_pos.x) or not(p.p_pos.y) or not(p.p_pos.z)) then
				return false
			end
			local meta = minetest.get_meta(p.p_pos)
			if(not(meta)) then
				return false
			end
			inv = meta:get_inventory()
			if(not(inv)) then
				return false
			end
			inv_name = p.p_inv_list_name
		end
		if(    p.p_itemstack and p.p_value == "inv_contains") then
			return inv:contains_item(inv_name, p.p_itemstack)
		elseif(p.p_itemstack and p.p_value == "inv_does_not_contain") then
			return not(inv:contains_item(inv_name, p.p_itemstack))
		elseif(p.p_itemstack and p.p_value == "has_room_for") then
			return inv:room_for_item(inv_name, p.p_itemstack)
		elseif(p.p_value == "inv_is_empty") then
			return inv:is_empty(inv_name)
		end
		return false
	elseif(p.p_type == "player_offered_item") then
		local pname = player:get_player_name()
		local inv = minetest.get_inventory({type="detached", name="yl_speak_up_player_"..pname})
		local stack_got = inv:get_stack("npc_wants",1)
		if(stack_got:is_empty()) then
			return false -- empty stack
		end
		local stack_wanted = ItemStack(p.p_value)
		-- check group
		if(p.p_item_group and p.p_item_group ~= "") then
			local g = minetest.get_item_group(stack_got:get_name(), p.p_item_group)
			if(not(g) or g == 0) then
				return false -- wrong group
			end
		-- or: check item name
		elseif(stack_got:get_name() ~= stack_wanted:get_name()) then
			return false -- wrong item
		end
		-- independent of that: check stack size
		if(p.p_match_stack_size and p.p_match_stack_size ~= "") then
			local c_got = stack_got:get_count()
			local c_wanted = stack_wanted:get_count()
			if(    p.p_match_stack_size == "exactly" and c_got ~= c_wanted) then
				return false -- not exactly the same amount as the wanted one
			elseif(p.p_match_stack_size == "less"    and c_got >= c_wanted) then
				return false -- didn't get less than the given number
			elseif(p.p_match_stack_size == "more"    and c_got <= c_wanted) then
				return false -- didn't get more than the given number
			elseif(p.p_match_stack_size == "another" and c_got == c_wanted) then
				return false -- got the same than the given number
			end
		end
		-- check quest_id
		if(p.p_item_quest_id and p.p_item_quest_id ~= "") then
			local meta = stack_got:get_meta()
			-- we don't check here if the item was given by the right NPC;
			-- only the quest id has to fit
			if(meta:get_string("yl_speak_up:quest_id") ~= p.p_item_quest_id) then
				return false -- wrong quest_id
			end
			-- was this quest item given to another player?
			if(meta:get_string("yl_speak_up:quest_item_for") ~= pname) then
				return false -- wrong player
			end
		end
		-- all ok
		return true
	elseif(p.p_type == "custom") then
		-- execute the custom function
		return yl_speak_up.precondition_custom(player, p.p_value)
	elseif(p.p_type == "other") then
		-- are the preconditions of another option fulfilled?
		return (p.p_value
		    and other_options_true_or_false
		    and other_options_true_or_false[ p.p_value ] ~= nil
		    and tostring(other_options_true_or_false[ p.p_value ]) == tostring(p.p_fulfilled))
	end
	-- fallback - unknown type
	return false
end


-- these are only wrapper functions for those in fs_edit_general.lua

yl_speak_up.input_fs_edit_preconditions = function(player, formname, fields)
	return yl_speak_up.input_fs_edit_option_related(player, formname, fields,
		"p_", "o_prerequisites", yl_speak_up.max_prerequirements,
		"pre(C)ondition", "tmp_prereq",
		"Please punch the block you want to check in your precondition!",
		values_what, values_operator, values_block, values_trade, values_inv,
		check_what, check_operator, check_block, check_trade, check_inv,
		-- player variables with read access
		yl_speak_up.get_sorted_player_var_list_read_access,
		"edit_preconditions"
		)
end


yl_speak_up.get_fs_edit_preconditions = function(player, table_click_result)
	return yl_speak_up.get_fs_edit_option_related(player, table_click_result,
		"p_", "o_prerequisites", yl_speak_up.max_prerequirements,
		"pre(C)ondition", "tmp_prereq",
		"What do you want to check in this precondition?",
		values_what, values_operator, values_block, values_trade, values_inv,
		check_what, check_operator, check_block, check_trade, check_inv,
		-- player variables with read access
		yl_speak_up.get_sorted_player_var_list_read_access,
		-- show one precondition element
		yl_speak_up.show_precondition,
		"table_of_preconditions",
		"The following expression shall be true:", "Operator:", "Value to compare with:",
		"The following shall be true about the block:"
		)
end