players = {}

registered_on_wielditem_changes = {function(...)
    local _, previous_item, _, item = ...
    if previous_item then
        ((previous_item:get_definition()._modlib or {}).un_wield or modlib.func.no_op)(...)
    end
    if item then
        ((item:get_definition()._modlib or {}).on_wield or modlib.func.no_op)(...)
    end
end}

--+ Registers an on_wielditem_change callback: function(player, previous_item, previous_index, item)
--+ Will be called once with player, nil, index, item on join
register_on_wielditem_change = modlib.func.curry(table.insert, registered_on_wielditem_changes)

minetest.register_on_mods_loaded(function()
    -- Other on_joinplayer / on_leaveplayer callbacks should execute first
    minetest.register_on_joinplayer(function(player)
        local item, index = player:get_wielded_item(), player:get_wield_index()
        players[player:get_player_name()] = {
            wield = {
                item = item,
                index = index
            }
        }
        modlib.table.icall(registered_on_wielditem_changes, player, nil, index, item)
    end)
    minetest.register_on_leaveplayer(function(player)
        players[player:get_player_name()] = nil
    end)
end)

minetest.register_globalstep(function()
    for _, player in pairs(minetest.get_connected_players()) do
        local item, index = player:get_wielded_item(), player:get_wield_index()
        local playerdata = players[player:get_player_name()]
        if not playerdata then return end
        local previous_item, previous_index = playerdata.wield.item, playerdata.wield.index
        if item:get_name() ~= previous_item or index ~= previous_index then
            playerdata.wield.item = item
            playerdata.wield.index = index
            modlib.table.icall(registered_on_wielditem_changes, player, previous_item, previous_index, item)
        end
    end
end)