local function update_formspec(meta)
    local group = meta:get_string("group")

    meta:set_string("formspec", [[
        size[10,8.3]
        real_coordinates[true]
        field[0.1,0.4;8.8,0.8;group;Group;]] .. group .. [[]
        button_exit[9,0.4;0.9,0.8;set;Set]
        list[context;main;0.1,1.4;8,1;]
        list[current_player;main;0.1,3;8,4;]
        listring[]
    ]])

    local txt = "Replacement node"
    if group and group ~= "" then
        txt = txt .. " (group: '" .. group .. "')"
    end
    meta:set_string("infotext", txt)
end

minetest.register_node("pick_and_place:replacement", {
	description = "Replacement node",
	tiles = {"pick_and_place.png^[colorize:#ff0000"},
    drawtype = "allfaces",
    use_texture_alpha = "blend",
    paramtype = "light",
    paramtype2 = "facedir",
    sunlight_propagates = true,
	groups = {
		oddly_breakable_by_hand = 3
	},

	on_construct = function(pos)
		local meta = minetest.get_meta(pos)
        local inv = meta:get_inventory()
        inv:set_size("main", 8)

        update_formspec(meta)
	end,

    on_receive_fields = function(pos, _, fields)
        if fields.set then
            local meta = minetest.get_meta(pos)
            meta:set_string("group", fields.group)
            update_formspec(meta)
        end
    end
})

function pick_and_place.get_replacement_nodeid(ctx, metadata)
    local group = metadata.fields.group
    local selected_name
    if group and group ~= "" and ctx[group] then
        -- group placement
        selected_name = metadata.inventory.main[ctx[group]]
    else
        -- random placement
        local replacement_names = {}
        for _, name in ipairs(metadata.inventory.main) do
            if name ~= "" then
                table.insert(replacement_names, name)
            end
        end

        if #replacement_names == 0 then
            -- no replacement
            return
        end

        local i = math.random(#replacement_names)
        selected_name = replacement_names[i]

        -- set group context
        if group and group ~= "" then
            ctx[group] = i
        end
    end

    local stack = ItemStack(selected_name)
    local nodename = stack:get_name()

    if not minetest.registered_nodes[nodename] then
        -- node not found
        return
    end

    local nodeid = minetest.get_content_id(nodename)
    return nodeid
end