Minetest: move_items

local function move_items(s_inv, s_listname, d_inv, d_listname)

    local s_size = s_inv:get_size(s_listname)
    local s_index = 1

    while s_index <= s_size do
        local stack = s_inv:get_stack(s_listname, s_index)
        if stack and not stack:is_empty() then
            local leftover = d_inv:add_item(d_listname, stack)
            s_inv:set_stack(s_listname, s_index, leftover)
        end
        s_index = s_index + 1
    end

end

minetest.register_tool(":default:asdf", {
    inventory_image = "default_wood.png",
    on_use = function(itemstack, user, pointed_thing)
        local inv = user:get_inventory()
        move_items(inv, "main", inv, "craft")
    end,
})