Before confirming package installation/removal, zypper prints a nice summary that looks like: "Overall download size X.Y M, after the operation A.B M will be used" (similarly for pkg removal). Zypper calculates these numbers by iterating over all resolvables with to_be_installed/to_be_removed flag and summing up their sizes. 

Total download size is calculated also in YCP pkg-bindings (where it is slightly more complicated by summing up the sizes by different repositories - anyway, don't fully understand that :) ).

Now I'd like to re-use this functionality (show the user how much MBs will be downloaded, installed, deleted) also in ncurses-pkg. There is several years old bloated code in patch selector that i) calculates total download size, ii) does the iteration over all selectables for every single patch status change, which is extremely clumsy iii) this is actually the third place where +- the same code is used

Could the code which is used in zypper please be moved somewhere else, so that it can be re-used in pkg selectors and/or pkg-bindings? It could, for example, have the form of the functions returning the sums in zypp::ByteCount for total download size, installed size, and removed size (if applicable)