in src/annoyluamodule.cc [205:211]
static int get_nns_by_item(lua_State* L) {
Searcher s(L);
int item = getItemIndex(L, 2, s.self->get_n_items());
s.self->get_nns_by_item(item, s.n, s.search_k, &s.result,
s.include_distances ? &s.distances : NULL);
return s.pushResults(L);
}