Hi Tyler! I think starting with a basic List.rotate is very welcome. My questions are:
1. Which of those indexes can be negative? 2. What happens if the middle index is less than or equal to the start index? 3. What happens if the last index is within start and middle? 4. Could List.rotate(list, range, index) be a better API? I.e. move the list represented by indexes in range to index? Thanks for the proposal! On Thu, Sep 2, 2021 at 3:38 AM Tyler Young <s3c...@gmail.com> wrote: > Hi folks! > > I'd like to propose a List.rotate/4 function (and maybe some convenience > wrappers for it as well). > > Rotate is one of those algorithms that, once I learned about it, I started > seeing it everywhere. It's somewhat complicated to grasp (it takes 4 > parameters, after all), but in situations where you need it, it's still > *much* simpler than the equivalent imperative version. > > The classic use case is this: Suppose you have a list of to-do items, > which the user has ordered by priority: > > 1. Apply to college > 2. Brush the dog > 3. Change the car's oil > 4. Deliver flowers > 5. Exchange gifts > > A "rotate" or "slide" occurs when the user selects some number of elements > and drags them to a new place in the list. Let's say they selected items 3 > & 4 from the preceding and dragged them above item 2. When they release the > mouse, the new order should be: > > 1. Apply to college > 2. Change the car's oil > 3. Deliver flowers > 4. Brush the dog > 5. Exchange gifts > > Doing this without the named algorithm requires 3 splits (one at the > insertion point, one at the start of the selected range, and one at the end > of the selected range). It's easy to get the index math wrong, and it's > even harder for readers of your code to grasp what's going on. Adding this > as a "vocabulary" algorithm would help a lot, I feel. > > A number of other languages > <https://twitter.com/code_report/status/1419900906062204939?s=20> have a > rotate algorithm, though it's still somewhat uncommon. I found Dave > Abrahams' comments > <https://forums.swift.org/t/proposal-implement-a-rotate-algorithm-equivalent-to-std-rotate-in-c/491/2> > valuable when this was discussed for inclusion in Swift. > > I've put together a first draft of an implementation > <https://github.com/s3cur3/elixir-xutil/blob/main/lib/x_util/list.ex>, > plus some basic tests > <https://github.com/s3cur3/elixir-xutil/blob/main/test/list_test.exs>. > > If this is something folks decide we want, it might also be worth > considering a few variants (also implemented in the file linked above): > > - slide/4: Syntactic sugar over rotate, but it's useful in practice > because usages of rotate often need to be able to move a chunk of the list > either or backward. (Most usages I've seen in the wild end up doing an if > insertion_idx < range_start . . . else . . .) > - slide_one/3, where the second argument is the index of the single > element to move and the last argument the target index. This would just be > syntactic sugar over slide/4. (In my experience, about half the times > I use this algorithm, the conceptual requirements guarantee that it'll only > act on a single element.) > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-lang-core+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/01c63660-6a11-4e69-bdcb-6659579ef683n%40googlegroups.com > <https://groups.google.com/d/msgid/elixir-lang-core/01c63660-6a11-4e69-bdcb-6659579ef683n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. To unsubscribe from this group and stop receiving emails from it, send an email to elixir-lang-core+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4KvzF42e_4jY%3DvCKrN2F7K8s2UG0eDmiR%2BGT1gzeAW7pg%40mail.gmail.com.
