Add Enum.slide/3 #11349
Add Enum.slide/3 #11349
Conversation
Previous discussion from the elixir-lang-core mailing list: https://groups.google.com/g/elixir-lang-core/c/LYmkUopaWN4/m/SwBzRt2zBQAJ
|
This does look useful, but looking at the description and comparing the implementation with the C++ and Rust functions, this seems to me like it performs a completely different operation. In particular the C++ I find it relatively hard to understand how a C++ call to the |
|
Right... the C++ equivalent I've heard called "slide," and it's just a wrapper around rotate to do the correct ordering of your range versus the insertion point (such that the 3 indices passed to rotate are in strictly increasing order). There was some discussion on the mailing list about whether we should go with another name. I don't have a particularly strong preference there. 🤷♂️ |
|
Just a quick note before I can review the PR, but I think |
|
Slide works for me! Pushing the change now... |
lib/elixir/lib/enum.ex
Outdated
| iex> Enum.slide([0, 1, 2, 3, 4, 5, 6], 3..-1//1, 2) | ||
| [0, 1, 3, 4, 5, 6, 2] | ||
| iex> Enum.slide([0, 1, 2, 3, 4, 5, 6], -4..-2, 1) | ||
| [0, 3, 4, 5, 1, 2, 6] |
There was a problem hiding this comment.
I just realized something, by having the input list be numbers, someone can think that 1..3 is referring to the actual values and not the position. Shall we use [:a, :b, :c, ...] as input?
| end | ||
|
|
||
| {index + 1, pre, post} | ||
| end) |
lib/elixir/lib/enum.ex
Outdated
|
|
||
| # Like slide_any/4 above, this optimized implementation of slide for lists depends | ||
| # on the indices being sorted such that we're moving middle..last to be in front of start. | ||
| defp find_start([h | t], start, middle, last) |
There was a problem hiding this comment.
Let's call this slide_list_start and then slide_list_middle and slide_list_last?
|
@s3cur3 thank you for the PR! ❤️ the code looks great, I have dropped some minor comments on the docs and on the function naming, everything else is ready to go! |
Co-authored-by: José Valim <jose.valim@gmail.com>
…view Co-authored-by: José Valim <jose.valim@gmail.com>
|
Really appreciate the code review, @josevalim. |
|
If you are looking for a challenge, you can implement Stream.slide :) I believe the implementation would fit Stream.Reducers, so it should be a matter of converting the Enumerable implementation into the Stream.Reducers format! |
|
Sweet, I'll look into it! 👍 |
|
💚 💙 💜 💛 ❤️ |
[Below is my intro/pitch from the previous discussion on the elixir-lang-core mailing list.]
Rotate pulls a slice of an enumerable out and replaces that slice somewhere else.
The classic use case is this: Suppose you have a list of to-do items, which the user has ordered by priority:
A rotate 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:
Doing this without the named algorithm requires 3 calls to
Enum.slice/3(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... and it's also substantially slower than the implementation here.A number of other languages have a rotate algorithm, though it's still somewhat uncommon. I found Dave Abrahams' comments valuable when this was discussed for inclusion in Swift.