proposal: slices: functions Shift, Rotate.

[markusheukelom]

Proposal Details

// Shift return a slice with n zero values of E inserted at the given position.
func Shift[S ~[]E, E any](s S, at, n int) S 

// Rotate right-rotates the slice by n places. To left-rotate use -n.
func Rotate[S ~[]E, E any](s S, n int) S 

Judging from the source code of the slices package there already is an internal "rotateLeft/Right" function. I am unsure if there is a good reason not to export this useful function (especially as it is not trivial to implement without allocations).

A "Shift" function is not trivial to implement efficiently either, I think (without intermediate allocations, ie. making an zero slice of length and inserting it) . Maybe this can be done using other functions in the package that I am unaware of. In that case just an example would be nice.

(Looking at the implementation of Insert I think a Shift function could simplify its implementation a little. Also Insert checks for overlap but this seems covered by copy already? Although maybe there's more to it then I can see by glancing at it).

Updates after feedback

Apparently shift is commonly used to refer to a different operation on arrays/slices (especially in scripting languages) and bits. Alternative names for Shift such as InsertN or InsertZeros should be considered.

[gabyhelp]

Related Issues and Documentation

• proposal: slices: add Shift #64103
• slices: add Reverse #58565 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

Duplicate of #64103

[randall77]

The Shift proposed here is different from the Shift proposed in #64103.

[seankhliao]

Ah, sorry, though I struggle to see how useful this definition of Shift is in common code?

[randall77]

I agree, I'm not sure I understand where these would be useful.
(And I wrote slices.rotateLeft/Right :) )

[markusheukelom]

@randall77 @seankhliao

Regarding usefulness in "common code", here's what I would comment:

Shift is useful because you can make room to insert values into a slice without having to allocate an intermediate slice. This saves an allocation. Say for example I have n slices that I want to insert into some other slice. With Shift I could just make room for those slices and then use Replace to copy the values. But with Insert I have to allocate an intermediate slice using Concat which I then insert using Insert.

Rotate is useful whenever a slice is used to represent a loop-like (data)structure (such as polygons, in my case).

(notice that Shift, Rotate also seem useful in the slices package implementation itself)

If that doesn't make the cut, that's understandable of course. Maybe it needs more evidence of usefulness.

[DeedleFake]

Why limit Shift() to just zero values? How about

func InsertN[Slice ~[]E, E any](s Slice, i, n int, v E) Slice

instead?

[markusheukelom]

@DeedleFake

I don't think there's a need, evidenced by the fact that make([]T) supports zero values only as well. Plus, it's trivial to set a value if needed (or maybe a slices.Set function). Then again, I wouldn't mind either.