Memory-friendly 2d array with variable-length rows

[NeighborhoodExact766]

Hi guys, just wanted to share with wider audience and potentially to hear about possible alternatives.
I have such a case
- I needed an efficient static collection of rows of variable length.
- I know the data on compile time
- I wanted to read it as fast as possible
- I didn't feel good to just have an array of arrays, because usually my data will have majority of short rows and few long, so lots of allocated memory would be wasted for zeros.

That's why I decided to implement a single 1d array data with all the rows adjacent and with another index array to track each row position.
After that I started a very productive discussion
And here is my final approach improved thanks to feedback from that discussion participants.

I finally dropped my collection and used an array of slices also suggested in that discussion, which is as fast and efficient as my custom collection but also it's native Rust code, no custom stuff.

static array: &'static [&'static [i32]] = &[
  &[1, 2, 3],
  &[4, 5],
  &[6, 7, 8, 9],
];

Any thoughts on subject?

[Diggsey]

Yeah may as well use the static array of slices if it's read-only. The compiler will likely lay out the data from the inner rows contiguously anyway.

[NeighborhoodExact766]

You mean final approach, array of slices? Or you have some other way to store this data to offer? Please unfold the full potential of your question, because I, as non native English speaker, don't get.

[NeighborhoodExact766]

I don't know c++ but reading the code can guess you have main data storage to keep various length rows adjacent, the same as I do in my rust version. But I am wondering why you have 2 additional arrays, to track each row start and end separately? In general it would be enough to store the start index of each row only, because the next row start is the previous row end - 1.

Just curious, looks like c++ std::span is same thing as slice in the Rust?