Representing strings with variable (but short, bounded) length

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1

I have some strings I would like to be able to parse without heap allocations.

I know ahead of time that the strings will be a maximum length of 127 bytes.

What type can I use that can be:

  1. Stack allocated
  2. Have string length information?

here is some pseudocode of what I would like to accomplish

pub fn get_string(port: *Port) ![128]u8 {
    const string_length: u8 = port.get_string_length();
    const buffer: [128]u8 = undefined;
    try port.read_string(&buffer[0..string_length]);
}

Perhaps I should use [128]?u8 ?

Or some sentinel stuff?

2

I usually just create a struct that holds both the buffer and the length:

const String = struct {
    buf: [127]u8,
    len: u7,
};

Or you can use std.BoundedArray.

1 Like
3

You can probably use a RingBuffer for that too, if you just need to read/write to a stack allocated slice that works well too.

4

Depends what you need to do with them after. You can wrap up arrays in a fixed buffer stream as well as other suggestions here.

You can also just take slices of an array backing and set the Len to the string Len.

5

The fact that this is one byte over two cache lines is causing me almost physical pain. If there’s a need for a zero terminator on the strings, then sure, but if not, a [127]u8 meets the stated goal, and the whole struct will Tetris into the rest of the program much better.

6 Likes
6

Could also pass a stack allocated buffer into the function, and have the function return a slice []u8 which would represent the part of the buffer actually used.

3 Likes