[wild]

can't you:

• Build a list of all file paths.
• Shuffle the list once.
• Iterate sequentially through the shuffled list.
• When exhausted, reshuffle again (optional).

This guarantees:

• every image appears exactly once per cycle
• no duplicates before all images are shown
• O(n) setup cost
• O(1) per image retrieval afterward

here it is a VERY stupid C example:

#include "stdio.h"
#include "stdlib.h"
#include "time.h"

void shuffle(char *arr[], int n)
{
    for (int i = n - 1; i > 0; --i)
    {
        int j = rand() % (i + 1);

        char *tmp = arr[i];
        arr[i] = arr[j];
        arr[j] = tmp;
    }
}

int main(void)
{
    char *images[] = {
        "img1.jpg",
        "img2.jpg",
        "img3.jpg",
        "img4.jpg",
        "img5.jpg"
    };

    int n = sizeof(images) / sizeof(images[0]);

    srand((unsigned)time(NULL));

    shuffle(images, n);

    // show all images exactly once
    for (int i = 0; i < n; ++i)
    {
        printf("%s\n", images[i]);
    }

    return 0;
}

now I see that you are using RUST:

extern crate rand;
use rand::rng;
use rand::seq::SliceRandom;

fn main() {
    let mut images = vec![
        "img1.jpg",
        "img2.jpg",
        "img3.jpg",
        "img4.jpg",
        "img5.jpg",
    ];

    let mut r = rng();

    // shuffle once
    images.shuffle(&mut r);

    // show all images exactly once
    for img in images {
        println!("{}", img);
    }
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=bf21ee09177a42b6d9a1245f59fe1fc4