use rayon::prelude::*; // 1.1.0

fn main() {
    let mut vecs = vec![
        vec![1, 2, 3],
        vec![1, 2, 3],
        vec![1, 2, 3],
        vec![1, 2, 3],
        vec![1, 2, 3],
        vec![1, 2, 3],
    ];

    let mut indices = vec![4, 3, 1];

    // Ensure all indices are unique:
    indices.sort_unstable();
    let len = indices.len();
    indices.dedup();
    assert!(indices.len() == len);

    let mut_vecs = get_multi_mut(&mut vecs, &indices);
    // This causes an underflow on debug and a different error on release
    //let mut_vecs = get_multi_mut(&mut vecs, &[0,0,0]);

    mut_vecs.into_par_iter().for_each(|vec| {
        vec.push(4);
    });

    println!("{:?}", vecs);
}

fn get_multi_mut<'a, T>(mut slice: &'a mut [T], indices: &[usize]) -> Vec<&'a mut T> {
    let mut offset = 0;
    let mut result = Vec::with_capacity(indices.len());

    for &i in indices {
        let (vec, rest) = slice.split_at_mut(i - offset + 1);
        slice = rest;
        offset = i + 1;
        result.push(vec.last_mut().unwrap());
    }

    result
}