[bash] Move array elements

Here is the task:
    1. move all array elements to offset (index) 'off'. I.e. such, that first
       array element will be at index 'off'.
    2. make holes (sequences of unset elements) in the array by moving
       different array parts to different offsets (indexes) off1, off2,
       off3,.. .

This can't be done by

    arr=( [i]="${arr[@]}" )

because "${[@]}" expansion works not as it should: it treats assignment prefix
specially, and in such case works the same as "${[*]}".

So, we should use some workaround, like

    #!/bin/bash

    declare -a arr=(
        1
        2
        3
        4
        5
    )
    declare -i off=15
    declare -p arr
    b=${arr[0]}
    unset arr[0]
    arr=( [off]=$b "${arr[@]}" )
    declare -p arr

This can be done simplier, but i'm not sure which version of bash this
requires

    #!/bin/bash

    declare -a arr=(
        1
        2
        3
        4
        5
    )
    declare -i off=15
    declare -p arr
    arr=( [off]=${arr[0]} "${arr[@]:1}" )
    declare -p arr

Second task can be solved like this

    #!/bin/bash

    declare -a arr=(
        1
        2
        3
        4
        5
        6
        7
        8
        9
        10
        11
        12
        13
    )
    declare -i off1=15 len1=2 i1=0
    declare -i off2=21 len2=3 i2=$((i1 + len1))
    declare -i off3=40 len3=4 i3=$((i2 + len2))
    declare -i off4=70        i4=$((i3 + len3))
    declare -p arr
    arr=( 
        [off1]=${arr[i1]}   "${arr[@]:i1 + 1: len1 - 1}"
        [off2]=${arr[i2]}   "${arr[@]:i2 + 1: len2 - 1}"
        [off3]=${arr[i3]}   "${arr[@]:i3 + 1: len3 - 1}"
        [off4]=${arr[i4]}   "${arr[@]:i4 + 1}"
    )
    declare -p arr

Note, that all existed holes in the array after such operations would be lost.