missing_integer() {
    local -n _a="$1"
    local -a _sorted=($(printf '%s\n' "${_a[@]}" | sort -nu))
    local _min=1 _v
    for _v in "${_sorted[@]}"; do
        if (( _v > 0 )); then
            if (( _min != _v )); then
                break
            fi
            ((_min++))
        fi
    done
    echo "$_min"
}