AS-3 - How To Insert A ByteArray In Another ByteArray - performance

Question

I would like to insert a ByteArray anywhere in between another ByteArray
var main: ByteArray = ...;
var bytes2insert: ByteArray = ...;
var index: int = ...;
// index is 0 to main.length and tells me where to write the bytes2insert in main
main.writeBytes(bytes2insert, index, bytes2insert.length);
If I try writeBytes with random index I get errors because of "IndexOutOfBounds" and stuff. How can I achieve the inserting ? I will probably get away with some for-loops but for performance reasons I would like to (mostly) use given methods.
EDIT: I think the AS-3 documentation is a bit lacky (didn't check adobe.com though)
// since the script needs to read from bytes2insert and write to main
// main.writeBytes(array, offset, length);
main.position = <start index for writing> // (seems to be important in this case)
offset = <start index for reading> // (I misunderstood that one)
length = <length for both>

Answer 1

Solution:
public function astest()
{
var main: ByteArray = new ByteArray();
var bytes2insert: ByteArray = new ByteArray();
var index: int = 5;
for(var i:int = 0; i < 20; i++)
main.writeByte(99);
for(var j:int = 0; j < 30; j++)
bytes2insert.writeByte(100);
trace("1", main);
insertBytes(main, bytes2insert, index);
trace("2", main);
}
private function insertBytes(target:ByteArray, insert:ByteArray, index:int):void
{
if(index < target.length)
{
var tmp:ByteArray = new ByteArray();
tmp.writeBytes(target, index);
target.position = index;
target.writeBytes(insert);
target.writeBytes(tmp);
}
}
//output:
//1 cccccccccccccccccccc
//2 cccccddddddddddddddddddddddddddddddccccccccccccccc