For those who are not following the code, the logic here is based on first trying to allocate elements which there is room for (this is the "if (i)" bit). If it cannot get them all in it then makes sure it has enough space, does some accounting, copies everything over, then inserts some new stuff. Note that japhy's comment about unused space is misleading, he means unused at the beginning of the array. There is also unused stuff at the other end, but we cannot directly use that.

For full details you have to also look at av.h. The call to AvMAX is just a macro to set xav_max which is the largest element which there is space for (adding to the front increases that), and AvFILL is setting xav_fill which is how many you have (adding obviously increases that as well).

The call to av_extend is where the array size increases. What it does is extends the definition of what the array is occupying. In fact it is in a buffer whose size is allocated in powers of two. If the extend takes the array beyond the size of the buffer, then a new buffer is allocated and the array is moved. If it does not then the array is left where it is.

Now it is clear how to make repeated calls to unshift fairly efficient. Right now it moves stuff by the minimum necessary. What we need is to have a new variable for how much to move it. That variable should be the maximum of num and the length of the array. This will cause space wastage, but it will also mean that when you hit an unshift and it has to move the array, it will not hit that copying logic again right away.

Even so building up an array using n calls to push will still be faster than unshift because there is less accounting to do. But both will be order n rather than having n calls to unshift being order n^2 as it is today.