Explain the Copy-on-Write principle when working with arrays and its application.

Copy-on-Write (COW) is an optimization where copying an array (or another collection type) does not happen immediately, but only at the moment of data modification. As long as the array is not changed, multiple variables can reference the same memory block, saving resources.

In Swift, arrays are implemented using COW. For example:

var array1 = [1, 2, 3]
var array2 = array1 // copy is not created immediately

array2.append(4) // here, the array is actually copied

Before calling append, both arrays reference the same data. When array2 is modified, a separate copy is created so that changes do not affect array1. This improves performance and reduces memory costs when working with large collections.