improve performance of XmlComplexContentImpl.arraySetterHelper

[pjfanning]

When saving large XLSX files via POI, arraySetterHelper iterates over N source elements and for each calls find_element_user(QName, int i) with a monotonically increasing i. Since find_element_user always rescans from _firstChild, this is O(n) per call — O(n²) total for N elements.

https://issues.apache.org/jira/browse/XMLBEANS-438

Changes

• commonSetterHelper / commonSetterHelper2: Pre-fetch all existing child elements once with find_all_element_users (O(n)) before the loop, then use O(1) list indexing instead of find_element_user(name, i) on each iteration.

• arraySetterHelper fast-path inner loop: Replaces the per-iteration find_element_user(name, j) call with existingList.get(j - originalI - insertCount). An insertCount counter tracks in-loop insertions that shift the original elements rightward in the store, keeping the index arithmetic correct without re-scanning.

• arraySetterHelper general-case final loop: Lazily builds a finalList after the excess-removal pass, then uses finalList.get(j) for existing positions instead of find_element_user(name, j).

Before:

// O(n) scan from _firstChild on every call — O(n²) total
for (int i = 0; i < n; i++) {
    user = store.find_element_user(elemName, i); // rescans from _firstChild each time
    ((XmlObjectBase) user).set(sources[i]);
}

After:

// O(n) once, then O(1) per element
List<XmlObject> existing = new ArrayList<>(m);
store.find_all_element_users(elemName, existing);

for (int i = 0; i < n; i++) {
    XmlObjectBase user = (i >= m) ? (XmlObjectBase) store.add_element_user(elemName)
                                   : (XmlObjectBase) existing.get(i);
    fun.accept(user, i);
}

Overall complexity for N-element array setter operations: O(n²) → O(n).

[pjfanning]

JMH testing in https://github.com/pjfanning/poi-bench shows none of the expected performance benefit