[COLLECTIONS-427] performance problem in SetUniqueList.retainAll()

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.2.1
• Fix Version/s: 4.0-alpha1, 4.0, 4.1

Environment

java 1.6.0_24
Ubuntu 11.10

• Assignee: Unassigned
• Reporter: Mert Guldur
• Created: 24/Jul/12 23:50
• Updated: 27/Nov/15 21:11
• Resolved: 27/Jan/15 15:09

Description

I am encountering a performance problem in SetUniqueList.retainAll().
It appears in version 3.2.1 and also in revision 1365132. I attached
a test that exposes this problem and a patch that fixes it. On my
machine, for this test, the patch provides a 621X speedup.

To run the test, just do:

$ java Test

The output for the un-patched version is:
Time is 6215

The output for the patched version is:
Time is 10

There are two problems here. First, "SetUniqueList.retainAll()"
should have similar implementation with the current implementation of
"ListOrderedSet.retainAll()", which is more optimized. Second, even
"ListOrderedSet.retainAll()" has a performance problem, which was
reported and explained in detail in COLLECTIONS-426.

The attached patch has two parts. The first part (the first loop) is
inspired from COLLECTIONS-426. The second part (everything after the
first loop) is in fact the current implementation of
"ListOrderedSet.retainAll()", with some minor changes to adapt it for
the current code. Overall, the attached patch is very similar to the
COLLECTIONS-426 patch.

I will rehash some of the information from COLLECTIONS-426 (which
describes "ListOrderedSet.retainAll()") for the current
"SetUniqueList.retainAll()".

The current code for "SetUniqueList.retainAll()" is:

public boolean retainAll(Collection<?> coll) {
    boolean result = super.retainAll(coll);
    set.retainAll(coll);
    return result;
}

where both "super.retainAll(coll)" and "set.retainAll(coll)" can have
quadratic complexity, e.g., if "coll" is a List. Both these calls to
"retainAll" are in fact calls to
"java.util.AbstractCollection.retainAll()", which has the code:

public boolean retainAll(Collection<?> c) {
    boolean modified = false;
    Iterator<E> e = iterator();
    while (e.hasNext()) {
        if (!c.contains(e.next())) {
            e.remove();
            modified = true;
        }
    }
    return modified;
}

which iterates over "this" and calls "contains()" on "c". Mapping
this code back to "SetUniqueList.retainAll()" means that the code
iterates over "this" and "set" and calls "contains()" on "coll". If
"coll" has slow "contains()" (e.g., if "coll" is a list), then
"SetUniqueList.retainAll()" has quadratic complexity.

The patch iterates over "coll" and calls "contains()" on "set", which
we know is fast, because "set" is a Set. For a more detailed
discussion of the patch and the problem, see the current
implementation of "ListOrderedSet.retainAll()", the discussion for
COLLECTIONS-426, and the patch for COLLECTIONS-426.

Is this a bug, or am I misunderstanding the intended behavior? If so,
can you please confirm if the patch is correct?