vasiliy-mikhailov opened a new pull request, #692:
URL: https://github.com/apache/commons-collections/pull/692
## Problem
`AbstractMapBag.retainAll(Bag)` should keep, for each element, the minimum
of this bags cardinality and the other bags cardinality. The current logic:
```java
final int myCount = getCount(current);
final int otherCount = other.getCount(current);
if (1 <= otherCount && otherCount <= myCount) {
excess.add(current, myCount - otherCount);
} else {
excess.add(current, myCount); // removes ALL copies
}
```
The `else` branch removes all copies of an element whenever the guarded case
(`1 <= otherCount <= myCount`) does not apply. That branch also covers
`otherCount > myCount`, so when the other bag contains **more** copies than
this bag, every copy is removed instead of all being kept.
Example: `bag{A:2, B:3}.retainAll(other{A:5, B:10})` empties the bag, when
it should leave it unchanged (the retained count is `min(2,5)=2` and
`min(3,10)=3`).
## Fix
Only remove all copies when `otherCount == 0`. When `otherCount > myCount`,
no copies are removed, leaving this bags cardinality intact.
## Test
Adds `testBagRetainAllOtherHasMoreCopies` to `AbstractBagTest`, so it runs
against every `Bag` implementation. It fails on `master` (`expected: <2> but
was: <0>`) and passes with this change; the existing `HashBagTest` suite (51
tests, including `testBagRetainAll`) stays green.
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