[Cbc] Problem in CbcCompareUser/invalid heap

[Torsten Fahle]

Hi,

there seems to be a problem in CbcCompareUser::test(...)
We received an "invalid heap" message from STL. 
STL was trying to insert a new B&B node into the node tree and detected that the heap was no longer correct
It turned out that test(CbcNode*x, CbcNode* y) in  CbcCompareUser.cpp changes the node ordering without re-ordering the heap.


Here is our analysis and workaround. Any comment or better solution is appreciated.

In CbcCompareUser, test() is a methods that compares two nodes from the node tree and indicates which node is considered better.
As the node tree is a heap, any change in the compare strategy requires a heap re-organization.
Methods CbcCompareUser::newSolution() and CbcCompareUser::every1000Nodes() request such a re-organization by returning true.

In CbcCompareUser.cpp,  test() starts by selecting either a depth oriented strategy (before a solution is found) or an objective oriented strategy (after a solution is found).
That selection depends on weight_ which is manipulated in CbcCompareUser::newSolution()  and any change to weight_ made in CbcCompareUser::newSolution() also requests a heap reorganisation.
However, that selection also depends on x->depth()/y->depth(). Depth is a property of the node that is independent of any decisions made in 
CbcCompareUser::newSolution() and CbcCompareUser::every1000Nodes().

here is the code snippet from CbcCompareUser.cpp as provided in the CBC_277-release. 

188	// Returns true if y better than x
189	bool
190	CbcCompareUser::test (CbcNode * x, CbcNode * y)
191	{
192	  if (weight_==-1.0&&(y->depth()>7||x->depth()>7)) {
(N.B.: there is another implementation of test() in that files, starting at line 85 that does not use the "x->depth()...", but that method is commented out )


At some point we had 3 nodes A, B, C where test() indicates: "A better than B", "B better than C", and "C better than A".
The reason was that two of these three comparisons had been conducted in the "if"-part (weight_ = 1 and at least one node was deeper than 7),
whereas the last comparison had been performed in the "else "-part (weight_ = -1, either node had deep <= 5).
As there was no heap re-organization in between, these evaluations were not compatible and thus corrupted the heap.

A quick solution is to remove the depth-test from the if. 
Another would be to switch strategy in ::newSolution()/::every1000Node(), indicating that via some enum and only evaluation that in ::test().

The problem exists in cbc 277. The code in CbcCompareUser looks the same also in older versions of Cbc (checked randomly down to version 2.3.0).

Best regards,

   Torsten







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Dr. Torsten Fahle
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