[Coin-lpsolver] basisIsAvailable()

[Francois Margot]

On Thu, 10 Nov 2005, Matthew Saltzman wrote:

> On Wed, 9 Nov 2005, Francois Margot wrote:
>> 
>> I must add that the Doxygen doc of OsiSolverInterface could be improved (I 
>> know, I can do it myself; except that to figure out what is supposed to
>> be returned by isProvenOptimal() you have to look at solver implementations
>> and they are not consistent with each other ...). The current doc on
>> isProvenOptimal() is the terse:
>> 
>>   Is optimality proven?
>
> Of course, what that means is algorithm dependent.
>

Well, this is exactly what I don't like. The purpose of Osi, I thought, 
is to have an interface that is solver independent. To obtain that,
Osi has to impose return codes that are solver independent, and have
enough documentation so that people implementing a solver interface 
know exactly what is required.

For example my impression is that IsProvenOptimal() is supposed to
return "true" if and only if the current solution that can be obtained
through getColSolution() is optimal for the model currently stored
in the solver. This is fairly obvious if you are merely loading
a LP, solve it, and access the solution. What is unclear is what 
happens when you start modifying the model (changing bounds on variables,
for example). This should not be left to the implementer of the solver 
interface. For example, it would not be completely insane that
IsProvenOptimal() returns the status of the last call to resolve() or
initialSolve() (in fact, I believed it was so until yesterday ...), 
independently of later modifications.

The same is true for the methods getBInvRow() and others. The choice I made
when writing the OsiCpxSolverInterface::getBInvRow() method is to make
changes to the native Cplex function getBInvRow() method so that both
OsiClp and OsiCpx return identical results, making Cplex behave like Clp.
However, the behavior of Clp is not really defined anywhere in the doc I could
find. Here, the problem is the way solvers add slack variables to the original
tableau. There should be an Osi description of the way slacks are supposed
to be handled in the original tableau, for all simplex based Osi solvers.

Francois