[Coin-lpsolver] OsiSimplexInterface behaviour

[Mikhail Nediak]

On Fri, 11 Apr 2003, Lou Hafer wrote:

>
>  << Context is behaviour of solvers when OsiSimplexInterface is enabled.
>     Question (broadly) is the effect/implications of asking a solver to
>     perform a pivot sequence that might normally be considered a violation
>     of the rules for the current simplex phase. >>
>
> 	Does a solver really need to be `in a phase' at all times? Well, no,
> not really. The various checks (in dylp, at least) really are sanity checks.
> Given a phase, you can assume certain things shouldn't happen. That allows
> some simplifications in the code. Violations are normally a good indication
> of a coding error. So being in a phase serves a useful purpose.
>
> 	Now I've written that last paragraph, I guess I'd say the real problem
> is that dylp assumes that if it's doing simplex pivots, then it's in one of
> primal I, primal II, or dual II. It probably wouldn't be too difficult to add
> a `no phase' phase (:-) to various specific routines (pivot, factorization).

OK. But then it probably makes sense to have a "richer" enabling routine
in OsiSimplexInterface. We should have at least three modes: primal, dual
and "neither". There is also an issue of what OsiSolverInterface calls
would be allowed in either of these modes. E.g., would adding a row while
in "primal" or "column" while in "dual" be allowed? How about changing
objective coefficients, right-hand-side, bounds? Can resolve() be called?
I though it would be reasonable to, at least, be able to call resolve().
Etc, etc, ...

>
> 	What sort of behaviour would be expected at the transition back to
> solver control? Would the solver be allowed to assess it's state and proceed
> accordingly?

What does transition mean? If it is a call to resolve(), should not
solver state assessment happen automatically?

Best,
Mikhail

>
> 							Lou
>
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