restarting an ODE solver that has been stopped by an event doesn't work
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Key: MATH-421
URL: https://issues.apache.org/jira/browse/MATH-421
Project: Commons Math
Issue Type: Bug
Affects Versions: 2.1
Environment: Linux
Reporter: Luc Maisonobe
Assignee: Luc Maisonobe
Priority: Minor
Fix For: 2.2
If an ODE solver is setup with an EventHandler that return STOP when the even
is triggered, the integrators stops (which is exactly the expected behavior).
If however the user want to restart the solver from the final state reached at
the event with the same configuration (expecting the event to be triggered
again at a later time), then the integrator may fail to start. It can get stuck
at the previous event.
The occurrence of the bug depends on the residual sign of the g function which
is not exactly 0, it depends on the convergence of the first event.
As this use case is fairly general, event occurring less than epsilon after the
solver start in the first step should be ignored, where epsilon is the
convergence threshold of the event. The sign of the g function should be
evaluated after this initial ignore zone, not exactly at beginning (if there
are no event at the very beginning g(t0) and g(t0+epsilon) have the same sign,
so this does not hurt ; if there is an event at the very beginning, g(t0) and
g(t0+epsilon) have opposite signs and we want to start with the second one. Of
course, the sign of epsilon depend on the integration direction (forward or
backward).
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