Stopping

There are two methods to halt the solver when a certain condition is met: you can either stop at a specific time or stop when a certain event occurs.

Stopping with high-level solve methods

The high-level solve and solve_dense methods stop the solver when either the final time tfinal is reached, or when one of the elements of the root function returns a value that changes sign. The internal state of the solver (t, y, dy etc.) is set to the time that the zero-crossing occured.

If a reset function is also defined, solve and solve_dense apply the reset automatically at the root time and continue integrating to the requested final time or final evaluation point. In solve_dense, any user-specified evaluation time that lands exactly on the event receives the pre-reset state.

The public solve_dense_sensitivities method also applies resets automatically when the equations provide sensitivity-aware root and reset operators. The lower-level step method, staged solves via solve_soln / solve_soln_sensitivities, and current adjoint workflows still report RootFound and leave it to the caller to decide how to resume.

Manual time-stepping

Stopping at a specific time is straightforward, as you can use the set_stop_time method on the OdeSolverMethod trait and then just check if the return value of the step method is Ok(OdeSolverStopReason::TstopReached)

Stopping at a certain event requires you to set a root function in your system of equations, see Root Finding for more information. During time-stepping, you can check if the solver has discovered a root by checking if the return value of the step method is Ok(OdeSolverStopReason::RootFound). RootFound holds the time at which the root was found, and the index of the root that was found. You can use the interpolate method to obtain the solution at the root time.

use diffsol::{OdeEquations, OdeSolverMethod, OdeSolverStopReason};

pub fn solve_match_step<'a, Solver, Eqn>(solver: &mut Solver)
where
    Solver: OdeSolverMethod<'a, Eqn>,
    Eqn: OdeEquations<T = f64> + 'a,
{
    solver.set_stop_time(10.0).unwrap();
    loop {
        match solver.step() {
            Ok(OdeSolverStopReason::InternalTimestep) => continue,
            Ok(OdeSolverStopReason::TstopReached) => break,
            Ok(OdeSolverStopReason::RootFound(_t, _idx)) => break,
            Err(e) => panic!("Solver failed to converge: {e}"),
        }
    }
    println!("Solver stopped at time: {}", solver.state().t);
}