Fix SBDF order ramp, history-shift timing, and dt-change reset - #4366
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Fix SBDF order ramp, history-shift timing, and dt-change reset#4366niy-ati wants to merge 1 commit into
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SBDF (SBDF2/3/4) is a fixed-coefficient IMEX multistep method and requires a uniform step-size history. Three issues combined to make it silently run below its nominal convergence order: - The bootstrap counter allowed the higher-order formula one step before enough trusted history existed. - The buffer history shift was gated on the bootstrap counter instead of the algorithm order, so high-order runs could skip a needed shift during ramp-up. - No reset occurred when dt changed, so the fixed-coefficient formula was silently applied across a non-uniform step history (e.g. on a clipped final step), inflating the error by orders of magnitude. Fixes SciML#4329.
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Summary
SBDF (SBDF2/3/4) is a fixed-coefficient IMEX multistep method and requires a uniform step-size history. Three issues combined to make it silently run below its nominal convergence order:
Fixes #4329.
Test plan
SBDFconvergence testset measuring empirical order for pure-BDF and genuine IMEX splits, out-of-place and in-place, across SBDF2/SBDF3/SBDF4.tstops-forced mid-integration dt change,reinit!+ re-solve consistency, and in-place order-4 correctness.