MARSS
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MARSSsimulate
de Jong has a fast algorithm for sampling x from the conditional distribution.
There are better algorithms for the unconditional distribution too.
Durbin J, Koopman SJ (2002). “A Simple and Efficient Simulation Smoother for State Space Time Series Analysis.” Biometrika, 89(3), 603–615. doi:10.1093/biomet/89.3.603.