simmer 4.0.0

The 4.0.0 release of simmer, the Discrete-Event Simulator for R, is on CRAN under a new license: we decided to switch to GPL >= 2. Most notably in this major release, the C++ core has been refactorised and exposed under inst/include. This is not a big deal for most users, but it enables extensions. As an example of this, simmer.mon is an experimental package that links to simmer and extends its monitoring facilities to provide a new DBI-based backend. Not a very efficient one, but it demonstrates how to extend the simmer core from another package.

Exception handling has been remarkably improved. In previous releases, errors were reported to happen in the run() method, which is… everything that can happen, obviously. In this version, errors are catched and more information is provided, particularly about the simulation time, the arrival and the activity involved:

library(simmer)

bad.traj <- trajectory() %>%
  timeout(function() NA)

simmer() %>%
  add_generator("dummy", bad.traj, at(pi)) %>%
  run()
## Error: 'dummy0' at 3.14 in 'Timeout':
##  missing value (NA or NaN returned)

Another improvement has to do with attributes. These are commonly used to build incremental indices, but some boilerplate was needed to initialise them. Now this is automatic (and configurable):

index.traj <- trajectory() %>%
  set_global("index", 1, mod="+", init=10)

simmer() %>%
  add_generator("dummy", index.traj, at(1:3), mon=2) %>%
  run() %>%
  get_mon_attributes()
##   time name   key value replication
## 1    1      index    11           1
## 2    2      index    12           1
## 3    3      index    13           1

Finally, the log_ activity was created for occasional debugging, but we noticed that simmer users use it a lot more to know what is happening when they build models, but so much output is annoying when a model is complete. Therefore, we have implemented simulation-scoped logging levels to be able to turn on and off specific messages on demand:

log.traj <- trajectory() %>%
  log_("This will be always printed") %>% # level=0
  log_("This can be disabled", level=1)

simmer(log_level=1) %>%
  add_generator("dummy", log.traj, at(pi)) %>%
  run() %>% invisible()
## 3.14159: dummy0: This will be always printed
## 3.14159: dummy0: This can be disabled
simmer() %>% # log_level=0
  add_generator("dummy", log.traj, at(pi)) %>%
  run() %>% invisible()
## 3.14159: dummy0: This will be always printed

See below for a comprehensive list of changes.