NEWS.md

In simmer: Discrete-Event Simulation for R

simmer 4.4.6.2

New features

• Add support for activity tags via tag argument (as part of #287).
• Add support for named rollbacks via tags. The amount argument is deprecated in favor of the more generic name target. The former will still work, but raises a warning (#287 addressing #197).
• Switch to C++17, drop Boost usage (#297).

Minor changes and fixes

• Fix call identification on error (#286).
• Make tests less verbose by default.
• Fix deprecated functionality in vignette.
• Fix vignette to feed character input to numeric_version.
• Fixes for Apple Clang 10.

simmer 4.4.5

New features

• Add arithmetic support for schedule objects (#272).
• New get_batch_size() getter allows a batch to retrieve its size (#263).
• New get_activity_time() and get_activity_time_selected() getters allow an arrival to retrieve the amount of activity time spent in resources (#186).

Minor changes and fixes

• Apply magrittr workaround and avoid copying simmer and monitor objects in function environments for generators too (#279).
• Fix file removal on object destruction for file-backed monitors (#277).
• Remove cap on the number of nested branches for trajectory printing (#262).
• Fix duplicate time-based batch trigger (as part of #263).
• Fix typo and trajectory in bank vignettes (#283).
• Fix clone synchronization with wait=FALSE, i.e., arrivals don't need to trigger the same synchronize() activity to be in sync (#275).

simmer 4.4.4

Minor changes and fixes

• Fix memory issues ("not previously seized" error) when reneging from multiple resources (#268).
• Use 'given' instead of 'middle' in CITATION (#273).

simmer 4.4.3

New features

• Add support for functions in when_activated() (#250).
• Add support for dynamic batch sizes (#258 addressing #245).

Minor changes and fixes

• Fix multiple reporting of ongoing arrivals (#240).
• Set package namespace as parent of simmer and wrap environments (#241).
• Fix arrival restart after queue drops (#257).
• Fix segfault printing activities with empty vectors (#253).
• Fix source behaviour with missing values, which now stop arrival generation in the same way as negative values do (#256).
• Fix deactivate(), which now unschedules future arrivals (#249).

simmer 4.4.2

Minor changes and fixes

• Fix memory issues in trap(), synchronize() and rollback(). These are stateful activities that require storing information about passing arrivals to manage clones or redirections. These activities were not properly cleaning their storage when arrivals were rejected at some point in the trajectory. As a result, certain simulations with these activities involved may show random improper behaviour depending on how memory reuse happens. This patch unifies storage management for stateful activities, adds a new interface to register these activities and another interface for arrivals to notify their termination, so that the stored information is properly cleaned up (#231).
• Fix state sharing in round-robin policy operation in select() (#233).

simmer 4.4.1

Minor changes and fixes

• Fix pause status of reneging arrivals that are kicked from a resource queue (#228 addressing #227).
• Set the minimum execution priority for the timeout() activity. This makes possible to set a null timeout so that the next event is processed in the last place if several more events happen at the same time (#229).
• Extend the Queueing Systems vignette with a section about custom service policies (as part of #229).

simmer 4.4.0

New features

• Add out and keep_seized parameters to leave() with the same behaviour as in renege_in() and renege_if(). Code and documentation of these functions are now integrated under help(renege) (#208, #217).
• Convenience functions from, to and from_to accept dynamic parameters for arguments start_time, stop_time and every (#219).
• Activities to interact with sources have been vectorised to modify multiple sources at once (#222).
• Several generators or resources with the same parameters can be added with a single call to add_generator() and add_resource() respectively if a vector of names is provided (#221).

Minor changes and fixes:

• Fix get_mon_*() dispatch for named lists (#210).
• Get/put the RNG state when random numbers are required in the backend (#218).
• Fix convenience functions from, to and from_to to preserve the environment of the supplied functions (as part of #219).
• Documentation improvements (#212, #220).
• Fix queueing in multiple resources after preemption (#224 addressing #206).

simmer 4.3.0

New features

• Add ability to keep_seized resources after reneging (#204 addressing #200).
• Add ability to define a range of arrival priorities that are allowed to access a resource's queue if there is no room in the server (#205 addressing #202).

Minor changes and fixes:

• Drop R6 as a dependency (#193 addressing #190).
• Small fix in from and from_to + documentation update (75a9569).
• Move activity usage examples to help pages (#194).
• Fix shortest-queue selection policies (#196).
• Fix batch triggering (#203).
• Update JSS paper, CITATION, references and DOI.

simmer 4.2.2

Minor changes and fixes:

• Fix batches with an infinite timeout (#184).
• Fix preemption for arrivals previously stopped by a signal (#187).
• Fix handler loop for two consecutive signals (#188).
• Fix incorrect handler linking (#189).

simmer 4.2.1

Minor changes and fixes:

• Fix memtest notes on CRAN (e741686).

simmer 4.2.0

New features:

• New handle_unfinished() activity sets a drop-out trajectory for unfinished arrivals, i.e., those dropped from a resource (due to preemption, resource shrinkage or a rejected seize) or those that leave a trajectory (#178 addressing #177).
• New release_all() and release_selected_all() activities automatically retrieve the amount of resources seized and release it (#180 addressing #25).
• New get_seized() and get_seized_selected() getters allow an arrival to retrieve the amount of resources seized (#180 addressing #179).
• New stop_if() activity sets a conditional breakpoint (#181 addressing #100).

Minor changes and fixes:

• Fix performance issues in data sources (#176).
• Update CITATION.
• Fix monitored activity for preempted arrivals (as part of #178).
• Fix seizes/releases with a null amount (as part of #180).
• Rename internal status codes (as part of #181).
• Provide more context on error or warning (as part of #181).
• Extend the Queueing Systems vignette with a section about state-dependent service rates.
• Fix performance issues in getters (#183).

simmer 4.1.0

New features:

• New getter get_selected() retrieves names of selected resources via the select() activity (#172 addressing #171).
• Source and resource getters have been vectorised to retrieve parameters from multiple entities (as part of #172).
• Simplify C++ Simulator interface for adding processes and resources (#162). The responsibility of building the objects has been moved to the caller.
• New add_global() method to attach global attributes to a simulation environment (#174 addressing #158).

Minor changes and fixes:

• Remove 3.8.0 and 4.0.1 deprecations (#170 addressing #165).
• Fix get_global() to work outside trajectories (#170 addressing #165).
• Fix rollback() with an infinite amount (#173).
• Fix and improve schedules and managers (as part of #174).
• Fix reset() to avoid overwriting the simulation environment (#175).

simmer 4.0.1

New features:

• New getters (#159):
  • get_sources() and get_resources() retrieve a character vector of source/resource names defined in a simulation environment.
  • get_trajectory() retrieves a trajectory to which a given source is attached.
• New resource selection policies: shortest-queue-available, round-robin-available, random-available (#156). These are the same as the existing non-available ones, but they exclude unavailable resources (capacity set to zero). Thus, if all resources are unavailable, an error is raised.

Minor changes and fixes:

• Rename -DRCPP_PROTECTED_EVAL (Rcpp >= 0.12.17.4) as -DRCPP_USE_UNWIND_PROTECT (6d27671).
• Keep compilation quieter with -DBOOST_NO_AUTO_PTR (70328b6).
• Improve log_ print (7c2e3b1).
• Add when_activated() convenience function to easily generate arrivals on demand from trajectories (#161 closing #160).
• Enhance schedule printing (9c66285).
• Fix generator-manager name clashing (#163).
• Deprecate set_attribute(global=TRUE), get_attribute(global=TRUE) and timeout_from_attribute(global=TRUE) (#164), the *_global versions should be used instead.

simmer 4.0.0

• The simmer license has been changed to GPL >= 2.

New features:

• The C++ core has been refactorised into a header-only library under inst/include (#147 closing #145). Therefore, from now on it is possible to extend the C++ API from another package by listing simmer under the LinkingTo field in the DESCRIPTION file.
• New generic monitor constructor enables the development of new monitoring backends in other packages (179f656, as part of #147).
• New simulation-scoped logging levels. The log_ activity has a new argument level which determines whether the message is printed depending on a global log_level defined in the simmer constructor (#152).
• set_attribute and set_global gain a new argument to automatically initialise new attributes (#157). Useful to update counters and indexes in a single line, without initialisation boilerplate.

Minor changes and fixes:

• Enhanced exception handling, with more informative error messages (#148).
• Refactorisation of the printing methods and associated code (#149).
• Allow empty trajectories in sources and activities with sub-trajectories (#151 closing #150).
• Enable -DRCPP_PROTECTED_EVAL (Rcpp >= 0.12.17.3), which provides fast evaluation of R expressions by leveraging the new stack unwinding protection API (R >= 3.5.0).
• Replace backspace usage in vector's ostream method (2b2f43e).
• Fix namespace clashes with rlang and purrr (#154).

simmer 3.8.0

New features:

• New data source add_dataframe enables the attachment of precomputed data, in the form of a data frame, to a trajectory. It can be used instead of (or along with) add_generator. The most notable advantage over the latter is that add_dataframe is able to automatically set attributes and prioritisation values per arrival based on columns of the provided data frame (#140 closing #123).
• New set_source activity deprecates set_distribution(). It works both for generators and data sources (275a09c, as part of #140).
• New monitoring interface allows for disk offloading. The simmer() constructor gains a new argument mon to provide different types of monitors. By default, monitoring is performed in-memory, as usual. Additionally, monitoring can be offloaded to disk through monitor_delim and monitor_csv, which produce flat delimited files. But more importantly, the C++ interface has been refactorised to enable the development of new monitoring backends (#146 closing #119).

Minor changes and fixes:

• Some documentation improvements (1e14ed7, 194ed05).
• New default until=Inf for the run method (3e6aae9, as part of #140).
• branch and clone now accept lists of trajectories, in the same way as join, so that there is no need to use do.call (#142).
• The argument continue (present in seize and branch) is recycled if only one value is provided but several sub-trajectories are defined (#143).
• Fix process reset: sources are reset in strict order of creation (e7d909b).
• Fix infinite timeouts (#144).

simmer 3.7.0

New features:

• New timeout_from_attribute() activity makes it easier to set a timeout based on an attribute (#129).
• The activities set_attribute(), set_prioritization(), set_capacity() and set_queue_size() get a new argument mod which, if set to "+" or "*", modifies the corresponding value instead of substituting it. This makes it easier to increment, decrement or scale one of these values (#130).
• New *_selected() versions for the already available resource getters: get_capacity(), get_queue seize(), get_server_count() and get_queue_count() (#134).

Minor changes and fixes:

• Broadcast signals with higher priority to prevent an arrival to catch its own signal with a trap() after a send() (#135).
• Generate new arrivals with minimum priority to avoid wrong interactions with simultaneous activities (#136).
• Remove v3.6.x deprecations: the old attribute retrieval system (see notes for v3.6.3), as well as methods create_trajectory() and onestep() (#117).
• Remove get_mon_resources()'s data argument. It was there for historical reasons and probably nobody was using it (851d34b).
• New vignette, "simmer: Discrete-Event Simuation for R", paper accepted for publication in the Journal of Statistical Software. Remove "Terminology" vignette (#127).
• New vignette, "Design and Analysis of 5G Scenarios", supplementary materials for a paper accepted for publication in the IEEE Communications Magazine (#137).

simmer 3.6.5

New features:

• set_attribute() (and set_global() by extension) can set multiple attributes at once by providing vectors of keys and values (or functions returning such keys and/or values). get_attribute() (and get_global() by extension) can retrieve multiple keys (#122).
• New stepn() method deprecates onestep() (e452975).

Minor changes and fixes:

• Restore ostream after formatting (9ff11f8).
• Fix arrival cloning to copy attributes over to the clone (#118).
• Fix self-induced preemption through set_capacity() (#125).
• Update "Queueing Systems" vignette (a0409a0, 8f03f4f).
• Update "Advanced Trajectory Usage" vignette (4501927).
• Fix print methods to return the object invisibly (#128).
• New "Dining Philosophers Problem" vignette (ff6137e).

simmer 3.6.4

Minor changes and fixes:

• Fix preemption in non-saturated multi-server resources when seizing amounts > 1 (#114).
• Fix queue priority in non-saturated finite-queue resources when seizing amounts > 1 (#115).
• Fix resource seizing: avoid jumping the queue when there is room in the server but other arrivals are waiting (#116).

simmer 3.6.3

New features:

• Show simulation progress via an optional progress callback in run() (#103).
• New "The Bank Tutorial: Part II" vignette, by Duncan Garmonsway @nacnudus (#106).
• New getters for running arrivals (#109), meant to be used inside trajectories:
  • get_name() retrieves the arrival name.
  • get_attribute() retrieves an attribute by name. The old method of retrieving them by providing a function with one argument is deprecated in favour of get_attribute(), and will be removed in version 3.7.x.
  • get_prioritization() retrieves the three prioritization values (priority, preemptible, restart) of the active arrival.
• New shortcuts for global attributes (#110): set_global() and get_global(), equivalent to set_attribute(global=TRUE) and get_attribute(global=TRUE) respectively.

Minor changes and fixes:

• Some code refactoring and performance improvements (2f4b484, ffafe1e, f16912a, fb7941b, 2783cd8).
• Use Rcpp::DataFrame instead of Rcpp::List (#104).
• Improve argument parsing and error messages (#107).
• Improve internal function make_resetable() (c596f73).

simmer 3.6.2

Minor changes and fixes:

• Update "The Bank Tutorial: Part I" vignette (@nacnudus in #90).
• Fix trap()'s handler cloning and associated test (#91).
• Apply select()'s policy also when resources is a function (#92).
• Accept dynamic timeouts in batches (#93).
• Change rollback()'s default behaviour to times=Inf, i.e., infinite loop (#95).
• Stop and throw an error when timeout() returns a missing value (#96 and #97).
• Fix memory management: resetting the environment was clearing but not deallocating memory (#98, fixed in #99).
• Fix object destruction: workaround for tidyverse/magrittr#146 (#98, fixed in effcb6b).

simmer 3.6.1

Minor changes and fixes:

• Recycle logical indexes when subsetting (2526e75).
• Implement replacement operators, [<- and [[<- (#88).
• Provide rep() S3 method for trajectories (7fa515e).
• Remove plotting functions (bb9656b), deprecated since v3.6.0. The new simmer.plot package (on CRAN) already covers these features among others.
• Don't evaluate vignette chunks if Suggests are not installed (e40e5b6).
• Rewrite DESCRIPTION (3f26516).
• Add an every parameter to the from_to() convenience function (9d68887).

simmer 3.6.0

New features:

• New subsetting operators, [ and [[, for trajectories (1847898). Think about trajectories as lists of activities and these operators will do (almost) everything you expect. As a side effect, the generics head() and tail() automatically work with trajectories also as expected.
• New length() method to obtain the number of first-level activities in a trajectory (f86375a). Useful in combination with the subsetting operators.

Minor changes and fixes:

• create_trajectory() has been deprecated in favor of trajectory() (76c1317).
• plot_resource_usage(), plot_resource_utilization(), plot_evolution_arrival_times() and plot_attributes() have been deprecated and will be removed in the next release in order to minimise dependencies (5b43f2b). We plan to release a new package on CRAN covering these features and new ones.
• All methods are now S3 methods, so that a nice intelligible error is displayed if you apply a method to the wrong object (e891045).
• All the activity management -related stuff has been removed, i.e, get_head(), get_tail(), print_activity(), get_next_activity(), get_prev_activity() (f86375a). These methods were only useful for development purposes and nobody should be using them. And it was never a good idea to directly expose external pointers.
• Clone all trajectories before passing them to the C++ core (f655cae).
• Update "Advanced Trajectory Usage" vignette.
• Update "Queueing Systems" vignette.

simmer 3.5.1

New features:

• New renege_if() activity triggers reneging upon reception of a signal broadcasted with send() (#84).
• Add support for uninterruptible handlers in trap() (bb2aa46).
• Add support for global attributes in set_attribute() (#82).

Minor changes and fixes:

• Fix bug in set_queue_size() with queue_size_strict=TRUE: arrivals were not being dropped (#83).
• Fix bug in set_capacity() with a preemptive resource when the old value was Inf: arrivals were not being preempted (63beb2c).
• Fix bug in per-resource activity monitoring: activity was not being reset (55097c9).
• Fix trap() printed information.
• Update "Advanced Trajectory Usage" vignette.
• New "Other SimPy Examples" vignette.

simmer 3.5.0

New features:

• set_capacity() and set_queue_size() become activities (#77). Just like seize() and release(), they have the associated set_capacity_selected() and set_queue_size_selected() for a joint use together with select().
• New activate() and deactivate() activities allow an arrival to start or stop a generator, respectively, from inside a trajectory (#80).
• New set_trajectory() and set_distribution() activities allow an arrival to install a new trajectory or distribution, respectively, in a generator from inside a trajectory (#80).
• Refactorised and improved arrival monitoring.
• New interarrival communication activities allowing asynchronous programming: send(), trap(), untrap() and wait() can be used to send signals, wait for signals, trap them and launch asynchronous handlers.
• New log_() activity simply prints messages for debugging purposes (eaa4554).

Minor changes and fixes:

• Store inline trajectory objects inside the simulation environment to prevent them to be garbage-collected.
• Update "Advanced Trajectory Usage" vignette.

simmer 3.4.4

Minor changes and fixes:

• Fix non-defined behaviour caused by a race condition in object destruction under some platforms.

simmer 3.4.3

Minor changes and fixes:

• Remove warnings for unused arguments in seize() and seize_selected() (1c8c3bb).
• Fix crash on arrival reneging in non-triggered batches (8713d95).
• Fix crash on batches triggered before the timer expires (8713d95).
• Fix crash on non-released preemptive resources when capacity decreases (#75).
• Leaving without releasing a resource throws a warning (#76).

simmer 3.4.2

New features:

• Ongoing (unfinished) arrivals are reported with get_mon_arrivals(ongoing = TRUE) (#73).

Minor changes and fixes:

• Simplify Rcpp glue: remove unnecessary as<>() calls (ec4e51a).
• Simplify trajectory's head/tail management (06432a8).
• Now, run(until) runs the simulation exactly until until, instead of until the first event scheduled at a time >= until (e7264f6).
• Fix batch cloning (c20bc1d).
• Coverage improved.

simmer 3.4.1

Minor changes and fixes:

• Fix memtest notes on CRAN (heap-use-after-free).
• Fix memory leaks.

simmer 3.4.0

New features:

• Prioritization (priority, preemptible, restart) has been moved from seize() to add_generator() (#69). This leads to a more natural interpretation of prioritization values as attributes of arrivals from the same generator, rather than attributes of a seize(). Still, prioritization values can be redefined dynamically from inside a trajectory with the new activity set_prioritization().
• New optional post.seize and reject sub-trajectories in seize() and seize_selected() (#49). This feature allows us to fine-tune what happens to an arrival if it cannot seize a resource: instead of getting dropped, it may execute a given sub-trajectory.
• New clone() and synchronize() activities (#71). clone() implements the workflow pattern in which an entity is processed in multiple parallel threads. The user can define a different sub-trajectory for each clone. With synchronize(), multiple parallel clones converge and are synchronized: only one continues (the first or the last to arrive), and the others are removed.
• New batch() and separate() activities (#45). They can be used to implement a rollercoaster process: batch() collects a number of arrivals before they can continue processing as a block, and separate() splits a previousl established batch.
• New renege_in() and renege_abort() activities (#58). They can be used to set or unset a timer after which the arrival will abandon.

Minor changes and fixes:

• If a branch()'s option returns 0, the arrival skips the branch() and continues to the next activity instead of throwing an index out of range error (#70).
• Throw errors on incorrect releases (#72).
• Remove deprecated convenience function every() (#65) and branch()'s deprecated argument merge (#57).
• New "The Bank Tutorial: Part I" vignette, by Duncan Garmonsway @nacnudus (#68).
• Update "Advanced Trajectory Usage" vignette.

simmer 3.3.0

New features:

• New join() activity to concatenate trajectories (#50).
• Batched generation: the generation function can return more than one interarrival value at a time (#65).
• Add the option queue_size_strict to add_resource() to guarantee the queue size limit with preemption (#59).
• New select(), seize_selected() and release_selected() activities (#52).
• Modify resources (capacity, queue size) from inside a trajectory (#66).
• New leave() activity (#63).

Major fixes:

• Fix per-resource activity time monitoring (#67). The problem emerged when an arrival revisited a resource and it was enqueued. An uninitialised variable could lead to an activity time greater than end_time - start_time. All versions 3.2.x are affected by this bug.

Minor changes and fixes:

• Fix the description of preemptible in the documentation of seize() and force preemptible to be equal or greater than priority (#53).
• Reset finite generators (#51).
• Fix the handling of a capacity change when the new value is infinite (#60).
• Various doc fixes (#61).
• Change branch()'s merge parameter name to continue. The old name is deprecated (#57).
• Use match.arg() in multiple-choice arguments (#55).
• Fix branch() backwards linking and count (#56).
• Split release() in two steps to deal properly with capacity changes at the same point in time (#64).
• The convenience function every() is deprecated due to #65.
• Update and extend previous vignettes.

simmer 3.2.1

New features:

• Add time-specific resource availability support (#21). Both resources' capacity and queue_size can change over time following a user-defined scheduling, which can be generated with the new function schedule().
• Advanced peek: inspect any number of future events in the event queue (8147820). For more details, see ?peek.

Minor changes and fixes:

• Fix steps grouping in plot_resource_usage() (8da9b97).
• Fix incorrect trajectory behaviour when a rejection occurs inside a branch with merge=TRUE (#46).
• Fix a couple of segmentation faults in preemptive resources (f64f6b2).
• Improve verbose output (9013db0).
• New multiset-based event queue with unscheduling capabilities (a615fea and d6a9d67).
• A simulation may run forever (until the user interrupts it), that is, until=Inf is allowed now (f47baa9).
• New vignette on queueing systems.
• New vignette on Continuous-Time Markov Chains.
• Update and extend previous vignettes.

simmer 3.2.0

Major fix:

• In previous versions, resources were monitored before performing the corresponding seize/release activity, before changing the status of the system. Thus, t=3, queue=2 meant that, until t=3, the queue had 2 customers, and at t=3 the system changed (because of a new arrival or a new departure). The idea was to keep the values and time vectors aligned (see #28). But from this moment on, the resources are monitored after changing the status of the system. This is more consistent with what a user would expect, and more consistent with the behaviour of other related R functions (e.g., see stepfun(), from the stats package). Wrapping up and from now on, t=3, queue=2 means that some event happened at t=3 whose immediate and subsequent result was a queue with 2 customers.

New features:

• Add preemption functionality (#34). Preemption comes into play when a resource is specified as preemptive=TRUE. Arrivals in the server can be preempted on a preempt_order="fifo" or preempt_order="lifo" basis. Each seize() has three basic properties:
  • priority: already present in previous versions.
  • preemptible: another seize() with a priority value greater than this may preempt the present seize().
  • restart: whether the current task (a timeout() activity, for instance) should be restarted if the arrival is preempted.

Minor changes and fixes:

• Remove deprecated functions show_activity() and show_trajectory().
• Add every(), to() and from_to() convenience functions (8e524cd).
• Fix colour scale in plot_resource_usage() (6b034a7).
• Fix compatibility with the upcoming version of testthat (#41).
• The branch() activity now provides attributes to its option function, as the other activities (#42).
• Implement error handling in plot_*() functions (#44).

simmer 3.1.2

New features:

• Monitor arrivals' start/activity/end times on a per-resource basis (#38). So far, the function get_mon_arrivals() returned the start/activity/end times per arrival for the whole trajectory. This behaviour remains, but additionally, get_mon_arrivals(per_resource = TRUE) returns these times per resource, so that it is possible to retrieve queueing/system times per resource.

Minor changes and fixes:

• Fix testing ERRORs reported on platforms using clang and Sparc Solaris.
• get_mon_*() functions accept a single simulation environment as well as a list of environments representing several replications (5ee2725). A new column (replication) in the resulting data frame indicates the corresponding replication number.
• Monitoring subsystem refactored (as a consequence of #38).

simmer 3.1.1

New features:

• Add attributes to arrivals and new set_attribute() activity (#16).
• New rollback() activity (#17 and #22).
• Add priorities to resources' queue (#20).

Minor changes and fixes:

• Performance improvements with Boost (1b654fd).
• Fix arrivals' timing issues (#24).
• Nicer object printing (#26).
• Return self visibly, instead of invisibly (#35).
• Add at() and from() convenience functions (29cccd2 and 7cfdd90).
• Some work on vignettes (#29).
• Fix ggplot2 2.0.0 compatibility issues (#37).

simmer 3.0.1

Minor changes and fixes:

• Complete test coverage (#12).
• Minor fix in the documentation (42363d2).
• Set finalizer in the simulator object (#14).
• Fix test errors under Windows r-oldrelease (#15).

simmer 3.0.0

New features:

• First major release submitted to CRAN. The philosophy and workflow of the pre-release remain with a more robust event-based C++ backend and a more flexible frontend.
• Enhanced programmability. The timeout activity is more than just a delay. It admits a user-defined function, which can be as complex as needed in order to interact with the simulation model. The old v2.0 was no more than a queueing network simulator. This feature makes simmer a flexible and generic DES framework. Moreover, we have finally got rid of the infamous add_skip_event() function to implement a more flexible and user-friendly branching method.
• Robustness. The event-based core design is rigorous and simple, which makes simmer faster and less error-prone, at the same level of other state-of-the-art DES frameworks.
• Much better performance. Instead of creating n arrivals beforehand, this release leverages the concept of generator of arrivals, which is faster and more flexible. At the same time, the concept of trajectory as a chain of activities is implemented entirely in C++ internally. Our tests show that simmer is even faster than SimPy when it comes to simulate queueing networks.
• Replication. In the pre-release, replication was implemented inside simmer. This no longer makes sense since, with the current design, it is more than straightforward to replicate and even parallelize the execution of replicas using standard R tools.

simmer documentation built on Aug. 22, 2023, 5:09 p.m.