Implement post.seize and reject subtrajectories in seize

[Enchufa2]

Many models require an arrival to do something if it gets rejected when trying to seize a resource. So we need something in between a seize and a branch: an ifseize activity would work similarly to the R function ifelse. For instance,

ifseize(traj, resource, amount = 1, retry = FALSE, merge = c(TRUE, TRUE),
           yes = NULL, 
           no = NULL,
           priority = 0, preemptible = 0, restart = FALSE)

Use case: a WiFi station trying to seize the wireless channel to transmit a frame:

frame <- create_trajectory() %>%
  timeout(backoff) %>%
  ifseize("channel", 1, retry=7, merge=c(T, T),
          yes = create_trajectory() %>%
            timeout(send_frame) %>%
            release("channel", 1),
          no = create_trajectory() %>%
            timeout(backoff))

@bart6114, review this proposal, please.

[Enchufa2]

By virtue of #56, retries can be implemented using rollback, so this argument can be suppressed.

[Enchufa2]

Linking with #52, there should be a ifseize_selected too.

[Enchufa2]

Following #57, the merge parameter must be called continue instead.

[Enchufa2]

#69 alleviates seize's syntax and simplifies things for this enhancement. Instead of defining new verbs, I've opted to add this new feature to the existing seize and seize_selected. This is their new signature:

seize(traj, resource, amount = 1, 
      continue = NULL, post.seize = NULL, reject = NULL)

seize_selected(traj, amount = 1, id = 0, 
               continue = NULL, post.seize = NULL, reject = NULL)

so that their default behaviour is the same. But now, we can execute a post.seize trajectory, that is, only if the resource is seized:

t <- create_trajectory() %>% 
  seize("dummy", 1, continue=FALSE, 
        post.seize = create_trajectory() %>% 
          timeout(2) %>%
          release("dummy", 1)) %>%
  timeout(1) # this is not executed in any case, as continue=FALSE

simmer() %>%
  add_resource("dummy", 1, 0) %>%
  add_generator("arrival", t, at(0, 1)) %>%
  run() %>% get_mon_arrivals()
#>       name start_time end_time activity_time finished replication
#> 1 arrival1          1        1             0    FALSE           1
#> 2 arrival0          0        2             2     TRUE           1

or a reject trajectory, that is, only if the resource cannot be seized, which is useful if we want to perform retries, for instance:

t <- create_trajectory() %>% 
  seize("dummy", 1, continue=FALSE, 
        reject = create_trajectory() %>% 
          timeout(1) %>%        # wait 
          rollback(2, Inf)) %>% # and then retry, until the resource is seized
  timeout(2) %>%
  release("dummy", 1)

simmer() %>%
  add_resource("dummy", 1, 0) %>%
  add_generator("arrival", t, at(0, 1)) %>%
  run() %>% get_mon_arrivals()
#>       name start_time end_time activity_time finished replication
#> 1 arrival0          0        2             2     TRUE           1
#> 2 arrival1          1        4             3     TRUE           1

or we can do both and then continue:

t <- create_trajectory() %>% 
  seize("dummy", 1, continue=c(TRUE, TRUE), 
        post.seize = create_trajectory() %>% 
          timeout(2) %>%
          release("dummy"),
        reject = create_trajectory() %>% 
          timeout(3)) %>%
  timeout(3) # all arrivals, rejected or not, execute this, as continue=c(TRUE, TRUE)

simmer() %>%
  add_resource("dummy", 1, 0) %>%
  add_generator("arrival", t, at(0, 1)) %>%
  run() %>% get_mon_arrivals()
#>       name start_time end_time activity_time finished replication
#> 1 arrival0          0        5             5     TRUE           1
#> 2 arrival1          1        7             6     TRUE           1