How to enumerate intervals in a sequence, like periods when cryptids come to visit

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Also, here's an alternative solution that also uses cumsum(), but in a less direct way:

visits |> 
  # flag Sasquatch days
  mutate(sasquatch = visitor == 'Sasquatch') |> 
  # if we assign a value of 0 to Sasquatch days, and 1 to the rest,
  # the cumulative sum will be constant during Sasquatch visits, but 
  # also different for each visit
  mutate(visit = if_else(sasquatch, cumsum(!sasquatch), NA)) |>
  # remove 'sasquatch' column since no longer needed
  select(!sasquatch) |> 
  # convert 'visit' to a factor so corresponding ordinal can be extracted
  mutate(visit = visit |> factor() |> as.numeric())

This:

visit |> factor() |> as.numeric()

is a bit of a hack, but I'm not sure of a way to convert to ordinal values more cleanly.