# This is a function for dividing a time series into periods, with allowence for gaps (threshold)
# data = dataset
# variable = time for measurement, e.g. year (duplicates allowed)
# threshold = number of consecutive years (e.g.) allowed missing in a period
# filter_less_than = filters periods with less than N values (not the length from min to max!)

periods <- function(data,
                    variable,
                    threshold = 3,
                    filter_less_than = NA){
  variable_enquo <- enquo(variable)
  groups_vec <- data %>% group_vars() %>% syms
  out <- data %>%
    dplyr::arrange(!!variable_enquo) %>%
    mutate(period = cumsum(c(1, diff(!!variable_enquo) >= threshold+1))) %>%
    group_by(period, add=T) %>%
    mutate(n_year_period = n_distinct(!!variable_enquo))

  if(!is_empty(groups_vec)){out <- out %>% group_by(!!!groups_vec)}else{out <- out %>% ungroup}

  if(!is.na(filter_less_than)){out <- out %>%
    filter(n_year_period >= filter_less_than) %>%
    mutate(period = cumsum(c(1, diff(!!variable_enquo) >= threshold+1)))
  }

  return(out)
}