##############################################################################
# Determine symptom onset to admission distribution from case data
# - will be used to backsample unknown symptom onsets from admission date
##############################################################################


determine_SO2admission <- function(data, date, start_date = NULL) {

  data <- data %>% 
    filter(admission_date_reported <= date,
           admission_date <= date,
           symptom_onset <= date) %>% 
           {if(length(start_date)!=0) filter(., admission_date >= start_date) else .}
  
  interval <- data %>% 
    mutate(interval = as.integer(admission_date - symptom_onset)) %>%
    pull(interval)
  
  interval_cdf <- ecdf(interval[interval >= 0 & interval <= 30]) # don't trust symptom onset after day of admission and admission nor admission more than 30 days after symptom onset
  interval_cdf <- interval_cdf(0:30)
  interval_pdf <- diff(c(0, interval_cdf))
  names(interval_pdf) <- 0:30
  
  return(interval_pdf)
}