Create (and optionally recenter) day of year from date

recenter_doy.R

#' Convert a date to a day of year, optionally setting an arbitrary date as the 
#' first day in a year.
#'
#' @param date a vector of dates in a standard unambigous format ("YYYY/MM/DD")
#' @param start_doy the first day of the year ("mm/dd"). Default January 1 ("01/01"), but can be set 
#' to any month/day to set an arbitrary start date from which to calculate day of year
#'
#' @return integer vector with values from 1:365 (or 366 if a leap year)
#' @export
#' 
#' @examples
#' dates <- c("1965/02/05", "1981/01/01", "2017/12/31")
#' doy(dates)
#' doy(dates, start_doy = "10/01")
doy <- function(date, start_doy = "01/01") {
  if (!require(lubridate)) stop("package lubridate required but not available")
  date <- as.Date(date)
  doy <- yday(date)
  if (start_doy != "01/01") {
    year <- year(date)
    doy <- recenter_doy(year, doy, start_doy)
  }
  doy
}

#' Convert a day of year based on a calendar year to a day of year using a
#' different start date than January 1. 
#'
#' @param year four-digit year
#' @param doy integer day of year
#' @param start_doy the first day of the year ("mm/dd"). Can be set 
#' to any month/day to set an arbitrary start date from which to calculate day of year
#'
#' @return integer vector with values from 1:365 (or 366 if a leap year)
#' @export
#'
#' @examples
#' data <- data.frame(year = c(1996, 1997, 1998),
#'                    doy = c(56, 72, 275))
#' recenter_doy(data$year, data$doy, "10/01")
recenter_doy <- function(year, doy, start_doy) {
  if (!require(lubridate)) stop("package lubridate required but not available")
  days_in_year <- yday(ymd(paste0(year, "12/31")))
  start_doy <- yday(ymd(paste0(year, start_doy)))
  ifelse(doy > start_doy, 
         doy - start_doy,
         days_in_year - start_doy + doy)
}