# yrfraction.R
# fraction of the year for a date, includes leap year
# type = 'monthly', 'weekly' or 'daily' (default)
# Jan 2014



#' Fraction of the Year
#' 
#' Calculate the fraction of the year for a date variable (after accounting for
#' leap years) or for month.
#' 
#' Returns the fraction of the year in the range [0,1).
#' 
#' @param date a date variable if type=\sQuote{\code{daily}}, or an integer
#' between 1 and 12 if type=\sQuote{\code{monthly}}.
#' @param type \sQuote{daily} for dates, or \sQuote{\code{monthly}} for months.
#' @return \item{yrfrac}{Fraction of the year.}
#' @author Adrian Barnett \email{a.barnett@qut.edu.au}
#' @examples
#' 
#' # create fractions for the start, middle and end of the year
#' date = as.Date(c(0, 181, 364), origin='1991-01-01')
#' # create fractions based on these dates
#' yrfraction(date)
#' yrfraction(1:12, type='monthly')
#' 
#' @export yrfraction
yrfraction<-function(date,type='daily'){
  if (type=='daily'){
    if (class(date)!="Date"){stop("Date variable for annual data must be in date format, see ?Dates")} 
    year<-as.numeric(format(date,'%Y'));
    lastday<-ISOdate(year,12,31); # last day in December
    day<-as.numeric(format(date,'%j')); # Day of year as decimal number (001-366)
    yrlength<-as.numeric(format(lastday,'%j'));
    yrfrac<-(day-1)/yrlength;
  }
  if (type=='weekly'){
    if (max(date)>53|min(date)<1){stop("Date variable for weekly data must be month integer (1 to 53)")} 
    yrfrac<-(date-1)/(365.25/7);
  }
  if (type=='monthly'){
    if (max(date)>12|min(date)<1){stop("Date variable for monthly data must be month integer (1 to 12)")} 
    yrfrac<-(date-1)/12;
  }
  return(yrfrac)
}