# R functions for labeling Date-indexed axes

# Days to the beginning of each month
ltt.private.dim <- c(0,31,59,90,120,151,181,212,243,273,304,334)
ltt.private.dim.leap <- c(0,31,60,91,121,152,182,213,244,274,305,335)

# Convert a string formatted as yyyy/mm/dd to a fraction of a year
date.string.to.year.fraction <- function(Dstring) {
    year <- as.integer(substr(Dstring,1,4))
    month <- 1
    if(nchar(Dstring)>=7) month <- as.integer(substr(Dstring,6,7))
    day <- 1
    if(nchar(Dstring)>=10) day <- as.integer(substr(Dstring,9,10))
    fraction <- 0
    if(is.leap.year(year)) { 
        fraction <- year + (ltt.private.dim.leap[month]+day-0.5)/366
    }
    else {
        fraction <- year + (ltt.private.dim[month]+day-0.5)/365
    }
    return(fraction)
}

is.leap.year <- function(year) {
    if(year/400-as.integer(year/400)==0) return(T)
    if(year/4-as.integer(year/4)==0 && 
       year/100-as.integer(year/100)) return(T)
    return(F)
}

# Calculate a set of locations from a set of labels
xaxis.label.to.at <- function(label) {
    at <- rep(0.0,length(label))
    for(i in seq(1,length(label))) { at[i] <- date.string.to.year.fraction(label[i]) }
    return(at) 
}