#' Extract Dates from (MODIS) Files
#' 
#' @description 
#' This function helps to extract dates from a vector of files.
#' 
#' @param files A \code{character} vector of filenames from which to extract 
#' dates. Alternatively, a \code{Raster*} with date information in its layer 
#' \code{\link[raster]{names}}.
#' @param pos1,pos2 Start and end of date string in \code{files} as 
#' \code{integer}. If missing, positions are tried to be retrieved from a 
#' look-up table provided that 'files' comply with the MODIS standard naming 
#' convention.
#' @param asDate \code{logical}. If \code{TRUE}, the result is converted to a 
#' \code{Date} object.
#' @param format \code{character}, date format. Used only if \code{asDate = TRUE}. 
#' Defaults to MODIS date style (i.e., \code{"\%Y\%j"} for year and julian day). 
#' See \code{\link{strptime}} for modifications.
#' 
#' @return 
#' A \code{list} with the following entries: 'inputLayerDates', 'pos1', 'pos2', 
#' 'asDate' and, optionally, 'format'. If \code{asDate = FALSE}, 
#' 'inputLayerDates' are represented as \code{character}, else as \code{Date}.
#' 
#' @author 
#' Matteo Mattiuzzi
#' 
#' @examples 
#' # example on HDF files
#' files <- c("MOD13Q1.A2010209.h18v03.005.2010239071130.hdf",
#'            "MOD13Q1.A2010225.h18v03.005.2010254043849.hdf")
#' extractDate(files)
#' extractDate(files, asDate = TRUE)
#' 
#' # on any other file 
#' files <- c("Myfile_20010101.XXX", "Myfile_20010115.XXX", "Myfile_20010204.XXX")
#' extractDate(files, pos1 = 8, pos2 = 15)
#' extractDate(files, pos1 = 8, pos2 = 15, asDate = TRUE, format = "%Y%m%d")
#'  
#' @export extractDate
#' @name extractDate
extractDate <- function(files, pos1, pos2, asDate = FALSE, format = "%Y%j")
{
  if (inherits(files, "Raster")) {
    files <- names(files)
  }
  
  files <- basename(files)
  
  ## if any position indication is missing, try to retrieve it from look-up table
  if (any(missing(pos1), missing(pos2))) {
    ids = positionIndication(files)
    pos1 = ids[[1]]; pos2 = ids[[2]]
  }

  date  <- sapply(files,function(x){
    substr(x, pos1, pos2)
  })
  
  if(asDate)
  {
    date <- as.Date(date, format=format)
    return(list(inputLayerDates = date, pos1=pos1, pos2=pos2, asDate = asDate, format=format))
  } else 
  {
    return(list(inputLayerDates = date, pos1 = pos1, pos2 = pos2, asDate = asDate))
  }
}