R/read.TSD_getindtFirstLast.R

In arnejohannesholmin/TSD: Read and write in the Time Step Data (TSD) format

#*********************************************
#*********************************************
#' A function for extracting the first and last value of 'indt'.
#'
#' @param labl  is a vector of the TSD labels of the file.
#' @param con  is a connection to a binary file, as returned by file(x,"rb").
#' @param index  is a matrix of the positions in the type of the variables and time steps.
#' @param datatypes	The data types of each element of 'labl'.
#' @param datasizes	The data sizes of each element of 'labl'.
#' @param endian  is the endianness of the file, changed if the float is not read properly.
#'
#' @return
#'
#' @examples
#' \dontrun{}
#'
#' @export
#' @rdname read.TSD_getindtFirstLast
#'
read.TSD_getindtFirstLast <- function(labl, con, index, datatypes, datasizes, endian=.Platform$endian){
	
	############### LOG: ###############
	# Start: 2014-10-17 - Clean version.
	
	# If 'indt' is requested, a file of many pings may cause reading 'indt' to be a slow process. Thus a test is made for continnuity of 'indt', in which case only the first and last value is read, and the sequence between these is returned as 'indt':
	indtat <- which("indt"==labl)
	# Get the first 'indt':
	seek(con, index[1, indtat], origin="start")
	indt_first <- readBin(con, datatypes[indtat], n=1, size=datasizes[indtat], endian=endian, signed=TRUE)
	# Get the last 'indt':
	seek(con, index[nrow(index), indtat], origin="start")
	indt_last <- readBin(con, datatypes[indtat], n=1, size=datasizes[indtat], endian=endian, signed=TRUE)
	# If the first and the last value of the indt variable span a sequence of step 1, we automatically have the entire sequence:
	if(length(indt_first)>0 && length(indt_last)>0 && (indt_last-indt_first+1) == nrow(index)){
		indt_con <- seq(indt_first, indt_last)
	}
	else{
		indt_con <- NULL
	}
	indt_con
}