R/ala_cache_filename.R

In AtlasOfLivingAustralia/ALA4R: Atlas of Living Australia (ALA) Data and Resources in R

#' Returns the name of the cache file associated with the given URL.
#' Note that this file may not actually exist, this function just provides
#' the mapping from URL to filename
#'
#' @references \url{https://api.ala.org.au/}
#' @seealso \code{ala_config} for cache settings, particularly the cache
#' directory
#'
#' @param url string: the URL
#' @return string: the file path and name
#'
#' @examples
#' ala_cache_filename("https://biocache-ws.ala.org.au/ws/index/fields")
#'
#' @export ala_cache_filename
ala_cache_filename <- function(url) {
    assert_that(is.string(url))
    ## returns the cache filename associated with the given url
    ## note that this file may not actually exist, this function just provides
    ## the mapping from URL to filename

    ## make sure that URL query parms are sorted, so that the same URL with
    ## different query ordering hits the same cache file
    this_url <- parse_url(url) ## decompose URL into components
    if (! is.null(this_url$query)) {
        ## sort query terms by name
        this_url$query <- this_url$query[order(names(this_url$query))]
        ## may also wish to sort items within some query parms, e.g. fl
        ## is a comma-separated list of fields
        if (! is.null(this_url$query$fl)) {
            this_url$query$fl <-
                str_c(sort(str_split(this_url$query$fl, ",")[[1]]),
                      collapse = ",")
        }
        ## Note that this means that the ordering of columns coming back
        ## from the query may not match the ordering that the user provided
        ## the list of fields in. Need to decide if this is acceptable
    }
    url <- build_url(this_url) ## reconstruct URL string

    ## use md5 hash of url as the cache filename, because it's an easy way of
    ## generating unique filenames based on the URL
    x <- digest(url)
    ## the downside is that we can't easily go from a cache filename back to
    ## its URL
    file.path(ala_config()$cache_directory, x)
}