#' Download random images from the web
#'
#' Generate n number of high-definition images by category from the web
#'
#' @param fp CHARACTER. storage directory
#' @param cat CHARACTER. category of image to download
#' @param n NUMERIC. number of images to download, maximum n is 99
#' @param w.px NUMERIC. width in pixels
#' @param h.px NUMERIC. height in pixels
#' @param ext CHARACTER. file extension eg jpg, png
#'
#' @section Sources & References:
#' The random images are downloaded from www.unsplash.com
#'
#' @section Category Choices:
#' Categories for 'cat' argument include
#' "3D", "animals", "architecture", "backgrounds", "beauty", "experimental",
#' "fashion", "film", "food", "interior", "nature", "people", "renders",
#' "school", "sports", "travel", "unsplash", "wallpapers".\cr\cr
#' Image categories can be captured in a separate vector as a cross-reference made available to the cat argument. \cr For example: \cr
#' imgcat= c("3D", "animals", "architecture", "backgrounds", "beauty", "experimental", "fashion", "film", "food", "interior", "nature", "people", "renders", "school", "sports", "travel", "unsplash", "wallpapers")\cr\cr
#' genRandImg(fp, cat = imgcat[9], n = 5)
#'
#'
#' @section Use case:
#' This functionality is great for developers trying to obtain one or more images for use in displays/analysis
#' or simply to build robust web applications.
#'
#' @return downloaded image from a select image category
#'
#'
#' @examples
#' \donttest{
#' # download 2 image from the nature category
#' genRandImg(fp = tempdir(),cat = "nature", n = 2)
#'
#' # download 4 random images with width = 600px and hight 100px
#' genRandImg(
#'   fp = tempdir(),
#'   cat = "fashion",
#'   w.px = 600,
#'   h.px = 100)
#'
#' # download 10 random images with extension png
#' genRandImg(fp = tempdir(),cat = "food", n = 10, ext = "png")
#'
#'
#' # download 200 random images from category of school
#' # Note that maximum download is 99, so the function will only download 99
#' genRandImg(fp = tempdir(),cat = "school", n = 200)
#' }
#' @export
#'
genRandImg <- function(fp, cat = imageCategories, n = 1, w.px = 500, h.px = 500, ext = "jpg") {
  # check existence of directory
  if(!dir.exists(fp)) stop(paste0("The directory path declared in the 'fp' argument must exist."))

  # match category
  cat <- match.arg(cat)

  # check that n does not exceed 99
  if(n > 99){
    warning("The value of n exceeds 99, so n was set to 99")
    n <- 99L
  }
  # check sum and file names
  checksum.files <- c()
  downloaded.files <- c()

  # download files to temp directory
  while (n > 0) {
    file.store <- file.path(fp,paste0(tolower(cat),"_",randString(1,5),n,".",ext))
    utils::download.file(
      url = paste0("https://source.unsplash.com/random/", w.px, "x", h.px, "/?", cat, "&", n),
      destfile = file.store, mode = "wb"
    )

    # make sure there is no duplication
    checksum <- as.vector(tools::md5sum(file.store))
    if (checksum %in% checksum.files) {
      unlink(file.store)
    } else {
      vector_push(checksum.files, checksum)
      vector_push(downloaded.files, file.store)
      minus(n)
    }
  }
  message(paste0("Downloaded ",length(downloaded.files)," files to ",fp))
}