#' Convert to a slidy presentation
#'
#' Format for converting from R Markdown to a slidy presentation.
#'
#' See the \href{https://bookdown.org/yihui/rmarkdown/slidy-presentation.html}{online
#' documentation} for additional details on using the \code{slidy_presentation}
#' format.
#'
#' For more information on markdown syntax for presentations see the
#' \href{https://pandoc.org/README.html}{pandoc online documentation}.
#' @inheritParams pdf_document
#' @inheritParams html_document
#' @inheritParams beamer_presentation
#' @param duration Duration (in minutes) of the slide deck. This value is used
#'   to add a countdown timer to the slide footer.
#' @param footer Footer text (e.g. organization name and/or copyright)
#' @param font_adjustment Increase or decrease the default font size (e.g. -1 or
#'   +1). You can also manually adjust the font size during the presentation
#'   using the 'S' (smaller) and 'B' (bigger) keys.
#' @param ... Additional function arguments to pass to the base R Markdown HTML
#'   output formatter \code{\link{html_document_base}}
#' @return R Markdown output format to pass to \code{\link{render}}
#' @examples
#' \dontrun{
#' library(rmarkdown)
#'
#' # simple invocation
#' render("pres.Rmd", slidy_presentation())
#'
#' # specify an option for incremental rendering
#' render("pres.Rmd", slidy_presentation(incremental = TRUE))
#' }
#' @export
slidy_presentation <- function(number_sections = FALSE,
                               incremental = FALSE,
                               slide_level = NULL,
                               duration = NULL,
                               footer = NULL,
                               font_adjustment = 0,
                               fig_width = 8,
                               fig_height = 6,
                               fig_retina = 2,
                               fig_caption = TRUE,
                               dev = 'png',
                               df_print = "default",
                               self_contained = TRUE,
                               highlight = "default",
                               mathjax = "default",
                               template = "default",
                               css = NULL,
                               includes = NULL,
                               keep_md = FALSE,
                               lib_dir = NULL,
                               md_extensions = NULL,
                               pandoc_args = NULL,
                               extra_dependencies = NULL,
                               ...) {

  # base pandoc options for all reveal.js output
  args <- c()

  # template path and assets
  if (identical(template, "default"))
    template <- pkg_file("rmd/slidy/default.html")
  if (!is.null(template))
    args <- c(args, "--template", pandoc_path_arg(template))

  # html dependency for slidy
  extra_dependencies <- append(
    extra_dependencies,
    list(html_dependency_slidy(),
         html_dependency_slidy_shiny()))

  # incremental
  if (incremental)
    args <- c(args, "--incremental")

  # slide level
  if (!is.null(slide_level))
    args <- c(args, "--slide-level", slide_level)

  # duration
  if (!is.null(duration))
    args <- c(args, pandoc_variable_arg("duration", duration))

  # footer
  if (!is.null(footer))
    args <- c(args, pandoc_variable_arg("footer", footer))

  # font size adjustment
  if (font_adjustment != 0)
    args <- c(args, pandoc_variable_arg("font-size-adjustment",
                                        font_adjustment))

  # content includes
  args <- c(args, includes_to_pandoc_args(includes))

  # pagedtables
  if (identical(df_print, "paged")) {
    extra_dependencies <- append(extra_dependencies,
                                 list(html_dependency_pagedtable()))
  }

  # additional css
  for (css_file in css)
    args <- c(args, "--css", pandoc_path_arg(css_file, backslash = FALSE))

  # pre-processor for arguments that may depend on the name of the
  # the input file (e.g. ones that need to copy supporting files)
  pre_processor <- function(metadata, input_file, runtime, knit_meta, files_dir,
                            output_dir) {

    # use files_dir as lib_dir if not explicitly specified
    if (is.null(lib_dir))
      lib_dir <- files_dir

    # extra args
    args <- c()

    # highlight
    args <- c(args, pandoc_highlight_args(highlight, default = "pygments"))

    # return additional args
    args
  }

  # return format
  output_format(
    knitr = knitr_options_html(fig_width, fig_height, fig_retina, keep_md, dev),
    pandoc = pandoc_options(
      to = "slidy",
      from = from_rmarkdown(fig_caption, md_extensions),
      args = args,
      lua_filters = if (number_sections) pkg_file_lua("number-sections.lua")
    ),
    keep_md = keep_md,
    clean_supporting = self_contained,
    df_print = df_print,
    pre_processor = pre_processor,
    base_format = html_document_base(lib_dir = lib_dir,
                                     self_contained = self_contained,
                                     mathjax = mathjax,
                                     bootstrap_compatible = TRUE,
                                     pandoc_args = pandoc_args,
                                     extra_dependencies = extra_dependencies,
                                     ...))
}

html_dependency_slidy <- function() {
  htmlDependency(
    name = "slidy",
    version = "2",
    src = pkg_file("rmd/slidy/Slidy2"),
    script = "scripts/slidy.js",
    stylesheet = "styles/slidy.css"
  )
}

html_dependency_slidy_shiny <- function() {
  htmlDependency(
    name = "slidy_shiny",
    version = "1",
    src = pkg_file("rmd/slidy/"),
    script = "slidy_shiny.js",
    all_files = FALSE
  )
}
