#' Add Extra CSS Styles
#'
#' Adds css elements to target `outfile`, typically a xaringanthemer css file.
#' The `css` argument takes a list of CSS classes and definitions (see examples below)
#' and appends CSS rules to `outfile`.
#'
#' @section css list:
#' The `css` input must be a named list of css properties and values within a
#' named list of class identifiers, for example
#' `list(".class-id" = list("css-property" = "value"))`.
#'
#' @param css A named list of CSS definitions each containing a named list
#'   of CSS property-value pairs, i.e.
#'   `list(".class-id" = list("css-property" = "value"))`
#' @param append If `TRUE` output will be appended to `outfile`; otherwise,
#'   it will overwrite the contents of `outfile`.
#' @param heading Heading added above extra CSS. Use `NULL` to disable.
#'
#' @examples
#' style_extra_css(
#'   outfile = stdout(),
#'   css = list(
#'     ".red" = list(color = "red"),
#'     ".small" = list("font-size" = "90%"),
#'     ".full-width" = list(
#'       display = "flex",
#'       width = "100%",
#'       flex = "1 1 auto"
#'     )
#'   )
#' )
#' @inheritParams style_xaringan
#' @export
style_extra_css <- function(
  css,
  outfile = "xaringan-themer.css",
  append = TRUE,
  heading = "Extra CSS"
) {
  has_heading <- !is.null(heading)
  x <- paste0(
    if (has_heading) paste0("/* ", heading, " */\n"),
    list2css(css)
  )
  if (append) x <- paste0(if (has_heading) "\n\n" else "\n", x)
  if (is.null(outfile)) return(x)
  cat(
    x,
    file = outfile,
    append = append,
    sep = "\n"
  )
  invisible(x)
}

#' @inheritParams style_extra_css
#' @keywords internal
list2css <- function(css) {
  `%.%` <- function(x, y) paste0(x, y)
  error <- NULL
  if (is.null(names(css))) {
    stop("All elements in `css` list must be named", call. = FALSE)
  }
  if (purrr::vec_depth(css) != 3) {
    stop(
      "`css` list must be a named list within a named list, e.g.:\n",
      '  list(".class-id" = list("css-property" = "value"))'
    )
  }
  if (any(names(css) == "")) {
    not_named <- which(names(css) == "")
    if (length(not_named) > 1) {
      stop(
        call. = FALSE,
        "All elements in `css` list must be named. Items ",
        paste(not_named, collapse = ", "),
        " are unnamed."
      )
    } else {
      stop(
        call. = FALSE,
        "All elements in `css` list must be named. Item ",
        not_named,
        " is not named."
      )
    }
  }
  child_unnamed <- purrr::map_lgl(purrr::map(css, ~ {
    is.null(names(.)) || any(names(.) == "")
  }), ~ any(.))
  if (any(child_unnamed)) {
    has_unnamed <- names(css)[child_unnamed]
    msg <- paste(
      "All properties of elements in `css` list must be named.",
      if (length(has_unnamed) > 1) "Elements" else "Element",
      paste(has_unnamed, collapse = ", "),
      if (length(has_unnamed) > 1) "have" else "has",
      "unnamed property or properties."
    )
    stop(msg, call. = FALSE)
  }

  purrr::map_chr(names(css), function(el) {
    paste(
      sep = "\n",
      el %.% " {",
      paste(
        purrr::map_chr(names(css[[el]]), function(prop) {
          "  " %.% prop %.% ": " %.% css[[el]][[prop]] %.% ";"
        }),
        collapse = "\n"
      ),
      "}"
    )
  })
}

list2fonts <- function(fonts) {
  if (
    length(setdiff(names(google_font('fam')), names(fonts))) == 0 &&
    !inherits(fonts, "google_font")
  ) {
    # concatenating a string and a google_font() provides a wacky list
    stop(
      "Multiple fonts in `extra_fonts` must be specified inside a `list()`.",
      call. = FALSE
    )
  }
  if (inherits(fonts, "google_font")) {
    fonts <- list(fonts)
  }
  fonts <- purrr::map_chr(fonts, function(f) {
    if (inherits(f, "google_font")) {
      f$url
    } else if (inherits(f, "character")) {
      f
    } else {
      NA_character_
    }
  })
  paste0("@import url(", fonts[!is.na(fonts)], ");")
}