#' Create a graph object
#'
#' @description
#'
#' Generates a graph object with the option to use node data frames (ndfs)
#' and/or edge data frames (edfs) to populate the initial graph.
#'
#' @param nodes_df An optional data frame containing, at minimum, a column
#'   (called `id`) which contains node IDs for the graph. Additional columns
#'   (node attributes) can be included with values for the named node attribute.
#' @param edges_df An optional data frame containing, at minimum, two columns
#'   (called `from` and `to`) where node IDs are provided. Additional columns
#'   (edge attributes) can be included with values for the named edge attribute.
#' @param directed With `TRUE` (the default) or `FALSE`, either directed or
#'   undirected edge operations will be generated, respectively.
#' @param graph_name An optional string for labeling the graph object.
#' @param attr_theme The theme (i.e., collection of `graph`, `node`, and `edge`
#'   global graph attributes) to use for this graph. The default theme is called
#'   `default`; there are hierarchical layout themes called `lr`, `tb`, `rl`,
#'   and `bt` (these operate from left-to-right, top-to-bottom, right-to-left,
#'   and bottom-to-top); and, for larger graphs, the `fdp` theme provides a
#'   force directed layout. If this is set to `NULL` then no global graph
#'   attributes will be applied to the graph upon creation.
#' @param write_backups An option to write incremental backups of changing graph
#'   states to disk. If `TRUE`, a subdirectory within the working directory will
#'   be created and used to store `RDS` files. The default value is `FALSE` so
#'   one has to opt in to use this functionality.
#' @param display_msgs An option to display messages primarily concerned with
#'   changes in graph selections. By default, this is `FALSE`.
#'
#' @return A graph object of class `dgr_graph`.
#'
#' @examples
#' # With `create_graph()` we can
#' # simply create an empty graph (and
#' # add in nodes and edges later
#' # with other functions)
#' graph <- create_graph()
#'
#' # A graph can be created with
#' # nodes and without having any edges;
#' # this can be done in 2 steps:
#' # 1. create a node data frame (ndf)
#' #    using `create_node_df()`
#' ndf <-
#'   create_node_df(n = 4)
#'
#' # 2. create a new graph object with
#' #    `create_graph()` and then pass
#' #    in the ndf to `nodes_df`
#' graph <-
#'   create_graph(
#'     nodes_df = ndf)
#'
#' # Get information on the graph's nodes
#' graph %>%
#'   get_node_info()
#'
#' # You can create a similar graph with
#' # just nodes but also providing a
#' # range of attributes for the nodes
#' # (e.g., types, labels, or arbitrary
#' # 'values')
#' ndf <-
#'   create_node_df(
#'     n = 4,
#'     label = TRUE,
#'     type = c("type_1", "type_1",
#'              "type_5", "type_2"),
#'     shape = c("circle", "circle",
#'               "rectangle", "rectangle"),
#'     values = c(3.5, 2.6, 9.4, 2.7))
#'
#' graph <-
#'   create_graph(nodes_df = ndf)
#'
#' # Get information on the graph's
#' # internal node data frame (ndf)
#' graph %>%
#'   get_node_df()
#'
#' # A graph can also be created by
#' # specifying both the nodes and
#' # edges; create an edge data frame
#' # (edf) using the `create_edge_df()`
#' # function:
#' edf <-
#'   create_edge_df(
#'     from = c(1, 2, 3),
#'     to = c(4, 3, 1),
#'     rel = "leading_to",
#'     values = c(7.3, 2.6, 8.3))
#'
#' # Create the graph object with
#' # `create_graph()` and pass in the
#' # ndf and edf objects
#' graph <-
#'   create_graph(
#'     nodes_df = ndf,
#'     edges_df = edf)
#'
#' # Get information on the graph's
#' # internal edge data frame (edf)
#' graph %>% get_edge_df()
#'
#' # Get information on the graph's
#' # internal node data frame (ndf)
#' graph %>% get_node_df()
#'
#' @export
create_graph <- function(
    nodes_df = NULL,
    edges_df = NULL,
    directed = TRUE,
    graph_name = NULL,
    attr_theme = "default",
    write_backups = FALSE,
    display_msgs = FALSE
) {

  ## DF: `graph_info`

  # Get the time of graph creation
  graph_time <- Sys.time()

  # Get the locale's time zone
  graph_tz <- Sys.timezone()

  # Generate a random 8-character, alphanumeric
  # string to use as a graph ID
  graph_id <-
    replicate(8, sample(c(LETTERS, letters, 0:9), 1)) %>%
    paste(collapse = "")

  # Create the `graph_info` data frame
  graph_info <-
    data.frame(
      graph_id = as.character(graph_id),
      graph_name = as.character(paste0("graph_", graph_id)),
      graph_time = graph_time,
      graph_tz = graph_tz,
      write_backups = write_backups,
      display_msgs = display_msgs,
      stringsAsFactors = FALSE
    )

  # Insert a user-defined `graph_name` if supplied
  if (!is.null(graph_name)) {
    graph_info[1, 2] <- as.character(graph_name)
  }

  ## DF: `global_attrs`

  # Create an empty table for global graph attributes
  global_attrs <-
    data.frame(
      attr = character(0L),
      value = character(0L),
      attr_type = character(0L),
      stringsAsFactors = FALSE
    )

  # If `attr_theme` is `default` then populate the
  # `global_attrs` data frame with global graph attrs
  if (inherits(attr_theme, "character")) {

    global_attrs <-
      switch(
        attr_theme,
        default = attr_theme_default(),
        lr = attr_theme_lr(),
        tb = attr_theme_tb(),
        rl = attr_theme_rl(),
        bt = attr_theme_bt(),
        fdp = attr_theme_fdp(),
        kk = attr_theme_kk(),
        cli::cli_abort(
          "The value for `attr_theme` doesn't refer to any available theme."
        )
      )

  } else if (is.null(attr_theme)) {

    global_attrs <-
      data.frame(
        attr = character(0L),
        value = character(0L),
        attr_type = character(0L),
        stringsAsFactors = FALSE)
  }

  ## DF: `nodes_df`

  # Create an empty node data frame (`ndf`)
  ndf <-
    data.frame(
      id = integer(0L),
      type = character(0L),
      label = character(0L),
      stringsAsFactors = FALSE
    )

  ## DF: `edges_df`

  # Create an empty edge data frame (`edf`)
  edf <-
    data.frame(
      id = integer(0L),
      from = integer(0L),
      to = integer(0L),
      rel = character(0L),
      stringsAsFactors = FALSE
    )

  ## DF: `node_selection`

  # Create an empty node selection data frame (`nsdf`)
  nsdf <-
    data.frame(node = integer(0L), stringsAsFactors = FALSE)

  ## DF: `edge_selection`

  # Create an empty edge selection data frame (`esdf`)
  esdf <-
    data.frame(
      edge = integer(0L),
      from = integer(0L),
      to = integer(0L),
      stringsAsFactors = FALSE
    )

  ## DF: `graph_actions`

  # Create an empty `graph_actions` data frame
  graph_actions <-
    data.frame(
      action_index = integer(0L),
      action_name = character(0L),
      expression = character(0L),
      stringsAsFactors = FALSE
    )

  ## DF: `graph_log`

  # Create an empty `graph_log` data frame

  graph_log <-
    data.frame(
      version_id = integer(0L),
      function_used = character(0L),
      # Datetime of length 0
      time_modified = structure(numeric(0L), class = c("POSIXct", "POSIXt")),
      duration = numeric(0L),
      nodes = integer(0L),
      edges = integer(0L),
      d_n = integer(0L),
      d_e = integer(0L),
      stringsAsFactors = FALSE
    )

  ## list: `cache`

  # Create an empty `cache` list object
  cache <- list()

  ## Empty Graph

  # Initialize a graph object
  graph <-
    list(
      graph_info = graph_info,
      nodes_df = ndf,
      edges_df = edf,
      global_attrs = global_attrs,
      directed = directed, # TRUE or FALSE
      last_node = 0,
      last_edge = 0,
      node_selection = nsdf,
      edge_selection = esdf,
      cache = cache,
      graph_actions = graph_actions,
      graph_log = graph_log
    )

  attr(graph, "class") <- "dgr_graph"

  # If neither an ndf nor both ndf & edf provided,
  # create an initialized graph with no nodes or edges
  if (is.null(nodes_df) && is.null(edges_df)) {

    # Get the name of the function
    fcn_name <- get_calling_fcn()

    # Update the `graph_log` df with an action
    graph_log <-
      add_action_to_log(
        graph_log = graph_log,
        version_id = 1L,
        function_used = fcn_name,
        time_modified = graph_time,
        duration = graph_function_duration(graph_time),
        nodes = nrow(graph$nodes_df),
        edges = nrow(graph$edges_df),
        d_n = nrow(graph$nodes_df),
        d_e = nrow(graph$edges_df)
      )

  } else if (!is.null(nodes_df) && is.null(edges_df)) {

    # If only an ndf is provided, create a graph
    # just containing nodes

    # Transform any `tbl_df` object to a `data.frame`
    if (inherits(nodes_df, "tbl_df")) {
      nodes_df <- as.data.frame(nodes_df, stringsAsFactors = FALSE)
    }

    # Force the `type` and `label` columns
    # to be of the character class
    for (i in 2:3) {
      nodes_df[, i] <- as.character(nodes_df[, i])
    }

    # Bind the nodes to the `nodes_df` df in the graph
    graph$nodes_df <- dplyr::bind_rows(graph$nodes_df, nodes_df)

    # Modify the `last_node` vector
    graph$last_node <- nrow(nodes_df)

    # Get the name of the function
    fcn_name <- get_calling_fcn()

    # Update the `graph_log` df with an action
    graph_log <-
      add_action_to_log(
        graph_log = graph_log,
        version_id = 1L,
        function_used = fcn_name,
        time_modified = graph_time,
        duration = graph_function_duration(graph_time),
        nodes = nrow(graph$nodes_df),
        edges = nrow(graph$edges_df),
        d_n = nrow(graph$nodes_df),
        d_e = nrow(graph$edges_df)
      )

  } else if (!is.null(nodes_df) && !is.null(edges_df)) {

    # If an ndf and edf both provided, create a graph
    # initially populated with both nodes and edges

    # Transform any `tbl_df` object to a `data.frame`
    if (inherits(nodes_df, "tbl_df")) {
      nodes_df <- as.data.frame(nodes_df, stringsAsFactors = FALSE)
    }

    # Transform any `tbl_df` object to a `data.frame`
    if (inherits(edges_df, "tbl_df")) {
      edges_df <- as.data.frame(edges_df, stringsAsFactors = FALSE)
    }

    # Force the `type` and `label` columns
    # to be of the character class
    for (i in 2:3) {
      nodes_df[, i] <- as.character(nodes_df[, i])
    }

    # Bind the nodes to the `nodes_df` df in the graph
    graph$nodes_df <-
      dplyr::bind_rows(graph$nodes_df, nodes_df)

    # Modify the `last_node` vector
    graph$last_node <- nrow(nodes_df)

    # Ensure that the edf has the correct classes
    if (inherits(edges_df, "data.frame") && ncol(edges_df) > 2) {

      # Force the rel column to be of the character class
      edges_df$rel <- as.character(edges_df$rel)
    }

    # Bind the edges to the `edges_df` df in the graph
    graph$edges_df <-
      dplyr::bind_rows(graph$edges_df, edges_df)

    # Modify the `last_edge` vector
    graph$last_edge <- nrow(edges_df)

    # Get the name of the function
    fcn_name <- get_calling_fcn()

    # Update the `graph_log` df with an action
    graph_log <-
      add_action_to_log(
        graph_log = graph_log,
        version_id = 1L,
        function_used = fcn_name,
        time_modified = graph_time,
        duration = graph_function_duration(graph_time),
        nodes = nrow(graph$nodes_df),
        edges = nrow(graph$edges_df),
        d_n = nrow(graph$nodes_df),
        d_e = nrow(graph$edges_df))
  }

  # Add the `graph_log` df to the graph object
  graph$graph_log <- graph_log

  # Write graph backup if the option is set
  if (graph$graph_info$write_backups) {
    save_graph_as_rds(graph = graph)
  }

  # If neither an ndf nor both ndf & edf provided,
  # return the initialized graph with no nodes or edges
  graph
}