#' Functions for accessing skim_df attributes
#'
#' These functions simplify access to attributes contained within a `skim_df`.
#' While all attributes are read-only, being able to extract this information
#' is useful for different analyses. These functions should always be preferred
#' over calling base R's attribute functions.
#'
#' @param object A `skim_df` or `skim_list`.
#' @return Data contained within the requested `skimr` attribute.
#' @name skim-attr
NULL

#' @describeIn skim-attr Get the number of rows in the skimmed data frame.
#' @export
data_rows <- function(object) {
  attr(object, "data_rows")
}

#' @describeIn skim-attr Get the number of columns in the skimmed data frame.
#' @export
data_cols <- function(object) {
  attr(object, "data_cols")
}

#' @describeIn skim-attr Get the name of the skimmed data frame. This is only
#'   available in contexts where the name can be looked up. This is often not
#'   the case within a pipeline.
#' @export
df_name <- function(object) {
  attr(object, "df_name")
}

#' @describeIn skim-attr Get the key of the skimmed data.table. This is only
#'   available in contexts where `data` is of class `data.table`.
#' @export
dt_key <- function(object) {
  attr(object, "dt_key")
}

#' @describeIn skim-attr Get the names of the groups in the original data frame.
#'   Only available if the data was grouped. Otherwise, `NULL`.
#' @export
group_names <- function(object) {
  attr(object, "groups")
}

#' @describeIn skim-attr Get the names of the base skimming functions used.
#' @export
base_skimmers <- function(object) {
  attr(object, "base_skimmers")
}

#' @describeIn skim-attr Get the names of the skimming functions used, separated
#'   by data type.
#' @export
skimmers_used <- function(object) {
  attr(object, "skimmers_used")
}


#' Test if an object is compatible with `skimr`
#'
#' Objects within `skimr` are identified by a class, but they require additional
#' attributes and data columns for all operations to succeed. These checks help
#' ensure this. While they have some application externally, they are mostly
#' used internally.
#'
#' Most notably, a `skim_df` has columns `skim_type` and `skim_variable`. And
#' has the following special attributes
#'
#' * `data_rows`: n rows in the original data
#' * `data_cols`: original number of columns
#' * `df_name`: name of the original data frame
#' * `dt_key`: name of the key if original is a data.table
#' * `groups`: if there were group variables
#' * `base_skimmers`: names of functions applied to all skim types
#' * `skimmers_used`: names of functions used to skim each type
#'
#' The functions in these checks work like [all.equal()]. The return `TRUE` if
#' the check passes, or otherwise notifies why the check failed. This makes them
#' more useful when throwing errors.
#'
#' @param object Any `R` object.
#' @name skim-obj
NULL

#' @describeIn skim-obj Does the object have the `skim_type` column?
#' @export
has_type_column <- function(object) {
  make_issue("skim_type" %in% names(object), "missing column `skim_type`")
}

#' @describeIn skim-obj Does the object have the `skim_variable` column?
#' @export
has_variable_column <- function(object) {
  make_issue(
    "skim_variable" %in% names(object), "missing column `skim_variable`"
  )
}

#' @describeIn skim-obj Does the object have the appropriate `skimr` attributes?
#' @export
has_skimr_attributes <- function(object) {
  skimr_attrs <- c(
    "data_rows", "data_cols", "df_name", "dt_key", "base_skimmers",
    "skimmers_used"
  )
  missing <- !(skimr_attrs %in% names(attributes(object)))
  make_issue(!any(missing), show_missing_attributes(skimr_attrs, missing))
}

show_missing_attributes <- function(attributes, missing) {
  missing_msg <- if (length(missing) > 0) {
    paste0(attributes[missing], collapse = ", ")
  } else {
    ""
  }
  paste("missing attributes:", missing_msg)
}

#' @describeIn skim-obj Does the object have a `skim_type` attribute? This makes
#'   it a `one_skim_df`.
#' @export
has_skim_type_attribute <- function(object) {
  make_issue(
    "skim_type" %in% names(attributes(object)),
    "missing attribute: `skim_type`"
  )
}

#' @describeIn skim-obj Does the object have skimmers?
#' @export
has_skimmers <- function(object) {
  base <- base_skimmers(object)
  skimmers <- reconcile_skimmers(
    object,
    group_names(object),
    base
  )
  make_issue(
    any(lengths(skimmers) > 0, base %in% names(object)),
    "does not have defined skimmers."
  )
}

#' @describeIn skim-obj Is the object a data frame?
#' @export
is_data_frame <- function(object) {
  make_issue(inherits(object, "data.frame"), "not a data.frame")
}

make_issue <- function(check, message) {
  structure(check, message = if (!check) message else character())
}

#' @describeIn skim-obj Is the object a `skim_df`?
#' @export
is_skim_df <- function(object) {
  check_issues(
    "Object is not a `skim_df`",
    is_data_frame(object),
    has_type_column(object),
    has_variable_column(object),
    has_skimr_attributes(object),
    has_skimmers(object)
  )
}

#' @describeIn skim-obj Is the object a `one_skim_df`? This is similar to a
#'   `skim_df`, but does not have the `type` column. That is stored as an
#'   attribute instead.
#' @export
is_one_skim_df <- function(object) {
  check_issues(
    "Object is not a `one_skim_df`",
    is_data_frame(object),
    has_skim_type_attribute(object),
    has_variable_column(object),
    has_skimr_attributes(object)
  )
}

#' @describeIn skim-obj Is the object a `skim_list`?
#' @export
is_skim_list <- function(object) {
  check_issues(
    "Object is not a `skim_list`",
    has_skimr_attributes(object),
    !!!purrr::map(object, is_one_skim_df)
  )
}

#' @describeIn skim-obj Is this a data frame with `skim_variable` and
#'  `skim_type` columns?
#' @export
could_be_skim_df <- function(object) {
  check_issues(
    "Object cannot behave like a `skim_df`",
    is_data_frame(object),
    has_variable_column(object),
    has_type_column(object)
  )
}

check_issues <- function(condition, ...) {
  issues <- rlang::list2(...)
  msgs <- purrr::map(issues, ~ attr(.x, "message"))
  check <- all(purrr::flatten_lgl(issues))
  message <- if (check) {
    character()
  } else {
    paste0(condition, ": ", paste0(unlist(msgs), collapse = "; "))
  }
  structure(check, message = message)
}

#' @describeIn skim-obj Stop if the object is not a `skim_df`.
#' @export
assert_is_skim_df <- function(object) {
  assert(is_skim_df(object))
}

#' @describeIn skim-obj Stop if the object is not a `skim_list`.
#' @export
assert_is_skim_list <- function(object) {
  assert(is_skim_list(object))
}

#' @describeIn skim-obj Stop if the object is not a `one_skim_df`.
#' @export
assert_is_one_skim_df <- function(object) {
  assert(is_one_skim_df(object))
}

assert <- function(check) {
  if (check) {
    invisible(NULL)
  } else {
    stop(attr(check, "message"))
  }
}

#' Remove `skimr` class if not needed.
#' @noRd
strip_skim_attrs <- function(object) {
  attrs <- attributes(object)
  stripped <- purrr::list_modify(
    attrs,
    class = class(object)[-1],
    data_rows = NULL,
    data_cols = NULL,
    df_name = NULL,
    dt_key = NULL,
    base_skimmers = NULL,
    skimmers_used = NULL
  )
  attributes(object) <- stripped
  object
}

#' Pass attributes from a `skimr` object to a new object.
#' @noRd
reassign_skim_attrs <- function(object, skim_df, ...) {
  defaults <- list(
    class = c("skim_df", "tbl_df", "tbl", "data.frame"),
    data_rows = data_rows(skim_df),
    data_cols = data_cols(skim_df),
    df_name = df_name(skim_df),
    dt_key = dt_key(skim_df),
    groups = group_names(skim_df),
    base_skimmers = base_skimmers(skim_df),
    skimmers_used = skimmers_used(skim_df)
  )
  updated <- purrr::list_modify(defaults, ...)
  assign_new_attributes(object, !!!updated)
}

assign_new_attributes <- function(object, ...) {
  original <- attributes(object)
  attributes(object) <- purrr::list_modify(original, ...)
  object
}