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#' @include arrow-datum.R

#' @title Arrow scalars
#' @usage NULL
#' @format NULL
#' @docType class
#'
#' @description A `Scalar` holds a single value of an Arrow type.
#'
#' @section Methods:
#'   `$ToString()`: convert to a string
#'   `$as_vector()`: convert to an R vector
#'   `$as_array()`: convert to an Arrow `Array`
#'   `$Equals(other)`: is this Scalar equal to `other`
#'   `$ApproxEquals(other)`: is this Scalar approximately equal to `other`
#'   `$is_valid`: is this Scalar valid
#'   `$null_count`: number of invalid values - 1 or 0
#'   `$type`: Scalar type
#'
#' @name Scalar
#' @rdname Scalar
#' @examplesIf arrow_available()
#' Scalar$create(pi)
#' Scalar$create(404)
#' # If you pass a vector into Scalar$create, you get a list containing your items
#' Scalar$create(c(1, 2, 3))
#'
#' # Comparisons
#' my_scalar <- Scalar$create(99)
#' my_scalar$ApproxEquals(Scalar$create(99.00001)) # FALSE
#' my_scalar$ApproxEquals(Scalar$create(99.000009)) # TRUE
#' my_scalar$Equals(Scalar$create(99.000009)) # FALSE
#' my_scalar$Equals(Scalar$create(99L)) # FALSE (types don't match)
#'
#' my_scalar$ToString()
#' @export
Scalar <- R6Class("Scalar",
  inherit = ArrowDatum,
  # TODO: document the methods
  public = list(
    ToString = function() Scalar__ToString(self),
    type_id = function() Scalar__type(self)$id,
    as_vector = function() Scalar__as_vector(self),
    as_array = function(length = 1L) MakeArrayFromScalar(self, as.integer(length)),
    Equals = function(other, ...) {
      inherits(other, "Scalar") && Scalar__Equals(self, other)
    },
    ApproxEquals = function(other, ...) {
      inherits(other, "Scalar") && Scalar__ApproxEquals(self, other)
    }
  ),
  active = list(
    is_valid = function() Scalar__is_valid(self),
    null_count = function() sum(!self$is_valid),
    type = function() Scalar__type(self)
  )
)
Scalar$create <- function(x, type = NULL) {
  if (is.null(x)) {
    x <- vctrs::unspecified(1)
  } else if (length(x) != 1 && !is.data.frame(x)) {
    # Wrap in a list type
    x <- list(x)
  }
  Array__GetScalar(Array$create(x, type = type), 0)
}

#' @rdname array
#' @usage NULL
#' @format NULL
#' @export
StructScalar <- R6Class("StructScalar",
  inherit = Scalar,
  public = list(
    field = function(i) StructScalar__field(self, i),
    GetFieldByName = function(name) StructScalar__GetFieldByName(self, name)
  )
)

#' @export
length.Scalar <- function(x) 1L

#' @export
sort.Scalar <- function(x, decreasing = FALSE, ...) x