#' Combine Dice
#'
#' This function is a way to combine two independent dice rolls into a new die.
#'
#' @param x A die
#' @param y A die
#' @param fun A function that takes two vectors containing the possible
#'   combinations of die rolls of \code{x} and \code{y}; the output should be of
#'   the same length. Example - \code{pmax} is such a function and is used to
#'   take the maximum of two rolls.
#'
#' @return A new die with the possible possibilities and their corresponding
#'   probabilties
#' @export
#'
#' @examples
#' summed.die <- combineDice(d4, d6, `+`)
#' advantaged.die <- combineDice(d4, d4, pmax)
combineDice <- function(x, y, fun){
  rolls <- expand.grid(x.numbers = x$numbers, y.numbers = y$numbers)
  rolls$x.probs <- getProbabilitiy(x, rolls$x.numbers)
  rolls$y.probs <- getProbabilitiy(y, rolls$y.numbers)
  rolls$combined.prob <- rolls$x.probs*rolls$y.probs
  rolls$combined.num <- fun(rolls$x.numbers, rolls$y.numbers)

  new.die.probs <- rolls %>%
    group_by(combined.num) %>%
    summarize(prob = sum(combined.prob)) %>%
    arrange(combined.num)

  new.die <- makeDie(new.die.probs$combined.num, new.die.probs$prob)

  return(new.die)

}