#  Copyright (c) 2022 Merck & Co., Inc., Rahway, NJ, USA and its affiliates. All rights reserved.
#
#  This file is part of the gsDesign2 program.
#
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#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation, either version 3 of the License, or
#  (at your option) any later version.
#
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#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
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#' @importFrom dplyr last lag lead
#' @importFrom tibble tibble
#' @importFrom stats stepfun
NULL

#' Piecewise constant expected accrual
#'
#' \code{eAccrual()} computes the expected cumulative enrollment (accrual)
#' given a set of piecewise constant enrollment rates and times.
#' @param x times at which enrollment is to be computed.
#' @param enrollRates Piecewise constant enrollment rates expressed as a `tibble` with
#' `duration` for each piecewise constant period and the `rate` of enrollment for that period.
#' @section Specification:
#' \if{latex}{
#'  \itemize{
#'    \item Validate if input x is a vector of strictly increasing non-negative numeric elements.
#'    \item Validate if input enrollment rate is of type data.frame.
#'    \item Validate if input enrollment rate contains duration column.
#'    \item Validate if input enrollment rate contains rate column.
#'    \item Validate if rate in input enrollment rate is non-negative with at least one positive rate.
#'    \item Convert rates to step function.
#'    \item Add times where rates change to enrollment rates.
#'    \item Make a tibble of the input time points x, duration, enrollment rates at points, and
#'    expected accrual.
#'    \item Extract the expected cumulative or survival enrollment.
#'    \item Return \code{eAccrual}
#'   }
#' }
#' \if{html}{The contents of this section are shown in PDF user manual only.}
#'
#' @return A vector with expected cumulative enrollment for the specified `times`.
#' @examples
#' # Example: default
#' eAccrual_()
#' @export
#'
eAccrual_ <- function(x = 0:24,
                      enrollRates = tibble::tibble(
                        duration = c(3, 3, 18),
                        rate = c(5, 10, 20)
                      )) {
  # check input value
  # check input enrollment rate assumptions
  if (!is.numeric(x)) {
    stop("gsDesign2: x in `eAccrual()` must be a strictly increasing non-negative numeric vector")
  }
  if (!min(x) >= 0) {
    stop("gsDesign2: x in `eAccrual()` must be a strictly increasing non-negative numeric vector")
  }
  if (!min(dplyr::lead(x, default = max(x) + 1) - x) > 0) {
    stop("gsDesign2: x in `eAccrual()` must be a strictly increasing non-negative numeric vector")
  }

  # check enrollment rate assumptions
  if (!is.data.frame(enrollRates)) {
    stop("gsDesign2: enrollRates in `eAccrual()` must be a data frame")
  }
  if (!max(names(enrollRates) == "duration") == 1) {
    stop("gsDesign2: enrollRates in `eAccrual()` column names must contain duration")
  }
  if (!max(names(enrollRates) == "rate") == 1) {
    stop("gsDesign2: enrollRates in `eAccrual()` column names must contain rate")
  }

  # test that enrollment rates are non-negative with at least one positive
  if (!min(enrollRates$rate) >= 0) {
    stop("gsDesign2: enrollRates in `eAccrual()` must be non-negative with at least one positive rate")
  }
  if (!max(enrollRates$rate) > 0) {
    stop("gsDesign2: enrollRates in `eAccrual()` must be non-negative with at least one positive rate")
  }


  # convert rates to step function
  ratefn <- stepfun(
    x = cumsum(enrollRates$duration),
    y = c(enrollRates$rate, 0),
    right = TRUE
  )
  # add times where rates change to enrollRates
  xvals <- sort(unique(c(x, cumsum(enrollRates$duration))))
  # make a tibble
  xx <- tibble::tibble(
    x = xvals,
    duration = xvals - dplyr::lag(xvals, default = 0),
    rate = ratefn(xvals), # enrollment rates at points (right continuous)
    eAccrual = cumsum(rate * duration) # expected accrual
  )
  # return survival or CDF
  ind <- !is.na(match(xx$x, x))
  return(as.numeric(xx$eAccrual[ind]))
}