R/decp.R

In ecpdist: Extended Chen-Poisson Lifetime Distribution

#### Density function ####

#' Density function
#'
#' @description
#' Compute the density function of the extended Chen-Poisson (ecp) distribution.
#'
#' @param x vector of quantiles.
#'
#' @param lambda,gamma  parameter values > 0.
#'
#' @param phi parameter value != 0.
#'
#' @param log logical value
#'
#' @return Numeric value of the density function.
#'
#' @return If log = TRUE, numeric value of the logarithm of the function.
#'
#' @examples
#' decp(2, 1, 1, 1, log = FALSE) # density function
#'
#' @export
decp <- function(x, lambda, gamma, phi, log = FALSE) {

  # Check if arguments are numeric
  if (!all(sapply(list(x, lambda, gamma, phi), is.numeric))) {
    stop("non-numeric argument")
  }

  # Check for invalid arguments
  if (any(x < 0) || min(lambda <= 0) || min(gamma <= 0) ||
        phi == 0) {
    stop("Invalid arguments")
  }

  # Compute density function

  pdf <- (lambda * gamma * phi * x^(gamma - 1) *
            exp(-phi * exp(lambda * (1 - exp(x^gamma))) +
                  lambda * (1 - exp(x^gamma)) + x^gamma)) / (1 - exp(-phi))

  # Convert density function to log scale if log is TRUE

  if (log)
    pdf <- log(pdf)

  return(pdf)

}