log likelihood of beta nbd model

beta_nbd_ll.R

beta.nbd.ll <- function(x, n_x, r, alpha, a, b)
{
  # params:
  #   x: vector of counts
  #   n_x: number of observations for each count
  #   r, alpha: nbd params
  #   a, b: beta params
  # output:
  #   the log-likelihood of the model
  probs <- sapply(x, function(y) beta.nbd.density(y, r, alpha, a, b))
  lls   <- n_x * log(probs)
  ll    <- sum(lls)
  return(ll)
}