net_sensspec.R

library(Rfast)
library(utils)


prob_test_outcome_combination <- function(prob,
                                          n_choose) {
  
  p_comb <- combn(x = prob, m = n_choose)
  n_comb <- combn(x = 1 - prob, m = length(prob) - n_choose)
  
  p_probs <- colprods(p_comb)
  n_probs <- colprods(n_comb)
  
  return(p_probs %*% rev(n_probs))
}

net_sensspec <- function(pos_threshold,
                         sens,
                         spec) {
  
  if (length(sens) != length(spec))
    stop("Length of sens and spec must be the same.")
  
  N <- length(sens)
  net_sens <- net_spec <- 0
  
  for (j in seq(0, N)) {
    
    if (j >= pos_threshold) {
      
      net_sens <- net_sens + prob_test_outcome_combination(sens, j)
    }
    if (j > N - pos_threshold) {
      
      net_spec <- net_spec + prob_test_outcome_combination(spec, j)
    }  
  }
  
  list(net_sens = net_sens,
       net_spec = net_spec)
}

net_sensspec(pos_threshold = 2,
             sens = c(0.1,0.2,0.3,0.4),
             spec = c(0.1,0.2,0.3,0.4)) 

net_sensspec(pos_threshold = 2,
             sens = c(0.1,0.2,0.3),
             spec = c(0.1,0.2,0.3)) 

net_sensspec(pos_threshold = 2,
             sens = c(0.1,0.2),
             spec = c(0.1,0.2))