Birthday_paradox.fold

--
-- Birthday paradox
--
-- Source: https://en.wikipedia.org/wiki/OCaml#Birthday_paradox
--
-- The following program calculates the smallest number of people in a room for
-- whom the probability of completely unique birthdays is less than 50% (the
-- so-called birthday paradox, where for 1 person the probability is 365/365
-- (or 100%), for 2 it is 364/365, for 3 it is 364/365 × 363/365, etc.)
-- (answer = 23).


val year_size = 365.0

fun birthday_paradox prob people =
  let prob' =
    (year_size - float people) / year_size * prob
  in
    if prob' < 0.5 then
      print ("answer = %d" % (people + 1))
    else
      birthday_paradox prob' (people + 1)

fun main () =
  birthday_paradox 1.0 1