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The Miller–Rabin primality test or Rabin–Miller primality test is a primality test: an algorithm which determines whether a given number is prime or not.

The algorithm, as modified by Michael O. Rabin to avoid the generalized Riemann hypothesis, is a probabilistic algorithm.

The pseudocode, from Wikipedia is:

Input: n > 2, an odd integer to be tested for primality;
       k, a parameter that determines the accuracy of the test
Output: composite if n is composite, otherwise probably prime
write n − 1 as 2s·d with d odd by factoring powers of 2 from n − 1
LOOP: repeat k times:
   pick a randomly in the range [2, n − 1]
   x ← ad mod n
   if x = 1 or x = n − 1 then do next LOOP
   repeat s − 1 times:
      x ← x2 mod n
      if x = 1 then return composite
      if x = n − 1 then do next LOOP
   return composite
return probably prime

 The nature of the test involves big numbers, so the use of "big numbers" libraries (or similar features of the language of your choice) are suggested, but not mandatory.
 Deterministic variants of the test exist and can be implemented as extra (not mandatory to complete the task)


input "Input a number:";n
input "Input test:";k
 
test =  millerRabin(n,k)
if test = 0 then 
  print "Probably Prime"
 else
  print "Composite"
end if
wait
 
' ----------------------------------------
' Returns 
'  Composite     = 1
'  Probably Prime = 0
' ----------------------------------------
 FUNCTION millerRabin(n, k)
  if n = 2 then
    millerRabin = 0 'probablyPrime
    goto [funEnd]
  end if
 
  if n mod 2 = 0 or n < 2 then
    millerRabin = 1 'composite
    goto [funEnd]
  end if
 
d = n - 1
while d mod 2 = 0
  d = d / 2
  s = s + 1
wend
 
while k > 0
  k = k - 1
  base = 2 + int(rnd(1)*(n-3))
  x = (base^d) mod n
  if x <> 1 and x <> n-1 then
    for r=1 To s-1
      x =(x * x) mod n
      if x=1 then
       millerRabin = 1 ' composite 
       goto [funEnd]
      end if
      if x = n-1 then exit for
    next r
 
    if x<>n-1 then
      millerRabin =  1 ' composite 
      goto [funEnd]
    end if
  end if
wend
[funEnd]
END FUNCTION