fermat primality test

fermat-prime-test.hs

@@ -0,0 +1,15 @@
+import System.Random (randomRIO)
+import ModularArithmeticUtils (modExp)
+
+fermatPrimeTest :: Integer -> Integer -> IO Bool
+fermatPrimeTest n k
+  | n <= 3    = return (n == 2 || n == 3)
+  | even n    = return False
+  | otherwise = go k
+  where
+    go 0 = return True
+    go i = do
+      a <- randomRIO (2, n - 2)
+      if modExp a (n - 1) n /= 1
+        then return False
+        else go (i - 1)