Data commit

Task/Long-primes/Haskell/long-primes.hs

@@ -0,0 +1,38 @@
+import Data.List (elemIndex)
+
+longPrimesUpTo :: Int -> [Int]
+longPrimesUpTo n =
+  filter isLongPrime $
+    takeWhile (< n) primes
+  where
+    sieve (p : xs) = p : sieve [x | x <- xs, x `mod` p /= 0]
+    primes = sieve [2 ..]
+    isLongPrime n = found
+      where
+        cycles = take n (iterate ((`mod` n) . (10 *)) 1)
+        index = elemIndex (head cycles) $ tail cycles
+        found = case index of
+          (Just i) -> n - i == 2
+          _ -> False
+
+display :: Int -> IO ()
+display n =
+  if n <= 64000
+    then do
+      putStrLn
+        ( show n <> " is "
+            <> show (length $ longPrimesUpTo n)
+        )
+      display (n * 2)
+    else pure ()
+
+main :: IO ()
+main = do
+  let fiveHundred = longPrimesUpTo 500
+  putStrLn
+    ( "The long primes up to 35 are:\n"
+        <> show fiveHundred
+        <> "\n"
+    )
+  putStrLn ("500 is " <> show (length fiveHundred))
+  display 1000