Add Collatz exploration

src/3n+1/Main.hs

@@ -0,0 +1,47 @@
+import           Control.Monad ()
+import           Debug.Trace   (trace)
+
+f' :: Integer -> Integer
+f' n = f'' 0 n
+  where
+    f'' :: Integer -> Integer -> Integer
+    f'' i n'
+      | n' == 1 = t' i
+      | even n' = t $ f'' (i + 1) (n' `div` 2)
+      | odd  n' = t $ f'' (i + 1) (3*n' + 1)
+      where t   = trace ("i: " ++ show i ++ ", number: " ++ show n')
+            t'  = t'' $ trace ("END: " ++ show n ++ ", length: " <> show i) $ t''
+            t'' = trace "\n#######################################################\n"
+
+f :: Integer -> Integer
+f n = s 0 n
+  where
+    s :: Integer -> Integer -> Integer
+    s i n
+      | n == 1 = i
+      | even n = s i' (n `div` 2)
+      | odd  n = s i' (3*n + 1)
+      where i' = i + 1
+
+main :: IO ()
+main = print $ map f [2^361..]
+
+-- collatz collect
+-- generate the collatz sequence and return it
+
+cc :: Integer -> [Integer]
+cc n = cc' [] n
+  where
+    cc' :: [Integer] -> Integer -> [Integer]
+    cc' acc n
+      | n == 1 = 1:acc
+      | even n = cc' acc' (n `div` 2)
+      | odd  n = cc' acc' (3*n + 1)
+      where acc' = acc <> [n]
+
+primes :: [Integer]
+primes = sieve [2..]
+  where
+    sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0]
+
+isPrime k = (k > 1) && null [ x | x <- [2..k - 1], k `mod` x == 0]