September 2017 Update

Task/Fibonacci-sequence/Haskell/fibonacci-sequence-12.hs

@@ -1,20 +1,2 @@
-fibsteps (a,b) n
-    | n <= 0    = (a,b)
-    | otherwise = fibsteps (b, a+b) (n-1)
-
-fibnums :: [Integer]
-fibnums = map fst $ iterate (`fibsteps` 1) (0,1)
-
-fibN2 :: Integer -> (Integer, Integer)
-fibN2 m | m < 10 = fibsteps (0,1) m
-fibN2 m          = fibN2_next (n,r) (fibN2 n)
-                     where (n,r) = quotRem m 3
-
-fibN2_next (n,r) (f,g) | r==0 = (a,b)    -- 3n  ,3n+1
-                       | r==1 = (b,c)    -- 3n+1,3n+2
-                       | r==2 = (c,d)    -- 3n+2,3n+3   (*)
-  where
-      a = ( 5*f^3 + if even n then 3*f else (- 3*f) ) -- 3n
-      b = (   g^3 + 3 * g * f^2 - f^3               ) -- 3n+1
-      c = (   g^3 + 3 * g^2 * f + f^3               ) -- 3n+2
-      d = ( 5*g^3 + if even n then (- 3*g) else 3*g ) -- 3(n+1)   (*)
+ *Main> (length &&& take 20) . show . fst $ fibN2 (10^6)
+(208988,"19532821287077577316")