Demo.hs

demo.hs

-- bar a b c = c + b - 2 * a
-- foo ... = bar ...
-- foo ... = ... bar ...

fib :: Integer -> Integer
fib n = fn n (0, 1)
  where
    fn 0 (a, b) = a
    fn n (a, b) = fn (n - 1) (a + b, a)

-- [1,2,3,3,2,-1,-5,-10,-13,-13,-6,7,27,46,59,51,18,-49,-133,-218,-253]
seqA :: Integer -> Integer
seqA n = let
  curr a b c = a + b - 2 * c
  fn 0 (a, b, c) = a
  fn 1 (a, b, c) = b
  fn 2 (a, b, c) = c
  fn n' (a, b, c) = fn (n' - 1) (a + b - 2 * c, a, b)
  in fn n (3, 2, 1)


seqA' :: Integer -> Integer
seqA' n = let
  f' 0 = 1
  f' 1 = 2
  f' 2 = 3
  f' n = f' (n - 1) + f' (n - 2) - 2 * f' (n - 3)
  in f' n

-- a0 = 1; a1 = 2;
-- an = 3 * an-1 – 2 * an-2 + 1 при n = 2, 3,...
seqB :: Integer -> Integer
seqB n = let
    fn 0 _ p = p
    fn 1 c _ = c
    fn n c p = fn (n - 1) q c
                 where q = 3 * c - 2 * p + 1

  in fn n 2 1