{- Fibonacci functions in Elm
   CSci 450/503, Fall 2016
   H. Conrad Cunningham
   2016-11-16
-}

import Html exposing (..)

-- Inefficient double backward recursive version
fib : Int -> Int
fib n =
   if n <= 0 then
      0
   else if n == 1 then
      1
   else -- double, backward recursion
      fib (n-1) + fib (n-2)

-- Tail recursive version
fib2 : Int -> Int
fib2 n =
   let
      fibIter : Int -> Int -> Int -> Int
      fibIter n p q = 
         if n <= 0 then
            p
         else  -- tail recursion
            fibIter (n-1) q (p+q)
   in
      fibIter n 0 1

-- Simple Display Functions

newline = br [] []

displayLines : List String -> List (Html msg)
displayLines ss = 
   case ss of
      []      -> []
      x :: xs -> text x :: newline :: displayLines xs

display : List String -> Html msg
display ls = p [] (displayLines ls)

-- Some testing

main =
  display 
     [ "fib   5 = " ++ toString (fib   5)
     , "fib  13 = " ++ toString (fib  13)
     , "fib2  5 = " ++ toString (fib2  5)
     , "fib2 13 = " ++ toString (fib2 13)
     , "Done!"
     ]