-- | The Roots.Fast module contains faster implementations of the
--   'Roots.Simple' algorithms. Generally, we will pass precomputed
--   values to the next iteration of a function rather than passing
--   the function and the points at which to (re)evaluate it.

module Roots.Fast
where

import Data.List (find)

import Vector


has_root :: (Fractional a, Ord a, Ord b, Num b)
         => (a -> b) -- ^ The function @f@
         -> a       -- ^ The \"left\" endpoint, @a@
         -> a       -- ^ The \"right\" endpoint, @b@
         -> Maybe a -- ^ The size of the smallest subinterval
                   --   we'll examine, @epsilon@
         -> Maybe b -- ^ Precoumpted f(a)
         -> Maybe b -- ^ Precoumpted f(b)
         -> Bool
has_root f a b epsilon f_of_a f_of_b =
  if not ((signum (f_of_a')) * (signum (f_of_b')) == 1) then
    -- We don't care about epsilon here, there's definitely a root!
    True
  else
    if (b - a) <= epsilon' then
      -- Give up, return false.
      False
    else
      -- If either [a,c] or [c,b] have roots, we do too.
      (has_root f a c (Just epsilon') (Just f_of_a') Nothing) ||
        (has_root f c b (Just epsilon') Nothing (Just f_of_b'))
  where
    -- If the size of the smallest subinterval is not specified,
    -- assume we just want to check once on all of [a,b].
    epsilon' = case epsilon of
                 Nothing -> (b-a)
                 Just eps -> eps

    -- Compute f(a) and f(b) only if needed.
    f_of_a'  = case f_of_a of
                 Nothing -> f a
                 Just v -> v

    f_of_b'  = case f_of_b of
                 Nothing -> f b
                 Just v -> v

    c = (a + b)/2



bisect :: (Fractional a, Ord a, Num b, Ord b)
       => (a -> b) -- ^ The function @f@ whose root we seek
       -> a       -- ^ The \"left\" endpoint of the interval, @a@
       -> a       -- ^ The \"right\" endpoint of the interval, @b@
       -> a       -- ^ The tolerance, @epsilon@
       -> Maybe b -- ^ Precomputed f(a)
       -> Maybe b -- ^ Precomputed f(b)
       -> Maybe a
bisect f a b epsilon f_of_a f_of_b
  -- We pass @epsilon@ to the 'has_root' function because if we want a
  -- result within epsilon of the true root, we need to know that
  -- there *is* a root within an interval of length epsilon.
  | not (has_root f a b (Just epsilon) (Just f_of_a') (Just f_of_b')) = Nothing
  | f_of_a' == 0 = Just a
  | f_of_b' == 0 = Just b
  | (b - c) < epsilon = Just c
  | otherwise =
      -- Use a 'prime' just for consistency.
      let f_of_c' = f c in
        if (has_root f a c (Just epsilon) (Just f_of_a') (Just f_of_c'))
        then bisect f a c epsilon (Just f_of_a') (Just f_of_c')
        else bisect f c b epsilon (Just f_of_c') (Just f_of_b')
  where
    -- Compute f(a) and f(b) only if needed.
    f_of_a'  = case f_of_a of
                 Nothing -> f a
                 Just v -> v

    f_of_b'  = case f_of_b of
                 Nothing -> f b
                 Just v -> v

    c = (a + b) / 2



-- | Iterate the function @f@ with the initial guess @x0@ in hopes of
--   finding a fixed point.
fixed_point_iterations :: (a -> a) -- ^ The function @f@ to iterate.
                       -> a       -- ^ The initial value @x0@.
                       -> [a]     -- ^ The resulting sequence of x_{n}.
fixed_point_iterations f x0 =
  iterate f x0


-- | Find a fixed point of the function @f@ with the search starting
--   at x0. This will find the first element in the chain f(x0),
--   f(f(x0)),... such that the magnitude of the difference between it
--   and the next element is less than epsilon.
--
--   We also return the number of iterations required.
--
fixed_point_with_iterations :: (Vector a, RealFrac b)
                            => (a -> a)  -- ^ The function @f@ to iterate.
                            -> b        -- ^ The tolerance, @epsilon@.
                            -> a        -- ^ The initial value @x0@.
                            -> (Int, a) -- ^ The (iterations, fixed point) pair
fixed_point_with_iterations f epsilon x0 =
  (fst winning_pair)
  where
    xn = fixed_point_iterations f x0
    xn_plus_one = tail xn

    abs_diff v w = norm_2 (v - w)

    -- The nth entry in this list is the absolute value of x_{n} -
    -- x_{n+1}.
    differences = zipWith abs_diff xn xn_plus_one

    -- This produces the list [(n, xn)] so that we can determine
    -- the number of iterations required.
    numbered_xn = zip [0..] xn

    -- A list of pairs, (xn, |x_{n} - x_{n+1}|).
    pairs = zip numbered_xn differences

    -- The pair (xn, |x_{n} - x_{n+1}|) with
    -- |x_{n} - x_{n+1}| < epsilon. The pattern match on 'Just' is
    -- "safe" since the list is infinite. We'll succeed or loop
    -- forever.
    Just winning_pair = find (\(_, diff) -> diff < epsilon) pairs