```-- |
-- Module:      Math.NumberTheory.MoebiusInversion
-- Copyright:   (c) 2012 Daniel Fischer
-- Licence:     MIT
--
-- Generalised Möbius inversion

{-# LANGUAGE BangPatterns        #-}
{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Math.NumberTheory.MoebiusInversion
( generalInversion
, totientSum
) where

import Data.Proxy
import qualified Data.Vector.Generic as G
import qualified Data.Vector.Generic.Mutable as MG

import Math.NumberTheory.Roots
import Math.NumberTheory.Utils.FromIntegral

-- | @totientSum n@ is, for @n > 0@, the sum of @[totient k | k <- [1 .. n]]@,
--   computed via generalised Möbius inversion.
--   See <http://mathworld.wolfram.com/TotientSummatoryFunction.html> for the
--   formula used for @totientSum@.
--
-- >>> import Data.Proxy
-- >>> totientSum (Proxy :: Proxy Data.Vector.Unboxed.Vector) 100 :: Int
-- 3044
-- >>> totientSum (Proxy :: Proxy Data.Vector.Vector) 100 :: Integer
-- 3044
totientSum
:: (Integral t, G.Vector v t)
=> Proxy v
-> Word
-> t
totientSum _ 0 = 0
totientSum proxy n = generalInversion proxy (triangle . fromIntegral) n
where
triangle k = (k * (k + 1)) `quot` 2

-- | @generalInversion g n@ evaluates the generalised Möbius inversion of @g@
--   at the argument @n@.
--
--   The generalised Möbius inversion implemented here allows an efficient
--   calculation of isolated values of the function @f : N -> Z@ if the function
--   @g@ defined by
--
-- >
-- > g n = sum [f (n `quot` k) | k <- [1 .. n]]
-- >
--
--   can be cheaply computed. By the generalised Möbius inversion formula, then
--
-- >
-- > f n = sum [moebius k * g (n `quot` k) | k <- [1 .. n]]
-- >
--
--   which allows the computation in /O/(n) steps, if the values of the
--   Möbius function are known. A slightly different formula, used here,
--   does not need the values of the Möbius function and allows the
--   computation in /O/(n^0.75) steps, using /O/(n^0.5) memory.
--
--   An example of a pair of such functions where the inversion allows a
--   more efficient computation than the direct approach is
--
-- >
-- > f n = number of reduced proper fractions with denominator <= n
-- >
-- > g n = number of proper fractions with denominator <= n
-- >
--
--   (a /proper fraction/ is a fraction @0 < n/d < 1@). Then @f n@ is the
--   cardinality of the Farey sequence of order @n@ (minus 1 or 2 if 0 and/or
--   1 are included in the Farey sequence), or the sum of the totients of
--   the numbers @2 <= k <= n@. @f n@ is not easily directly computable,
--   but then @g n = n*(n-1)/2@ is very easy to compute, and hence the inversion
--   gives an efficient method of computing @f n@.
--
--   Since the function arguments are used as array indices, the domain of
--   @f@ is restricted to 'Int'.
--
--   The value @f n@ is then computed by @generalInversion g n@. Note that when
--   many values of @f@ are needed, there are far more efficient methods, this
--   method is only appropriate to compute isolated values of @f@.
generalInversion
:: (Num t, G.Vector v t)
=> Proxy v
-> (Word -> t)
-> Word
-> t
generalInversion proxy fun n = case n of
0 ->error "Möbius inversion only defined on positive domain"
1 -> fun 1
2 -> fun 2 - fun 1
3 -> fun 3 - 2*fun 1
_ -> runST (fastInvertST proxy (fun . intToWord) (wordToInt n))

fastInvertST
:: forall s t v.
(Num t, G.Vector v t)
=> Proxy v
-> (Int -> t)
-> Int
-> ST s t
fastInvertST _ fun n = do
let !k0 = integerSquareRoot (n `quot` 2)
!mk0 = n `quot` (2*k0+1)
kmax a m = (a `quot` m - 1) `quot` 2

small <- MG.unsafeNew (mk0 + 1) :: ST s (G.Mutable v s t)
MG.unsafeWrite small 0 0
MG.unsafeWrite small 1 \$! (fun 1)
when (mk0 >= 2) \$
MG.unsafeWrite small 2 \$! (fun 2 - fun 1)

let calcit :: Int -> Int -> Int -> ST s (Int, Int)
calcit switch change i
| mk0 < i   = return (switch,change)
| i == change = calcit (switch+1) (change + 4*switch+6) i
| otherwise = do
let mloop !acc k !m
| k < switch    = kloop acc k
| otherwise     = do
let nxtk = kmax i (m+1)
mloop (acc - fromIntegral (k-nxtk)*val) nxtk (m+1)
kloop !acc k
| k == 0    = do
MG.unsafeWrite small i \$! acc
calcit switch change (i+1)
| otherwise = do
val <- MG.unsafeRead small (i `quot` (2*k+1))
kloop (acc-val) (k-1)
mloop (fun i - fun (i `quot` 2)) ((i-1) `quot` 2) 1

(sw, ch) <- calcit 1 8 3
large <- MG.unsafeNew k0 :: ST s (G.Mutable v s t)

let calcbig :: Int -> Int -> Int -> ST s (G.Mutable v s t)
calcbig switch change j
| j == 0    = return large
| (2*j-1)*change <= n   = calcbig (switch+1) (change + 4*switch+6) j
| otherwise = do
let i = n `quot` (2*j-1)
mloop !acc k m
| k < switch    = kloop acc k
| otherwise     = do
let nxtk = kmax i (m+1)
mloop (acc - fromIntegral (k-nxtk)*val) nxtk (m+1)
kloop !acc k
| k == 0    = do
MG.unsafeWrite large (j-1) \$! acc
calcbig switch change (j-1)
| otherwise = do
let m = i `quot` (2*k+1)
val <- if m <= mk0