```{-# LANGUAGE NoImplicitPrelude #-}
{- |
Copyright    :   (c) Henning Thielemann 2007
Maintainer   :   numericprelude@henning-thielemann.de
Stability    :   provisional
Portability  :   portable

Lazy Peano numbers represent natural numbers inclusive infinity.
Since they are lazily evaluated,
they are optimally for use as number type of 'Data.List.genericLength' et.al.
-}
module Number.Peano where

import qualified Algebra.PrincipalIdealDomain as PID
import qualified Algebra.Units                as Units
import qualified Algebra.RealIntegral         as RealIntegral
import qualified Algebra.IntegralDomain       as Integral
import qualified Algebra.Absolute             as Absolute
import qualified Algebra.Ring                 as Ring
import qualified Algebra.ZeroTestable         as ZeroTestable
import qualified Algebra.Indexable            as Indexable
import qualified Algebra.Monoid               as Monoid

import qualified Algebra.ToInteger            as ToInteger
import qualified Algebra.ToRational           as ToRational
import qualified Algebra.NonNegative          as NonNeg

import qualified Algebra.EqualityDecision as EqDec
import qualified Algebra.OrderDecision    as OrdDec

import Data.Maybe (catMaybes, )
import Data.Array(Ix(..))

import qualified Prelude     as P98
{-
import qualified NumericPrelude.Base as P
import qualified NumericPrelude.Numeric as NP
-}
import Data.Tuple.HT (mapFst, )

import NumericPrelude.Base
import NumericPrelude.Numeric

data T = Zero
| Succ T

infinity :: T
infinity = Succ infinity

err :: String -> String -> a
err func msg = error ("Number.Peano."++func++": "++msg)

instance ZeroTestable.C T where
isZero Zero     = True
isZero (Succ _) = False

add :: T -> T -> T

sub :: T -> T -> T
sub x y =
let (sign,z) = subNeg y x
in  if sign
then err "sub" "negative difference"
else z

subNeg :: T -> T -> (Bool, T)
subNeg Zero y = (False, y)
subNeg x Zero = (True,  x)
subNeg (Succ x) (Succ y) = subNeg x y

mul :: T -> T -> T
mul Zero _ = Zero
mul _ Zero = Zero
mul (Succ x) y = add y (mul x y)

fromPosEnum :: (ZeroTestable.C a, Enum a) => a -> T
fromPosEnum n =
if isZero n
then Zero
else Succ (fromPosEnum (pred n))

toPosEnum :: (Additive.C a, Enum a) => T -> a
toPosEnum Zero = zero
toPosEnum (Succ x) = succ (toPosEnum x)

zero = Zero
(-) = sub
negate Zero     = Zero
negate (Succ _) = err "negate" "cannot negate positive number"

instance Ring.C T where
one = Succ Zero
(*) = mul
fromInteger n =
if n<0
then err "fromInteger" "Peano numbers are always non-negative"
else fromPosEnum n

instance Enum T where
pred Zero = err "pred" "Zero has no predecessor"
pred (Succ x) = x
succ = Succ
toEnum n =
if n<0
then err "toEnum" "Peano numbers are always non-negative"
else fromPosEnum n
enumFrom x = iterate Succ x
enumFromThen x y =
let (sign,d) = subNeg x y
in  if sign
then iterate (sub d) x
{-
enumFromTo =
enumFromThenTo =
-}

{- |
If all values are completely defined,
then it holds

> if b then x else y == ifLazy b x y

However if @b@ is undefined,
then it is at least known that the result is larger than @min x y@.
-}
ifLazy :: Bool -> T -> T -> T
ifLazy b (Succ x) (Succ y) = Succ (ifLazy b x y)
ifLazy b x y = if b then x else y

instance EqDec.C T where
(==?) x y = ifLazy (x==y)

instance OrdDec.C T where
(<=?) x y le gt = ifLazy (x<=y) le gt

{-
The default instance is good for compare,
but fails for min and max.
-}
instance Ord T where
compare (Succ x) (Succ y) = compare x y
compare Zero     (Succ _) = LT
compare (Succ _) Zero     = GT
compare Zero     Zero     = EQ

min (Succ x) (Succ y) = Succ (min x y)
min _        _        = Zero

max (Succ x) (Succ y) = Succ (max x y)
max Zero     y        = y
max x        Zero     = x

{-
This special implementation works also for undefined < Zero.
Thanks to Peter Divianszky for the hint.
-}
_      < Zero   = False
Zero   < _      = True
Succ n < Succ m = n < m

x > y  = y < x

x <= y = not (y < x)

x >= y = not (x < y)

{- | cf.
To how to find the shortest list in a list of lists efficiently,
this means, also in the presence of infinite lists.
-}
argMinFull :: (T,a) -> (T,a) -> (T,a)
argMinFull (x0,xv) (y0,yv) =
let recourse (Succ x) (Succ y) =
let (z,zv) = recourse x y
in  (Succ z, zv)
recourse Zero _ = (Zero,xv)
recourse _    _ = (Zero,yv)
in  recourse x0 y0

{- |
On equality the first operand is returned.
-}
argMin :: (T,a) -> (T,a) -> a
argMin x y = snd \$ argMinFull x y

argMinimum :: [(T,a)] -> a
argMinimum = snd . foldl1 argMinFull

argMaxFull :: (T,a) -> (T,a) -> (T,a)
argMaxFull (x0,xv) (y0,yv) =
let recourse (Succ x) (Succ y) =
let (z,zv) = recourse x y
in  (Succ z, zv)
recourse x Zero = (x,xv)
recourse _ y    = (y,yv)
in  recourse x0 y0

{- |
On equality the first operand is returned.
-}
argMax :: (T,a) -> (T,a) -> a
argMax x y = snd \$ argMaxFull x y

argMaximum :: [(T,a)] -> a
argMaximum = snd . foldl1 argMaxFull

-- isAscending - naive implementations

{- |
@x0 <= x1 && x1 <= x2 ... @
for possibly infinite numbers in finite lists.
-}
isAscendingFiniteList :: [T] -> Bool
isAscendingFiniteList [] = True
isAscendingFiniteList (x:xs) =
let decrement (Succ y) = Just y
decrement _ = Nothing
in  case x of
Zero -> isAscendingFiniteList xs
Succ xd ->
case mapM decrement xs of
Nothing -> False
Just xsd -> isAscendingFiniteList (xd : xsd)

isAscendingFiniteNumbers :: [T] -> Bool
isAscendingFiniteNumbers = and . mapAdjacent (<=)

-- isAscending - sophisticated implementations - explicit

toListMaybe :: a -> T -> [Maybe a]
toListMaybe a =
let recourse Zero     = [Just a]
recourse (Succ x) = Nothing : recourse x
in  recourse

{- |
In @glue x y == (z,(b,r))@
@z@ represents @min x y@,
@r@ represents @max x y - min x y@,
and @x<=y  ==  b@.

Cf. Numeric.NonNegative.Chunky
-}
glue :: T -> T -> (T, (Bool, T))
glue Zero ys = (Zero, (True, ys))
glue xs Zero = (Zero, (False, xs))
glue (Succ xs) (Succ ys) =
mapFst Succ \$ glue xs ys

{-
Implementation notes:
We check all pairs of adjacent numbers for correct order.
We obtain a set of booleans, which must all be True.
The order of checking these booleans is crucial.
Pairs of numbers that are infinitely big or infinitely far in the list
must be checked \"last\".
Thus we order the booleans according to their computation costs
(list position + magnitude of number)
using 'shearTranspose'.
-}
isAscending :: [T] -> Bool
isAscending =
and . catMaybes . concat .
shearTranspose .
let (costs0,(le,_)) = glue x y
in  toListMaybe le costs0)

-- isAscending - use a cost measuring data type (could generalized to a monad, when considered as Writer monad, see htam and unique-logic packages

-- following an idea of vixy http://moonpatio.com:8080/fastcgi/hpaste.fcgi/view?id=562

data Valuable a = Valuable {costs :: T, value :: a}
deriving (Show, Eq, Ord)

increaseCosts :: T -> Valuable a -> Valuable a
increaseCosts inc ~(Valuable c x) = Valuable (inc+c) x

{- |
Compute '(&&)' with minimal costs.
-}
infixr 3 &&~
(&&~) :: Valuable Bool -> Valuable Bool -> Valuable Bool
(&&~) (Valuable xc xb) (Valuable yc yb) =
let (minc,~(le,difc)) = glue xc yc
(bCheap,bExpensive) =
if le
then (xb,yb)
else (yb,xb)
in  increaseCosts minc \$
uncurry Valuable \$
if bCheap
then (difc, bExpensive)
else (Zero, False)

andW :: [Valuable Bool] -> Valuable Bool
andW =
foldr
(\b acc -> b &&~ increaseCosts one acc)
(Valuable Zero True)

leW :: T -> T -> Valuable Bool
leW x y =
let (minc,~(le,_difc)) = glue x y
in  Valuable minc le

isAscendingW :: [T] -> Valuable Bool
isAscendingW =

{-
test with

*Number.Peano> isAscendingW [0,infinity,infinity,5]
False
-}

-- instances

instance Absolute.C T where
signum Zero     = zero
signum (Succ _) = one
abs             = id

instance ToInteger.C T where
toInteger = toPosEnum

instance ToRational.C T where
toRational = toRational . toInteger

instance RealIntegral.C T where
quot = div
rem  = mod
quotRem = divMod

instance Integral.C T where
div x y = fst (divMod x y)
mod x y = snd (divMod x y)
divMod x y =
let (isNeg,d) = subNeg y x
in  if isNeg
then (zero,x)
else let (q,r) = divMod d y in (succ q,r)

instance Monoid.C T where
idt = zero

instance NonNeg.C T where
split = glue

instance Ix T where
range = uncurry enumFromTo
index (lower,_) i =
let (sign,offset) = subNeg lower i
in  if sign
then err "index" "index out of range"
else toPosEnum offset
inRange (lower,upper) i =
isAscending [lower, i, upper]
rangeSize (lower,upper) =
toPosEnum (sub lower (succ upper))

instance Indexable.C T where
compare = Indexable.ordCompare

instance Units.C T where
isUnit x  =  x == one
stdAssociate  =  id
stdUnit    _ = one
stdUnitInv _ = one

instance PID.C T where
gcd = PID.euclid mod
extendedGCD = PID.extendedEuclid divMod

instance Bounded T where
minBound = Zero
maxBound = infinity

legacyInstance :: a
legacyInstance =
error "legacy Ring.C instance for simple input of numeric literals"

instance P98.Num T where
fromInteger = Ring.fromInteger
negate = Additive.negate -- for unary minus
(-) = sub
(*) = mul
signum = legacyInstance
abs = legacyInstance

-- for use with genericLength et.al.
instance P98.Real T where
toRational = P98.toRational . toInteger

instance P98.Integral T where
rem  = div
quot = mod
quotRem = divMod
div x y = fst (divMod x y)
mod x y = snd (divMod x y)
divMod x y =
let (sign,d) = subNeg y x
in  if sign
then (0,x)
else let (q,r) = divMod d y in (succ q,r)
toInteger = toPosEnum
```