module Data.Group (
    Group(..)
  , Loop(..)
  , Quasigroup(..)
  , Monoid(..)
  , mreplicate
  , Semigroup(..)
  , Magma(..)
) where

import safe Data.Magma
import safe Data.Semigroup
import safe Numeric.Natural

--  x << y = x <> inv y
--  inv x = mempty << x
class (Loop a, Monoid a) => Group a where

  inv :: a -> a
  inv x = mempty << x

  greplicate :: Integer -> a -> a
  greplicate n a     
      | n == 0 = mempty
      | n > 0 = mreplicate (fromInteger n) a
      | otherwise = mreplicate (fromInteger $ abs n) (inv a)

instance (Semigroup a, Quasigroup (Maybe a)) => Group (Maybe a) where

-- | A generalization of 'Data.List.replicate' to an arbitrary 'Monoid'. 
--
-- Adapted from <http://augustss.blogspot.com/2008/07/lost-and-found-if-i-write-108-in.html>.
--
mreplicate :: Monoid a => Natural -> a -> a
mreplicate n a
    | n == 0 = mempty
    | otherwise = f a n
    where
        f x y 
            | even y = f (x <> x) (y `quot` 2)
            | y == 1 = x
            | otherwise = g (x <> x) ((y - 1) `quot` 2) x
        g x y z 
            | even y = g (x <> x) (y `quot` 2) z
            | y == 1 = x <> z
            | otherwise = g (x <> x) ((y - 1) `quot` 2) (x <> z)
{-# INLINE mreplicate #-}



{-
Every group is a loop, because a ∗ x = b if and only if x = a−1 ∗ b, and y ∗ a = b if and only if y = b ∗ a−1.
The integers Z with subtraction (−) form a quasigroup.
The nonzero rationals Q× (or the nonzero reals R×) with division (÷) form a quasigroup.
https://en.wikipedia.org/wiki/Mathematics_of_Sudoku#Sudokus_from_group_tables
-}
class Quasigroup a => Loop a where

  lempty :: a
  default lempty :: Monoid a => a
  lempty = mempty

  lreplicate :: Natural -> a -> a
  default lreplicate :: Group a => Natural -> a -> a
  lreplicate n = mreplicate n . inv

instance (Semigroup a, Quasigroup (Maybe a)) => Loop (Maybe a)

{-
Idempotent total symmetric quasigroups are precisely (i.e. in a bijection with) Steiner triples, so such a quasigroup is also called a Steiner quasigroup, and sometimes the latter is even abbreviated as squag; the term sloop is defined similarly for a Steiner quasigroup that is also a loop. Without idempotency, total symmetric quasigroups correspond to the geometric notion of extended Steiner triple, also called Generalized Elliptic Cubic Curve (GECC). 

< https://en.wikipedia.org/wiki/Steiner_system >
type Steiner a = (Symmetric a, IdempotentMagma a)
type Sloop a = (Steiner a, Loop a)

A quasigroup (Q, <<) is called totally anti-symmetric if for all c, x, y ∈ Q, both of the following implications hold:

    (c << x) << y == (c << y) << x ==> x == y
    x << y == y << x ==> x == y

It is called weakly totally anti-symmetric if only the first implication holds.[5]

This property is required, for example, in the < https://en.wikipedia.org/wiki/Damm_algorithm Damm algorithm >. 
class Quasigroup a => Antisymmetric a


-- https://en.wikipedia.org/wiki/Bol_loop
-- x << (y << (x << z)) = (x << (y << x)) << z     for each x, y and z in Q (a left Bol loop)
class Loop a => LeftBol a

-- ((z << x) << y) << x = z << ((x << y) << x)     for each x, y and z in Q (a right Bol loop)
class Loop a => RightBol a

A loop that is both a left and right Bol loop is a Moufang loop. 
This is equivalent to any one of the following single Moufang identities holding for all x, y, z:

x << (y << (x << z)) = ((x << y) << x) << z,
z << (x << (y << x)) = ((z << x) << y) << x,
(x << y) << (z << x) = x << ((y << z) << x), or
(x << y) << (z << x) = (x << (y << z)) << x.

-- The nonzero octonions form a nonassociative Moufang loop under multiplication.
class (LeftBol a, RightBol a) => Moufang a


< https://en.wikipedia.org/wiki/Planar_ternary_ring >

-- The structure ( R , ++ ) is a loop with identity element /zero'/. 
-- The set R 0 = R ∖ { 0 } is closed under this multiplication. The structure ( R 0 , ** ) {\displaystyle (R_{0},\otimes )} (R_{{0}},\otimes ) is also a loop, with identity element 1. 
class Ternary a where
  t :: a -> a -> a -> a
  zero' :: a
  one' :: a
a ++ b = t a one' b
a ** b = t a b zero'

-- ? type Planar a = ((Additive-Ternary) a, (Multiplicative-Ternary) a)

-- | < https://en.wikipedia.org/wiki/Planar_ternary_ring#Linear_PTR >
--
-- @ 't' a b c = (a '#' b) '%' c @
--
-- ? type Linear a = ((Additive-Loop) a, (Multiplicative-Loop) a)

instance Ternary a => (Additive-Loop) a where
-}

{-
-- A quasigroup is semisymmetric if the following equivalent identities hold:

    x << y = y // x,
    y << x = x \\ y,
    x = (y << x) << y,
    x = y << (x << y).

--class Quasigroup a => Semisymmetric a


A narrower class that is a totally symmetric quasigroup (sometimes abbreviated TS-quasigroup) in which all conjugates coincide as one operation: xy = x / y = x \ y. Another way to define (the same notion of) totally symmetric quasigroup is as a semisymmetric quasigroup which also is commutative, i.e. xy = yx.

-- x << y = x // y = x \\ y.

--type Symmetric a = (Semisymmetric a, AbelianMagma a)

Idempotent total symmetric quasigroups are precisely (i.e. in a bijection with) Steiner triples, so such a quasigroup is also called a Steiner quasigroup, and sometimes the latter is even abbreviated as squag; the term sloop is defined similarly for a Steiner quasigroup that is also a loop. Without idempotency, total symmetric quasigroups correspond to the geometric notion of extended Steiner triple, also called Generalized Elliptic Cubic Curve (GECC). 

< https://en.wikipedia.org/wiki/Steiner_system >

--type Steiner a = (Symmetric a, IdempotentMagma a)
--type Sloop a = (Steiner a, Loop a)

A quasigroup (Q, <<) is called totally anti-symmetric if for all c, x, y ∈ Q, both of the following implications hold:

    (c << x) << y == (c << y) << x ==> x == y
    x << y == y << x ==> x == y

It is called weakly totally anti-symmetric if only the first implication holds.[5]

This property is required, for example, in the < https://en.wikipedia.org/wiki/Damm_algorithm Damm algorithm >. 
--class Quasigroup a => Antisymmetric a
-}


-- (<<) has the < https://en.wikipedia.org/wiki/Latin_square_property >.
-- The unique solutions to these equations are written x = a \\ b and y = b // a. The operations '\\' and '//' are called, respectively, left and right division. 

-- https://en.wikipedia.org/wiki/Quasigroup
-- in a group (//) = (\\) = (<>)
class Magma a => Quasigroup a where
  (//) :: a -> a -> a
  default (//) :: Semigroup a => a -> a -> a
  (//) = (<>)

  (\\) :: a -> a -> a
  default (\\) :: Semigroup a => a -> a -> a
  (\\) = (<>)