-- Copyright (c) 2010, David Amos. All rights reserved.

{-# LANGUAGE  MultiParamTypeClasses, FlexibleInstances #-}

module Math.Algebras.GroupAlgebra where

import Math.Algebras.VectorSpace
import Math.Algebras.TensorProduct
import Math.Algebras.Structures

import Math.Algebra.Group.PermutationGroup hiding (action)

import Math.Algebra.Field.Base


instance Mon (Permutation Int) where
    munit = 1
    mmult = (*)

type GroupAlgebra k = Vect k (Permutation Int)

-- Monoid Algebra instance
instance Num k => Algebra k (Permutation Int) where
    unit 0 = zero -- V []
    unit x = V [(munit,x)]
    mult = nf . fmap (\(a,b) -> a `mmult` b)

-- Set Coalgebra instance
-- instance SetCoalgebra (Permutation Int) where {}

instance Num k => Coalgebra k (Permutation Int) where
    counit (V ts) = sum [x | (m,x) <- ts] -- trace
    comult = fmap (\m -> (m,m)) -- diagonal

instance Num k => Bialgebra k (Permutation Int) where {}
-- should check that the algebra and coalgebra structures are compatible

instance (Num k) => HopfAlgebra k (Permutation Int) where
    antipode (V ts) = nf $ V [(g^-1,x) | (g,x) <- ts]

-- inject permutation into group algebra
ip :: [[Int]] -> GroupAlgebra Q
ip cs = return $ p cs


instance Num k => Module k (Permutation Int) Int where
    action = nf . fmap (\(g,x) -> x .^ g)

-- use *. instead
-- r *> m = action (r `te` m)