HarryPotter.hs

module HarryPotter where

import qualified Data.Map.Strict as Map
import Data.Maybe (catMaybes) 

type Copies = Int
type Discount = Double
type Price = Double
type DiscountScheme = Int -> Discount

data Sale = Sale Copies Discount
  deriving (Eq, Show)

data Book = One | Two | Three | Four | Five
  deriving (Eq, Ord, Show)

type Books = Map.Map Book Copies

-- TODO: Fix this shite
discount :: DiscountScheme
discount 2 = 0.05
discount 3 = 0.10
discount 4 = 0.20
discount 5 = 0.25
discount _ = 0.00

booksFromList :: [Book] -> Books
booksFromList = foldl' (\acc x -> Map.insertWith (+) x 1 acc) Map.empty

sales :: Books -> [Sale]
sales = catMaybes . sales' -- Sorry!

sales' :: Books -> [Maybe Sale]
sales' books
  | Map.null books = []
  | otherwise      = let (books', sale) = bestValueSale books
                     in  sale : sales' books'

bestValueSale :: Books -> (Books, Maybe Sale)
bestValueSale books 
  | Map.null books = (Map.empty, Nothing)
  | otherwise      = (books', Just sale)
  where
    numTitles   = Map.size books
    leastCopies = minimum . Map.elems $ books
    sale        = Sale (numTitles * leastCopies) (discount numTitles)
    books'      = Map.filter (>0) . Map.map (subtract leastCopies) $ books

totalPrice :: [Sale] -> Price
totalPrice = sum . map salePrice

salePrice :: Sale -> Price
salePrice (Sale copies discount) = 8.0 * fromIntegral copies * (1 - discount)

{-
*HarryPotter> totalPrice . sales . booksFromList $ [One, One, Two, Two, Three, Three, Four, Five]
51.6
-}