{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Yesod.Paginator.Pages
    (
    -- * Type safe @'Natural'@s
    -- |
    --
    -- N.B. @'PageNumber'@ and @'PerPage'@ will currently allow @0@, but it's
    -- unclear if that's correct and may not be the case in the future.
    --
      PageNumber
    , PerPage
    , ItemsCount
    , pageOffset

    -- * Page
    , Page
    , pageItems
    , pageNumber
    , toPage

    -- * Pages
    , Pages
    , pagesCurrent
    , pagesLast
    , toPages

    -- * Safely accessing Pages data
    , takePreviousPages
    , takeNextPages
    , getPreviousPage
    , getNextPage
    )
where

import Yesod.Paginator.Prelude

import Text.Blaze (ToMarkup)
import Web.PathPieces

newtype PageNumber = PageNumber Natural
    deriving (Enum, Eq, Integral, Num, Ord, Real)
    deriving newtype (Show, ToMarkup)

newtype PerPage = PerPage Natural
    deriving (Enum, Eq, Integral, Num, Ord, Real)
    deriving newtype (Read, Show, PathPiece)

newtype ItemsCount = ItemsCount Natural
    deriving (Enum, Eq, Integral, Num, Ord, Real)
    deriving newtype (Read, Show, PathPiece)

data Page a = Page
    { pageItems :: [a]
    , pageNumber :: PageNumber
    }
    deriving (Eq, Show)

-- | @'Page'@ constructor
toPage :: [a] -> PageNumber -> Page a
toPage = Page

data Pages a = Pages
    { pagesCurrent :: Page a
    , pagesPrevious :: [PageNumber]
    , pagesNext :: [PageNumber]
    , pagesLast :: PageNumber
    }
    deriving (Eq, Show)

-- | Take previous pages, going back from current
--
-- >>> takePreviousPages 3 $ Pages (Page [] 5) [1,2,3,4] [6] 6
-- [2,3,4]
--
takePreviousPages :: Natural -> Pages a -> [PageNumber]
takePreviousPages n = reverse . genericTake n . reverse . pagesPrevious

-- | Take next pages, going forward from current
--
-- >>> takeNextPages 3 $ Pages (Page [] 2) [1] [3,4,5,6] 6
-- [3,4,5]
--
takeNextPages :: Natural -> Pages a -> [PageNumber]
takeNextPages n = genericTake n . pagesNext

-- | The previous page number, if it exists
--
-- >>> getPreviousPage $ Pages (Page [] 1) [] [2,3,4] 4
-- Nothing
--
-- >>> getPreviousPage $ Pages (Page [] 2) [1] [3,4] 4
-- Just 1
--
getPreviousPage :: Pages a -> Maybe PageNumber
getPreviousPage pages = do
    let prevPage = pageNumber (pagesCurrent pages) - 1
    firstPage <- headMay $ pagesPrevious pages
    prevPage <$ guard (prevPage >= firstPage)

-- | The next page number, if it exists
--
-- >>> getNextPage $ Pages (Page [] 4) [1,2,3] [] 4
-- Nothing
--
-- >>> getNextPage $ Pages (Page [] 3) [1,2] [4] 4
-- Just 4
--
getNextPage :: Pages a -> Maybe PageNumber
getNextPage pages = do
    let nextPage = pageNumber (pagesCurrent pages) + 1
    lastPage <- lastMay $ pagesNext pages
    nextPage <$ guard (nextPage <= lastPage)

-- | Construct a @'Pages' a@ from paginated data
--
-- >>> toPages 4 3 10 []
-- Pages {pagesCurrent = Page {pageItems = [], pageNumber = 4}, pagesPrevious = [1,2,3], pagesNext = [], pagesLast = 4}
--
toPages :: PageNumber -> PerPage -> ItemsCount -> [a] -> Pages a
toPages number per total items = Pages
    { pagesCurrent = toPage items number
    , pagesPrevious = [1 .. (number - 1)]
    , pagesNext = [(number + 1) .. lastPage]
    , pagesLast = lastPage
    }
    where lastPage = getLastPage total per

-- | Calculate the last page of some paginated data
--
-- >>> getLastPage 10 3
-- 4
--
-- >>> getLastPage 10 5
-- 2
--
getLastPage :: ItemsCount -> PerPage -> PageNumber
getLastPage total = fromIntegral . carry . (total `divMod`) . fromIntegral
  where
    carry (q, 0) = q
    carry (q, _) = q + 1

-- | Calculate a page's zero-based offset in the overall items
--
-- >>> pageOffset 4 3
-- 9
--
pageOffset :: PageNumber -> PerPage -> ItemsCount
pageOffset p per = fromIntegral $ (fromIntegral p - 1) * per