{-# LANGUAGE DeriveFoldable    #-}
{-# LANGUAGE DeriveFunctor     #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE NamedFieldPuns    #-}
{-# LANGUAGE OverloadedStrings #-}

module Web.Slack.Pager
  ( Response
  , conversationsHistoryAllBy
  , repliesFetchAllBy
  , LoadPage
  , loadingPage
  ) where

-- base
import           Control.Monad.IO.Class (MonadIO, liftIO)
import           Data.IORef             (newIORef, readIORef, writeIORef)
import           Data.Maybe             (isNothing)

-- slack-web
import qualified Web.Slack.Common       as Common
import qualified Web.Slack.Conversation as Conversation
import           Web.Slack.Types        (Cursor)


-- | Public only for testing.
conversationsHistoryAllBy
  :: MonadIO m
  => (Conversation.HistoryReq -> m (Response Conversation.HistoryRsp))
  -- ^ Response generator
  -> Conversation.HistoryReq
  -- ^ The first request to send. _NOTE_: 'Conversation.historyReqCursor' is silently ignored.
  -> m (LoadPage m Common.Message)
  -- ^ An action which returns a new page of messages every time called.
  --   If there are no pages anymore, it returns an empty list.
conversationsHistoryAllBy sendRequest initialRequest =
  genericFetchAllBy
    sendRequest
    (\cursor -> initialRequest { Conversation.historyReqCursor = cursor })


-- | Public only for testing.
repliesFetchAllBy
  :: MonadIO m
  => (Conversation.RepliesReq -> m (Response Conversation.HistoryRsp))
  -- ^ Response generator
  -> Conversation.RepliesReq
  -- ^ The first request to send. _NOTE_: 'Conversation.historyReqCursor' is silently ignored.
  -> m (LoadPage m Common.Message)
  -- ^ An action which returns a new page of messages every time called.
  --   If there are no pages anymore, it returns an empty list.
repliesFetchAllBy sendRequest initialRequest =
  genericFetchAllBy
    sendRequest
    (\cursor -> initialRequest { Conversation.repliesReqCursor = cursor })


type Response a = Either Common.SlackClientError a


-- | Represents an action which returns a paginated response from Slack.
--   Every time calling the action, it performs a request with a new cursor
--   to get the next page.
--   If there is no more response, the action returns an empty list.
type LoadPage m a = m (Response [a])


-- | Utility function for 'LoadPage'. Perform the 'LoadPage' action to call
--   the function with the loaded page, until an empty page is loaded.
loadingPage :: (Monad m, Monoid n) => LoadPage m a -> (Response [a] -> m n) -> m n
loadingPage loadPage usePage = go mempty
 where
  go result = do
    epage <- loadPage
    case epage of
        Right page ->
          if null page
            then return result
            else (go $!) . (result <>) =<< usePage epage
        Left e -> (result <>) <$> usePage (Left e)


genericFetchAllBy
  :: MonadIO m
  => (a -> m (Response Conversation.HistoryRsp))
  -> (Maybe Cursor -> a)
  -> m (LoadPage m Common.Message)
genericFetchAllBy sendRequest requestFromCursor = do
  cursorRef <- liftIO $ newIORef Nothing

  let collectAndUpdateCursor
        Conversation.HistoryRsp
          { Conversation.historyRspMessages
          , Conversation.historyRspResponseMetadata
          } = do
        let newCursor = Conversation.responseMetadataNextCursor =<< historyRspResponseMetadata
            -- emptyCursor is used for the marker to show that there are no more pages.
            cursorToSave = if isNothing newCursor then emptyCursor else newCursor
        writeIORef cursorRef cursorToSave
        return historyRspMessages

  return $ do
    cursor <- liftIO $ readIORef cursorRef
    if cursor == emptyCursor
      then
        return $ Right []
      else
        traverse (liftIO . collectAndUpdateCursor)
          =<< sendRequest (requestFromCursor cursor)
 where
  -- Used for the marker to show that there are no more pages.
  emptyCursor = Just $ Common.Cursor ""