module Amazon.SNS.Verify
  ( verifySNSMessage
  , verifySNSMessageEither
  , verifySNSMessageJSON
  , verifySNSMessageJSONEither
  , SNSNotificationValidationError(..)
  ) where

import Amazon.SNS.Verify.Prelude

import Amazon.SNS.Verify.Payload
import Amazon.SNS.Verify.Validate
import Control.Error (hoistEither, runExceptT)
import Data.Aeson (FromJSON, Value, eitherDecode)
import Data.Aeson.Types (Result(Error, Success), fromJSON)
import Data.Bifunctor (first)
import Data.ByteString.Lazy (fromStrict)
import Data.Text.Encoding (encodeUtf8)

-- | Decode and verify an SNS message as JSON
--
-- The same as 'verifySNSMessage', but decodes the message as `JSON`.
--
verifySNSMessageJSON :: (FromJSON a, MonadIO m) => Value -> m a
verifySNSMessageJSON = unTryIO id <=< verifySNSMessageJSONEither

verifySNSMessageJSONEither
  :: (FromJSON a, MonadIO m)
  => Value
  -> m (Either SNSNotificationValidationError a)
verifySNSMessageJSONEither value =
  join
    . traverse (first BadJSONParse . eitherDecode . fromStrict . encodeUtf8)
    <$> verifySNSMessageEither value

-- | Decode and verify an SNS message
--
-- This function follows the process outlined in the following:
--
-- <https://docs.aws.amazon.com/sns/latest/dg/sns-verify-signature-of-message.html>
--
-- A `JSON` payload is:
--
-- 1. Parsed as an SNS message type 'Notification', `SubscriptionConfirmation`,
--    or `UnsubscribeConfirmation`.
-- 2. Verified against its signature.
-- 3. And in the case of subscription events responded to.
--
verifySNSMessage :: MonadIO m => Value -> m Text
verifySNSMessage = unTryIO id <=< verifySNSMessageEither

verifySNSMessageEither
  :: MonadIO m => Value -> m (Either SNSNotificationValidationError Text)
verifySNSMessageEither value = runExceptT $ do
  payload <- hoistEither $ parseSNSPayload value
  verified <- hoistEither =<< validateSnsMessage payload
  hoistEither =<< handleSubscription verified

parseSNSPayload :: Value -> Either SNSNotificationValidationError SNSPayload
parseSNSPayload = first BadJSONParse . fromResult . fromJSON

fromResult :: Result a -> Either String a
fromResult = \case
  Success a -> Right a
  Error str -> Left str