{-|
Module      :  GitHub.REST.Auth
Maintainer  :  Brandon Chinn <brandon@leapyear.io>
Stability   :  experimental
Portability :  portable

Definitions for handling authentication with the GitHub REST API.
-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedStrings #-}

module GitHub.REST.Auth
  ( Token(..)
  , fromToken
  -- * Helpers for using JWT tokens with the GitHub API
  , getJWTToken
  , loadSigner
  ) where

import Data.ByteString (ByteString)
import qualified Data.ByteString as ByteString
#if !MIN_VERSION_base(4,11,0)
import Data.Monoid ((<>))
#endif
import qualified Data.Text as Text
import qualified Data.Text.Encoding as Text
import Data.Time (addUTCTime, getCurrentTime)
import Data.Time.Clock.POSIX (utcTimeToPOSIXSeconds)
import qualified Web.JWT as JWT

-- | The token to use to authenticate with GitHub.
data Token
  = AccessToken ByteString
    -- ^ https://developer.github.com/v3/#authentication
  | BearerToken ByteString
    -- ^ https://developer.github.com/apps/building-github-apps/authenticating-with-github-apps/#authenticating-as-a-github-app
  deriving (Show)

fromToken :: Token -> ByteString
fromToken = \case
  AccessToken t -> "token " <> t
  BearerToken t -> "bearer " <> t

-- | The ID of your GitHub application
type AppId = Int

-- | Create a JWT token that expires in 10 minutes.
getJWTToken :: JWT.Signer -> AppId -> IO Token
getJWTToken signer appId = mkToken <$> getNow
  where
#if MIN_VERSION_jwt(0,10,0)
    signToken = flip JWT.encodeSigned mempty
#else
    signToken = JWT.encodeSigned
#endif
    mkToken now =
      let claims = mempty
            { JWT.iat = JWT.numericDate $ utcTimeToPOSIXSeconds now
            , JWT.exp = JWT.numericDate $ utcTimeToPOSIXSeconds now + (10 * 60)
            , JWT.iss = JWT.stringOrURI $ Text.pack $ show appId
            }
      in BearerToken . Text.encodeUtf8 $ signToken signer claims
    -- lose a second in the case of rounding
    -- https://github.community/t5/GitHub-API-Development-and/quot-Expiration-time-claim-exp-is-too-far-in-the-future-quot/m-p/20457/highlight/true#M1127
    getNow = addUTCTime (-1) <$> getCurrentTime

-- | Load a RSA private key as a Signer from the given file path.
loadSigner :: FilePath -> IO JWT.Signer
loadSigner file = maybe badSigner return . readSigner =<< ByteString.readFile file
  where
    badSigner = fail $ "Not a valid RSA private key file: " ++ file
    readSigner = fmap JWT.RSAPrivateKey . JWT.readRsaSecret