{-# LANGUAGE ViewPatterns #-}
module BlockCipher
    ( KAT_ECB(..)
    , KAT_CBC(..)
    , KAT_CFB(..)
    , KAT_CTR(..)
    , KAT_XTS(..)
    , KAT_AEAD(..)
    , KATs(..)
    , defaultKATs
    , testBlockCipher
    , CipherInfo
    ) where

import           Imports
import           Data.Maybe
import           Crypto.Error
import           Crypto.Cipher.Types
import           Data.ByteArray as B hiding (pack, null, length)
import qualified Data.ByteString as B hiding (all, take, replicate)

------------------------------------------------------------------------
-- KAT
------------------------------------------------------------------------

type BlockSize = Int
type KeySize = Int
type CipherInfo a = (BlockSize, KeySize, ByteString -> a)

instance Show (IV c) where
    show _ = "IV"

-- | ECB KAT
data KAT_ECB = KAT_ECB
    { ecbKey        :: ByteString -- ^ Key
    , ecbPlaintext  :: ByteString -- ^ Plaintext
    , ecbCiphertext :: ByteString -- ^ Ciphertext
    } deriving (Show,Eq)

-- | CBC KAT
data KAT_CBC = KAT_CBC
    { cbcKey        :: ByteString -- ^ Key
    , cbcIV         :: ByteString -- ^ IV
    , cbcPlaintext  :: ByteString -- ^ Plaintext
    , cbcCiphertext :: ByteString -- ^ Ciphertext
    } deriving (Show,Eq)

-- | CFB KAT
data KAT_CFB = KAT_CFB
    { cfbKey        :: ByteString -- ^ Key
    , cfbIV         :: ByteString -- ^ IV
    , cfbPlaintext  :: ByteString -- ^ Plaintext
    , cfbCiphertext :: ByteString -- ^ Ciphertext
    } deriving (Show,Eq)

-- | CTR KAT
data KAT_CTR = KAT_CTR
    { ctrKey        :: ByteString -- ^ Key
    , ctrIV         :: ByteString -- ^ IV (usually represented as a 128 bits integer)
    , ctrPlaintext  :: ByteString -- ^ Plaintext
    , ctrCiphertext :: ByteString -- ^ Ciphertext
    } deriving (Show,Eq)

-- | XTS KAT
data KAT_XTS = KAT_XTS
    { xtsKey1       :: ByteString -- ^ 1st XTS key
    , xtsKey2       :: ByteString -- ^ 2nd XTS key
    , xtsIV         :: ByteString -- ^ XTS IV
    , xtsPlaintext  :: ByteString -- ^ plaintext
    , xtsCiphertext :: ByteString -- ^ Ciphertext
    } deriving (Show,Eq)

-- | AEAD KAT
data KAT_AEAD = KAT_AEAD
    { aeadMode       :: AEADMode
    , aeadKey        :: ByteString -- ^ Key
    , aeadIV         :: ByteString -- ^ IV for initialization
    , aeadHeader     :: ByteString -- ^ Authenticated Header
    , aeadPlaintext  :: ByteString -- ^ Plaintext
    , aeadCiphertext :: ByteString -- ^ Ciphertext
    , aeadTaglen     :: Int        -- ^ aead tag len
    , aeadTag        :: ByteString -- ^ expected tag
    } deriving (Show,Eq)

-- | all the KATs. use defaultKATs to prevent compilation error
-- from future expansion of this data structure
data KATs = KATs
    { kat_ECB  :: [KAT_ECB]
    , kat_CBC  :: [KAT_CBC]
    , kat_CFB  :: [KAT_CFB]
    , kat_CTR  :: [KAT_CTR]
    , kat_XTS  :: [KAT_XTS]
    , kat_AEAD :: [KAT_AEAD]
    } deriving (Show,Eq)

defaultKATs = KATs [] [] [] [] [] []

{-
testECB (_, _, cipherInit) ecbEncrypt ecbDecrypt kats =
    testGroup "ECB" (concatMap katTest (zip is kats) {- ++ propTests-})
  where katTest (i,d) =
            [ testCase ("E" ++ show i) (ecbEncrypt ctx (ecbPlaintext d) @?= ecbCiphertext d)
            , testCase ("D" ++ show i) (ecbDecrypt ctx (ecbCiphertext d) @?= ecbPlaintext d)
            ]
          where ctx = cipherInit (ecbKey d)
        --propTest = testProperty "decrypt.encrypt" (ECBUnit key plaintext) =

        --testProperty_ECB (ECBUnit (cipherInit -> ctx) (toBytes -> plaintext)) =
        --    plaintext `assertEq` ecbDecrypt ctx (ecbEncrypt ctx plaintext)

testKatCBC cbcInit cbcEncrypt cbcDecrypt (i,d) =
    [ testCase ("E" ++ show i) (cbcEncrypt ctx iv (cbcPlaintext d) @?= cbcCiphertext d)
    , testCase ("D" ++ show i) (cbcDecrypt ctx iv (cbcCiphertext d) @?= cbcPlaintext d)
    ]
  where ctx = cbcInit $ cbcKey d
        iv  = cbcIV d

testKatCFB cfbInit cfbEncrypt cfbDecrypt (i,d) =
    [ testCase ("E" ++ show i) (cfbEncrypt ctx iv (cfbPlaintext d) @?= cfbCiphertext d)
    , testCase ("D" ++ show i) (cfbDecrypt ctx iv (cfbCiphertext d) @?= cfbPlaintext d)
    ]
  where ctx = cfbInit $ cfbKey d
        iv  = cfbIV d

testKatCTR ctrInit ctrCombine (i,d) =
    [ testCase ("E" ++ i) (ctrCombine ctx iv (ctrPlaintext d) @?= ctrCiphertext d)
    , testCase ("D" ++ i) (ctrCombine ctx iv (ctrCiphertext d) @?= ctrPlaintext d)
    ]
  where ctx = ctrInit $ ctrKey d
        iv  = ctrIV d

testKatXTS xtsInit xtsEncrypt xtsDecrypt (i,d) =
    [ testCase ("E" ++ i) (xtsEncrypt ctx iv 0 (xtsPlaintext d) @?= xtsCiphertext d)
    , testCase ("D" ++ i) (xtsDecrypt ctx iv 0 (xtsCiphertext d) @?= xtsPlaintext d)
    ]
  where ctx  = xtsInit (xtsKey1 d, xtsKey2 d)
        iv   = xtsIV d

testKatAEAD cipherInit aeadInit aeadAppendHeader aeadEncrypt aeadDecrypt aeadFinalize (i,d) =
    [ testCase ("AE" ++ i) (etag @?= aeadTag d)
    , testCase ("AD" ++ i) (dtag @?= aeadTag d)
    , testCase ("E" ++ i)  (ebs @?= aeadCiphertext d)
    , testCase ("D" ++ i)  (dbs @?= aeadPlaintext d)
    ]
  where ctx              = cipherInit $ aeadKey d
        (Just aead)      = aeadInit ctx (aeadIV d)
        aeadHeaded       = aeadAppendHeader aead (aeadHeader d)
        (ebs,aeadEFinal) = aeadEncrypt aeadHeaded (aeadPlaintext d)
        (dbs,aeadDFinal) = aeadDecrypt aeadHeaded (aeadCiphertext d)
        etag = aeadFinalize aeadEFinal (aeadTaglen d)
        dtag = aeadFinalize aeadDFinal (aeadTaglen d)
-}

testKATs :: BlockCipher cipher
         => KATs
         -> cipher
         -> TestTree
testKATs kats cipher = testGroup "KAT"
    (   maybeGroup makeECBTest "ECB" (kat_ECB kats)
     ++ maybeGroup makeCBCTest "CBC" (kat_CBC kats)
     ++ maybeGroup makeCFBTest "CFB" (kat_CFB kats)
     ++ maybeGroup makeCTRTest "CTR" (kat_CTR kats)
     -- ++ maybeGroup makeXTSTest "XTS" (kat_XTS kats)
     ++ maybeGroup makeAEADTest "AEAD" (kat_AEAD kats)
    )
  where makeECBTest i d =
            [ testCase ("E" ++ i) (ecbEncrypt ctx (ecbPlaintext d) @?= ecbCiphertext d)
            , testCase ("D" ++ i) (ecbDecrypt ctx (ecbCiphertext d) @?= ecbPlaintext d)
            ]
          where ctx = cipherInitNoErr (cipherMakeKey cipher $ ecbKey d)
        makeCBCTest i d =
            [ testCase ("E" ++ i) (cbcEncrypt ctx iv (cbcPlaintext d) @?= cbcCiphertext d)
            , testCase ("D" ++ i) (cbcDecrypt ctx iv (cbcCiphertext d) @?= cbcPlaintext d)
            ]
          where ctx = cipherInitNoErr (cipherMakeKey cipher $ cbcKey d)
                iv  = cipherMakeIV cipher $ cbcIV d
        makeCFBTest i d =
            [ testCase ("E" ++ i) (cfbEncrypt ctx iv (cfbPlaintext d) @?= cfbCiphertext d)
            , testCase ("D" ++ i) (cfbDecrypt ctx iv (cfbCiphertext d) @?= cfbPlaintext d)
            ]
          where ctx = cipherInitNoErr (cipherMakeKey cipher $ cfbKey d)
                iv  = cipherMakeIV cipher $ cfbIV d
        makeCTRTest i d =
            [ testCase ("E" ++ i) (ctrCombine ctx iv (ctrPlaintext d) @?= ctrCiphertext d)
            , testCase ("D" ++ i) (ctrCombine ctx iv (ctrCiphertext d) @?= ctrPlaintext d)
            ]
          where ctx = cipherInitNoErr (cipherMakeKey cipher $ ctrKey d)
                iv  = cipherMakeIV cipher $ ctrIV d
{-
        makeXTSTest i d  =
            [ testCase ("E" ++ i) (xtsEncrypt ctx iv 0 (xtsPlaintext d) @?= xtsCiphertext d)
            , testCase ("D" ++ i) (xtsDecrypt ctx iv 0 (xtsCiphertext d) @?= xtsPlaintext d)
            ]
          where ctx1 = cipherInitNoErr (cipherMakeKey cipher $ xtsKey1 d)
                ctx2 = cipherInitNoErr (cipherMakeKey cipher $ xtsKey2 d)
                ctx  = (ctx1, ctx2)
                iv   = cipherMakeIV cipher $ xtsIV d
-}
        makeAEADTest i d =
            [ testCase ("AE" ++ i) (etag @?= AuthTag (B.convert (aeadTag d)))
            , testCase ("AD" ++ i) (dtag @?= AuthTag (B.convert (aeadTag d)))
            , testCase ("E" ++ i)  (ebs @?= aeadCiphertext d)
            , testCase ("D" ++ i)  (dbs @?= aeadPlaintext d)
            ]
          where ctx  = cipherInitNoErr (cipherMakeKey cipher $ aeadKey d)
                aead = aeadInitNoErr (aeadMode d) ctx (aeadIV d)
                aeadHeaded     = aeadAppendHeader aead (aeadHeader d)
                (ebs,aeadEFinal) = aeadEncrypt aeadHeaded (aeadPlaintext d)
                (dbs,aeadDFinal) = aeadDecrypt aeadHeaded (aeadCiphertext d)
                etag = aeadFinalize aeadEFinal (aeadTaglen d)
                dtag = aeadFinalize aeadDFinal (aeadTaglen d)

        cipherInitNoErr :: BlockCipher c => Key c -> c
        cipherInitNoErr (Key k) =
            case cipherInit k of
                CryptoPassed a -> a
                CryptoFailed e -> error (show e)

        aeadInitNoErr :: (ByteArrayAccess iv, BlockCipher cipher) => AEADMode -> cipher -> iv -> AEAD cipher
        aeadInitNoErr mode ct iv =
            case aeadInit mode ct iv of
                CryptoPassed a -> a
                CryptoFailed _ -> error $ "cipher doesn't support aead mode: " ++ show mode
------------------------------------------------------------------------
-- Properties
------------------------------------------------------------------------

-- | any sized bytestring
newtype Plaintext a = Plaintext { unPlaintext :: B.ByteString }
    deriving (Show,Eq)

-- | A multiple of blocksize bytestring
newtype PlaintextBS a = PlaintextBS { unPlaintextBS :: B.ByteString }
    deriving (Show,Eq)

newtype Key a = Key ByteString
    deriving (Show,Eq)

-- | a ECB unit test
data ECBUnit a = ECBUnit (Key a) (PlaintextBS a)
    deriving (Eq)

-- | a CBC unit test
data CBCUnit a = CBCUnit (Key a) (IV a) (PlaintextBS a)
    deriving (Eq)

-- | a CBC unit test
data CFBUnit a = CFBUnit (Key a) (IV a) (PlaintextBS a)
    deriving (Eq)

-- | a CFB unit test
data CFB8Unit a = CFB8Unit (Key a) (IV a) (Plaintext a)
    deriving (Eq)

-- | a CTR unit test
data CTRUnit a = CTRUnit (Key a) (IV a) (Plaintext a)
    deriving (Eq)

-- | a XTS unit test
data XTSUnit a = XTSUnit (Key a) (Key a) (IV a) (PlaintextBS a)
    deriving (Eq)

-- | a AEAD unit test
data AEADUnit a = AEADUnit (Key a) B.ByteString (Plaintext a) (Plaintext a)
    deriving (Eq)

-- | Stream cipher unit test
data StreamUnit a = StreamUnit (Key a) (Plaintext a)
    deriving (Eq)

instance Show (ECBUnit a) where
    show (ECBUnit key b) = "ECB(key=" ++ show key ++ ",input=" ++ show b ++ ")"
instance Show (CBCUnit a) where
    show (CBCUnit key iv b) = "CBC(key=" ++ show key ++ ",iv=" ++ show iv ++ ",input=" ++ show b ++ ")"
instance Show (CFBUnit a) where
    show (CFBUnit key iv b) = "CFB(key=" ++ show key ++ ",iv=" ++ show iv ++ ",input=" ++ show b ++ ")"
instance Show (CFB8Unit a) where
    show (CFB8Unit key iv b) = "CFB8(key=" ++ show key ++ ",iv=" ++ show iv ++ ",input=" ++ show b ++ ")"
instance Show (CTRUnit a) where
    show (CTRUnit key iv b) = "CTR(key=" ++ show key ++ ",iv=" ++ show iv ++ ",input=" ++ show b ++ ")"
instance Show (XTSUnit a) where
    show (XTSUnit key1 key2 iv b) = "XTS(key1=" ++ show key1 ++ ",key2=" ++ show key2 ++ ",iv=" ++ show iv ++ ",input=" ++ show b ++ ")"
instance Show (AEADUnit a) where
    show (AEADUnit key iv aad b) = "AEAD(key=" ++ show key ++ ",iv=" ++ show iv ++ ",aad=" ++ show (unPlaintext aad) ++ ",input=" ++ show b ++ ")"
instance Show (StreamUnit a) where
    show (StreamUnit key b) = "Stream(key=" ++ show key ++ ",input=" ++ show b ++ ")"

-- | Generate an arbitrary valid key for a specific block cipher
generateKey :: Cipher a => Gen (Key a)
generateKey = keyFromCipher undefined
  where keyFromCipher :: Cipher a => a -> Gen (Key a)
        keyFromCipher cipher = do
            sz <- case cipherKeySize cipher of
                         KeySizeRange low high -> choose (low, high)
                         KeySizeFixed v -> return v
                         KeySizeEnum l  -> elements l
            Key . B.pack <$> replicateM sz arbitrary

-- | Generate an arbitrary valid IV for a specific block cipher
generateIv :: BlockCipher a => Gen (IV a)
generateIv = ivFromCipher undefined
  where ivFromCipher :: BlockCipher a => a -> Gen (IV a)
        ivFromCipher cipher = fromJust . makeIV . B.pack <$> replicateM (blockSize cipher) arbitrary

-- | Generate an arbitrary valid IV for AEAD for a specific block cipher
generateIvAEAD :: Gen B.ByteString
generateIvAEAD = choose (12,90) >>= \sz -> (B.pack <$> replicateM sz arbitrary)

-- | Generate a plaintext multiple of blocksize bytes
generatePlaintextMultipleBS :: BlockCipher a => Gen (PlaintextBS a)
generatePlaintextMultipleBS = choose (1,128) >>= \size -> replicateM (size * 16) arbitrary >>= return . PlaintextBS . B.pack

-- | Generate any sized plaintext
generatePlaintext :: Gen (Plaintext a)
generatePlaintext = choose (0,324) >>= \size -> replicateM size arbitrary >>= return . Plaintext . B.pack

instance BlockCipher a => Arbitrary (ECBUnit a) where
    arbitrary = ECBUnit <$> generateKey
                        <*> generatePlaintextMultipleBS

instance BlockCipher a => Arbitrary (CBCUnit a) where
    arbitrary = CBCUnit <$> generateKey
                        <*> generateIv
                        <*> generatePlaintextMultipleBS

instance BlockCipher a => Arbitrary (CFBUnit a) where
    arbitrary = CFBUnit <$> generateKey
                        <*> generateIv
                        <*> generatePlaintextMultipleBS

instance BlockCipher a => Arbitrary (CFB8Unit a) where
    arbitrary = CFB8Unit <$> generateKey <*> generateIv <*> generatePlaintext

instance BlockCipher a => Arbitrary (CTRUnit a) where
    arbitrary = CTRUnit <$> generateKey
                        <*> generateIv
                        <*> generatePlaintext

instance BlockCipher a => Arbitrary (XTSUnit a) where
    arbitrary = XTSUnit <$> generateKey
                        <*> generateKey
                        <*> generateIv
                        <*> generatePlaintextMultipleBS

instance BlockCipher a => Arbitrary (AEADUnit a) where
    arbitrary = AEADUnit <$> generateKey
                         <*> generateIvAEAD
                         <*> generatePlaintext
                         <*> generatePlaintext

instance StreamCipher a => Arbitrary (StreamUnit a) where
    arbitrary = StreamUnit <$> generateKey
                           <*> generatePlaintext

testBlockCipherBasic :: BlockCipher a => a -> [TestTree]
testBlockCipherBasic cipher = [ testProperty "ECB" ecbProp ]
  where ecbProp = toTests cipher
        toTests :: BlockCipher a => a -> (ECBUnit a -> Bool)
        toTests _ = testProperty_ECB
        testProperty_ECB (ECBUnit key (unPlaintextBS -> plaintext)) = withCtx key $ \ctx ->
            plaintext `assertEq` ecbDecrypt ctx (ecbEncrypt ctx plaintext)

testBlockCipherModes :: BlockCipher a => a -> [TestTree]
testBlockCipherModes cipher =
    [ testProperty "CBC" cbcProp
    , testProperty "CFB" cfbProp
    --, testProperty "CFB8" cfb8Prop
    , testProperty "CTR" ctrProp
    ]
  where (cbcProp,cfbProp,ctrProp) = toTests cipher
        toTests :: BlockCipher a
                => a
                -> ((CBCUnit a -> Bool), (CFBUnit a -> Bool), {-(CFB8Unit a -> Bool),-} (CTRUnit a -> Bool))
        toTests _ = (testProperty_CBC
                    ,testProperty_CFB
                    --,testProperty_CFB8
                    ,testProperty_CTR
                    )
        testProperty_CBC (CBCUnit key testIV (unPlaintextBS -> plaintext)) = withCtx key $ \ctx ->
            plaintext `assertEq` cbcDecrypt ctx testIV (cbcEncrypt ctx testIV plaintext)

        testProperty_CFB (CFBUnit key testIV (unPlaintextBS -> plaintext)) = withCtx key $ \ctx ->
            plaintext `assertEq` cfbDecrypt ctx testIV (cfbEncrypt ctx testIV plaintext)

{-
        testProperty_CFB8 (CFB8Unit (cipherInit -> ctx) testIV (unPlaintext -> plaintext)) =
            plaintext `assertEq` cfb8Decrypt ctx testIV (cfb8Encrypt ctx testIV plaintext)
-}

        testProperty_CTR (CTRUnit key testIV (unPlaintext -> plaintext)) = withCtx key $ \ctx ->
            plaintext `assertEq` ctrCombine ctx testIV (ctrCombine ctx testIV plaintext)

testBlockCipherAEAD :: BlockCipher a => a -> [TestTree]
testBlockCipherAEAD cipher =
    [ testProperty "OCB" (aeadProp AEAD_OCB)
    , testProperty "CCM" (aeadProp (AEAD_CCM 0 CCM_M16 CCM_L2))
    , testProperty "EAX" (aeadProp AEAD_EAX)
    , testProperty "CWC" (aeadProp AEAD_CWC)
    , testProperty "GCM" (aeadProp AEAD_GCM)
    ]
  where aeadProp = toTests cipher
        toTests :: BlockCipher a => a -> (AEADMode -> AEADUnit a -> Bool)
        toTests _ = testProperty_AEAD
        testProperty_AEAD mode (AEADUnit key testIV (unPlaintext -> aad) (unPlaintext -> plaintext)) = withCtx key $ \ctx ->
            case aeadInit mode' ctx iv' of
                CryptoPassed iniAead ->
                    let aead           = aeadAppendHeader iniAead aad
                        (eText, aeadE) = aeadEncrypt aead plaintext
                        (dText, aeadD) = aeadDecrypt aead eText
                        eTag           = aeadFinalize aeadE (blockSize ctx)
                        dTag           = aeadFinalize aeadD (blockSize ctx)
                     in (plaintext `assertEq` dText) && (eTag `B.eq` dTag)
                CryptoFailed err
                    | err == CryptoError_AEADModeNotSupported -> True
                    | otherwise                               -> error ("testProperty_AEAD: " ++ show err)
            where (mode', iv') = updateCcmInputSize mode (B.length plaintext) testIV
                  updateCcmInputSize aeadmode k iv = case aeadmode of
                    AEAD_CCM _ m l -> (AEAD_CCM k m l, B.take 13 (iv <> (B.replicate 15 0)))
                    aeadOther      -> (aeadOther, iv)

withCtx :: Cipher c => Key c -> (c -> a) -> a
withCtx (Key key) f =
    case cipherInit key of
        CryptoFailed e   -> error ("init failed: " ++ show e)
        CryptoPassed ctx -> f ctx

{-
testBlockCipherXTS :: BlockCipher a => a -> [TestTree]
testBlockCipherXTS cipher = [testProperty "XTS" xtsProp]
  where xtsProp = toTests cipher
        toTests :: BlockCipher a => a -> (XTSUnit a -> Bool)
        toTests _ = testProperty_XTS

        testProperty_XTS (XTSUnit (cipherInit -> ctx1) (cipherInit -> ctx2) testIV (toBytes -> plaintext))
            | blockSize ctx1 == 16 = plaintext `assertEq` xtsDecrypt (ctx1, ctx2) testIV 0 (xtsEncrypt (ctx1, ctx2) testIV 0 plaintext)
            | otherwise            = True
-}

-- | Test a generic block cipher for properties
-- related to block cipher modes.
testModes :: BlockCipher a => a -> [TestTree]
testModes cipher =
    [ testGroup "decrypt.encrypt==id"
--        (testBlockCipherBasic cipher ++ testBlockCipherModes cipher ++ testBlockCipherAEAD cipher ++ testBlockCipherXTS cipher)
        (testBlockCipherBasic cipher ++ testBlockCipherModes cipher ++ testBlockCipherAEAD cipher)
    ]

-- | Test IV arithmetic (based on the cipher block size)
testIvArith :: BlockCipher a => a -> [TestTree]
testIvArith cipher =
    [ testCase "nullIV is null" $
          True @=? B.all (== 0) (ivNull cipher)
    , testProperty "ivAdd is linear" $ \a b -> do
          iv <- generateIvFromCipher cipher
          return $ ivAdd iv (a + b) `propertyEq` ivAdd (ivAdd iv a) b
    ]
  where
    ivNull :: BlockCipher a => a -> IV a
    ivNull = const nullIV

    -- uses IV pattern <00 .. 00 FF .. FF> to test carry propagation
    generateIvFromCipher :: BlockCipher a => a -> Gen (IV a)
    generateIvFromCipher c = do
        let n = blockSize c
        i <- choose (0, n)
        let zeros = Prelude.replicate (n - i) 0x00
            ones  = Prelude.replicate i 0xFF
        return $ cipherMakeIV c (B.pack $ zeros ++ ones)

-- | Return tests for a specific blockcipher and a list of KATs
testBlockCipher :: BlockCipher a => KATs -> a -> TestTree
testBlockCipher kats cipher = testGroup (cipherName cipher)
    (  (if kats == defaultKATs  then [] else [testKATs kats cipher])
    ++ testModes cipher ++ testIvArith cipher
    )

cipherMakeKey :: Cipher cipher => cipher -> ByteString -> Key cipher
cipherMakeKey _ bs = Key bs

cipherMakeIV :: BlockCipher cipher => cipher -> ByteString -> IV cipher
cipherMakeIV _ bs = fromJust $ makeIV bs

maybeGroup :: (String -> t -> [TestTree]) -> TestName -> [t] -> [TestTree]
maybeGroup mkTest groupName l
    | null l    = []
    | otherwise = [testGroup groupName (concatMap (\(i, d) -> mkTest (show i) d) $ zip nbs l)]
  where nbs :: [Int]
        nbs = [0..]