module Text.Formlets ( input', inputM', optionalInput, inputFile, fmapFst, nothingIfNull
                     , check, ensure, ensures
                     , ensureM, checkM, pureM
                     , runFormState 
                     , massInput
                     , xml, plug, plug'
                     , Env , Form , Formlet
                     , File (..), ContentType (..), FormContentType (..)
                     )
                     where

import Data.Monoid
import Control.Applicative
import Control.Applicative.Error
import Control.Applicative.State
import Data.Maybe (isJust)
import Data.List (intercalate)
import qualified Text.Formlets.FormResult as FR
import qualified Data.ByteString.Lazy as BS
import qualified Data.Traversable as T

-- Form stuff
type Env = [(String, Either String File)]
type FormState = [Integer]
type Formlet xml m a = Maybe a -> Form xml m a
type Name = String
type S a = State FormState a
type Validator a = S (FR.FormResult a)
data FormContentType = UrlEncoded | MultiPart deriving (Eq, Show, Read)
newtype Form xml m a = Form { deform :: Env -> S (m (Validator a), m xml, FormContentType) }
data File = File {content :: BS.ByteString, fileName :: String, contentType :: ContentType} deriving (Eq, Show, Read)
data ContentType = ContentType { ctType :: String
                               , ctSubtype :: String
                               , ctParameters :: [(String, String)]
                               }
                               deriving (Eq, Show, Read)

-- | Apply a predicate to a value and return FR.Success or FR.Failure as appropriate
ensure :: Show a 
       => (a -> Bool) -- ^ The predicate
       -> String      -- ^ The error message, in case the predicate fails
       -> a           -- ^ The value
       -> Failing a
ensure p msg x | p x = Success x
               | otherwise = Failure [msg]

ensureM :: (Monad m, Show a)
       => (a -> m Bool) -- ^ The predicate
       -> String      -- ^ The error message, in case the predicate fails
       -> a           -- ^ The value
       -> m (Failing a)
ensureM p msg x = do result <- p x
                     return $ if result then Success x else Failure [msg]

-- | Apply multiple predicates to a value, return FR.Success or all the FR.Failure messages
ensures :: Show a
        => [(a -> Bool, String)] -- ^ List of predicate functions and error messages, in case the predicate fails
        -> a                     -- ^ The value
        -> Failing a
ensures ps x | null errors = Success x
             | otherwise   = Failure errors
    where errors = [ err | (p, err) <- ps, not $ p x ]

-- | Helper function for genereting input components based forms.
input' :: Monad m => (String -> String -> xml) -> Maybe String -> Form xml m String
input' i = inputM' (\n -> return . i n)

inputM' :: Monad m => (String -> String -> m xml) -> Maybe String -> Form xml m String
inputM' i defaultValue = Form $ \env -> mkInput env <$> freshName
   where mkInput env name = (lookupFreshName fromLeft env, i name (value name env), UrlEncoded)
         value name env = maybe (maybe "" id defaultValue) fromLeft' (lookup name env)
         fromLeft' (Left x) = x
         fromLeft' _        = ""
         fromLeft n Nothing         = FR.NotAvailable $ n ++ " is not in the data"
         fromLeft n (Just (Left x)) = FR.Success x
         fromLeft n _               = FR.Failure [n ++ " is a file."]

lookupFreshName f env = return $ (freshName >>= \name -> return $ f name $ (lookup name env)) 

optionalInput :: Monad m => (String -> xml) -> Form xml m (Maybe String)
optionalInput i = Form $ \env -> mkInput env <$> freshName
   where mkInput env name = (lookupFreshName fromLeft env, return (i name), UrlEncoded)
         fromLeft n Nothing         = FR.Success Nothing
         fromLeft n (Just (Left x)) = FR.Success (Just x)
         fromLeft n _               = FR.Failure [n ++ " could not be recognized."]

-- | A File input widget.
inputFile :: Monad m 
          => (String -> xml)  -- ^ Generates the xml for the file-upload widget based on the name
          -> Form xml m File
inputFile i = Form $ \env -> mkInput env <$> freshName
  where  mkInput env name    = (lookupFreshName fromRight env, return (i name), MultiPart)
         fromRight n Nothing          = FR.NotAvailable $ n ++ " is not in the data"
         fromRight n (Just (Right x)) = FR.Success x
         fromRight n _                = FR.Failure [n ++ " is not a file"]

-- | Runs the form state
runFormState :: Monad m 
             => Env               -- ^ A previously filled environment (may be empty)
             -> Form xml m a      -- ^ The form
             -> (m (Failing a), m xml, FormContentType)
runFormState e (Form f) = fmapFst3 (liftM FR.toE . liftM es) (es (f e))
  where es = flip evalState [0]

-- | Check a condition or convert a result
check :: (Monad m) => Form xml m a -> (a -> Failing b) -> Form xml m b
check (Form frm) f = Form $ fmap checker frm
 where checker = fmap $ fmapFst3 (liftM $ liftM $ f')
       f' (FR.Failure x)       = FR.Failure x
       f' (FR.NotAvailable x)  = FR.NotAvailable x
       f' (FR.Success x)       = FR.fromE $ f x

-- | Monadically check a condition or convert a result
checkM :: (Monad m) => Form xml m a -> (a -> m (Failing b)) -> Form xml m b
checkM (Form frm) f = Form $ \env -> checker f (frm env)
 where checker f frm = do currentState <- get
                          frm'         <- frm
                          return $ fmapFst3 (transform f. liftM (flip evalState currentState)) frm'
       transform f source = source >>= \x -> case x of 
                              FR.Success x      -> liftM return (convert f x)
                              FR.NotAvailable x -> return . return $ FR.NotAvailable x
                              FR.Failure x      -> return . return $ FR.Failure x
       convert :: Monad m => (a -> m (Failing b)) -> (a -> m (FR.FormResult b))
       convert f = fmap (liftM FR.fromE) f

instance (Functor m, Monad m) => Functor (Form xml m) where
  fmap f (Form a) = Form $ \env -> (fmap . fmapFst3 . liftM . liftM . fmap) f (a env)

fmapFst  f (a, b)    = (f a, b)
fmapFst3 f (a, b, c) = (f a, b, c)

instance (Monad m, Applicative m, Monoid xml) => Applicative (Form xml m) where
   pure = pureF
   (<*>) = applyF

-- | Pure xml
xml :: Monad m => xml -> Form xml m ()
xml x = Form $ \env -> pure (return (return $ FR.Success ()), return x, UrlEncoded)

-- | Transform the XML component
plug :: (Monad m, Monoid xml) => (xml -> xml1) -> Form xml m a -> Form xml1 m a
f `plug` (Form m) = Form $ \env -> pure plugin <*> m env
   where plugin (c, x, t) = (c, liftM f x, t)

plug' :: (Monad m, Monoid xml1) => (xml1 -> xml2) -> Formlet xml1 m a -> Formlet xml2 m a
plug' transformer formlet value = plug transformer (formlet value)

-- | This generates a single (or more) forms for a, and a parser function for a list of a's.
massInput :: (Applicative m, Monad m, Monoid xml)
          => (Formlet xml m a) -- ^ A formlet for a single a
          -> Formlet xml m [a]
massInput single defaults = Form $ \env -> do
  modify (\x -> 0:0:x)
  st <- get
  (collector, xml, contentType) <- (deform $ single Nothing) env
  resetCurrentLevel
  listXml <- generateListXml (single Nothing) env
  let newCollector = liftCollector st collector 
      xml' = case env of
               [] -> xml
               _  -> listXml
  x <- case maybe [] id defaults of
       [] -> return (newCollector, xml', contentType)
       xs -> do resetCurrentLevel
                xmls <- mapM (generateXml single env) xs
                let xmls' = sequence xmls 
                return (newCollector, liftM mconcat xmls', contentType)
  modify (tail.tail)
  return x

generateXml :: Monad m => (Maybe a -> Form xml m a) -> Env -> a -> S (m xml)
generateXml form env value = do (_, xml, _) <- (deform $ form $ Just value) env
                                modify nextItem
                                return xml

resetCurrentLevel :: S ()
resetCurrentLevel = do modify (tail . tail)
                       modify (\x -> 0:0:x)

generateListXml :: (Applicative m, Monad m, Monoid xml) => Form xml m a -> Env -> S (m xml)
generateListXml form env = do n <- currentName
                              case lookup n env of
                                Nothing -> return $ return mempty
                                Just _  -> do (_, xml, _) <- (deform form) env
                                              modify nextItem
                                              rest <- generateListXml form env
                                              return $ mappend <$> xml <*> rest

liftCollector :: (Monad m) => FormState -> m (Validator a) -> m (Validator [a])
liftCollector st coll = do coll' <- coll
                           let st'         = nextItem st
                               computeRest = liftCollector st' coll
                           case evalState coll' st of
                                 FR.Success x      -> do rest <- computeRest
                                                         return (fmap (fmap (x:)) rest)
                                 FR.NotAvailable x -> return (return (FR.Success []))
                                 FR.Failure x      -> do rest <- computeRest
                                                         return $ combineFailures x rest

nextItem st = flip execState st $ modify tail >> freshName >> modify (0:) >> get

combineFailures :: [String] -> Validator [a] -> Validator [a]
combineFailures msgs s = do x <- s
                            case x of
                                 FR.Success x -> return $ FR.Failure msgs
                                 FR.Failure f -> return $ FR.Failure (msgs ++ f)

                          
-- | Returns Nothing if the result is the empty String.
nothingIfNull :: (Monad m, Functor m) => Form xml m String -> Form xml m (Maybe String)
nothingIfNull frm = nullToMaybe <$> frm
 where nullToMaybe [] = Nothing
       nullToMaybe x  = Just x

-----------------------------------------------
-- Private methods
-----------------------------------------------

freshName :: S String
freshName = do n <- currentName
               modify (changeHead (+1))
               return n

-- TODO: think of a good name
changeHead f []     = error "changeHead: there is no head"
changeHead f (x:xs) = (f x) : xs

currentName :: S String
currentName = gets $ \xs ->  "fval[" ++ (intercalate "." $ reverse $ map show xs) ++ "]"

orT UrlEncoded x = x
orT x UrlEncoded = x
orT x y          = x

pureF :: (Monad m, Monoid xml) => a -> Form xml m a
pureF v = Form $ \env -> pure (return (return $ FR.Success v), return mempty, UrlEncoded)

pureM :: (Monad m, Monoid xml) => m a -> Form xml m a
pureM v = Form $ \env -> pure (liftM (return . FR.Success) v, return mempty, UrlEncoded)

applyF :: (Monad m, Applicative m, Monoid xml) => Form xml m (a -> b) -> Form xml m a -> Form xml m b
(Form f) `applyF` (Form v) = Form $ \env -> combine <$> f env <*> v env
  where combine (v1, xml1, t1) (v2, xml2, t2) = (first v1 v2, (mappend <$> xml1 <*> xml2), t1 `orT` t2)
        first :: Monad m
              => m (Validator (a -> b)) 
              -> m (Validator (a     )) 
              -> m (Validator (b     )) 
        first v1 v2 = do x <- v1
                         y <- v2
                         return $ do x'' <- x
                                     y'' <- y
                                     return (x'' <*> y'')