{-| JSON utility functions. -}

{-

Copyright (C) 2009, 2010, 2011, 2012 Google Inc.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301, USA.

-}

module Ganeti.HTools.JSON
  ( fromJResult
  , readEitherString
  , JSRecord
  , loadJSArray
  , fromObj
  , maybeFromObj
  , fromObjWithDefault
  , fromKeyValue
  , fromJVal
  , asJSObject
  , asObjectList
  , tryFromObj
  , toArray
  )
  where

import Control.Monad (liftM)
import Data.Maybe (fromMaybe)
import Text.Printf (printf)

import qualified Text.JSON as J

import Ganeti.BasicTypes

-- * JSON-related functions

-- | A type alias for the list-based representation of J.JSObject.
type JSRecord = [(String, J.JSValue)]

-- | Converts a JSON Result into a monadic value.
fromJResult :: Monad m => String -> J.Result a -> m a
fromJResult s (J.Error x) = fail (s ++ ": " ++ x)
fromJResult _ (J.Ok x) = return x

-- | Tries to read a string from a JSON value.
--
-- In case the value was not a string, we fail the read (in the
-- context of the current monad.
readEitherString :: (Monad m) => J.JSValue -> m String
readEitherString v =
  case v of
    J.JSString s -> return $ J.fromJSString s
    _ -> fail "Wrong JSON type"

-- | Converts a JSON message into an array of JSON objects.
loadJSArray :: (Monad m)
               => String -- ^ Operation description (for error reporting)
               -> String -- ^ Input message
               -> m [J.JSObject J.JSValue]
loadJSArray s = fromJResult s . J.decodeStrict

-- | Reads the value of a key in a JSON object.
fromObj :: (J.JSON a, Monad m) => JSRecord -> String -> m a
fromObj o k =
  case lookup k o of
    Nothing -> fail $ printf "key '%s' not found, object contains only %s"
               k (show (map fst o))
    Just val -> fromKeyValue k val

-- | Reads the value of an optional key in a JSON object.
maybeFromObj :: (J.JSON a, Monad m) =>
                JSRecord -> String -> m (Maybe a)
maybeFromObj o k =
  case lookup k o of
    Nothing -> return Nothing
    Just val -> liftM Just (fromKeyValue k val)

-- | Reads the value of a key in a JSON object with a default if missing.
fromObjWithDefault :: (J.JSON a, Monad m) =>
                      JSRecord -> String -> a -> m a
fromObjWithDefault o k d = liftM (fromMaybe d) $ maybeFromObj o k

-- | Reads a JValue, that originated from an object key.
fromKeyValue :: (J.JSON a, Monad m)
              => String     -- ^ The key name
              -> J.JSValue  -- ^ The value to read
              -> m a
fromKeyValue k val =
  fromJResult (printf "key '%s'" k) (J.readJSON val)

-- | Small wrapper over readJSON.
fromJVal :: (Monad m, J.JSON a) => J.JSValue -> m a
fromJVal v =
  case J.readJSON v of
    J.Error s -> fail ("Cannot convert value '" ++ show v ++
                       "', error: " ++ s)
    J.Ok x -> return x

-- | Converts a JSON value into a JSON object.
asJSObject :: (Monad m) => J.JSValue -> m (J.JSObject J.JSValue)
asJSObject (J.JSObject a) = return a
asJSObject _ = fail "not an object"

-- | Coneverts a list of JSON values into a list of JSON objects.
asObjectList :: (Monad m) => [J.JSValue] -> m [J.JSObject J.JSValue]
asObjectList = mapM asJSObject

-- | Try to extract a key from a object with better error reporting
-- than fromObj.
tryFromObj :: (J.JSON a) =>
              String     -- ^ Textual "owner" in error messages
           -> JSRecord   -- ^ The object array
           -> String     -- ^ The desired key from the object
           -> Result a
tryFromObj t o = annotateResult t . fromObj o

-- | Ensure a given JSValue is actually a JSArray.
toArray :: (Monad m) => J.JSValue -> m [J.JSValue]
toArray (J.JSArray arr) = return arr
toArray o = fail $ "Invalid input, expected array but got " ++ show o