{-# LANGUAGE RecordWildCards #-}

module Main where

import           Control.Applicative
import           Control.Exception
import           Data.Aeson
import           Data.Aeson.Diff
import qualified Data.ByteString.Char8     as BS
import qualified Data.ByteString.Lazy      as BSL
import           Data.Monoid
import           Options.Applicative       hiding (Success)
import           Options.Applicative.Types hiding (Success)
import           System.IO

type File = Maybe FilePath

-- | Command-line options.
data Options = Options
    { optionOut   :: File -- ^ JSON destination
    , optionPatch :: File -- ^ Patch input
    , optionFrom  :: File -- ^ JSON source
    }

data Configuration = Configuration
    { cfgOut   :: Handle
    , cfgPatch :: Handle
    , cfgFrom  :: Handle
    }

optionParser :: Parser Options
optionParser = Options
    <$> option fileP
        (  long "output"
        <> short 'o'
        <> metavar "OUTPUT"
        <> help "Destination for patched JSON."
        <> value Nothing
        )
    <*> argument fileP
        (  metavar "PATCH"
        <> help "Patch to apply."
        )
    <*> argument fileP
        (  metavar "FROM"
        <> help "JSON file to patch."
        )
  where
    fileP = do
        s <- readerAsk
        return $ case s of
            "-" -> Nothing
            _ -> Just s

jsonRead :: Handle -> IO Value
jsonRead fp = do
    s <- BS.hGetContents fp
    case decode (BSL.fromStrict s) of
        Nothing -> error "Could not parse as JSON"
        Just v -> return v

run :: Options -> IO ()
run opt = bracket (load opt) close process
  where
    openr :: Maybe FilePath -> IO Handle
    openr Nothing = return stdin
    openr (Just p) = openFile p ReadMode

    openw :: Maybe FilePath -> IO Handle
    openw Nothing = return stdout
    openw (Just p) = openFile p WriteMode

    load :: Options -> IO Configuration
    load Options{..} =
        Configuration
            <$> openw optionOut
            <*> openr optionPatch
            <*> openr optionFrom

    close :: Configuration -> IO ()
    close Configuration{..} = do
        hClose cfgPatch
        hClose cfgFrom
        hClose cfgOut

process :: Configuration -> IO ()
process Configuration{..} = do
    json_patch <- jsonRead cfgPatch
    json_from <- jsonRead cfgFrom
    case fromJSON json_patch >>= flip patch json_from of
        Error e -> error e
        Success d -> BS.hPutStrLn cfgOut $ BSL.toStrict (encode d)

main :: IO ()
main = execParser opts >>= run
  where
    opts = info (helper <*> optionParser)
     (  fullDesc
     <> progDesc "Generate a patch between two JSON documents.")