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+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE QuasiQuotes #-}
+
+module Main
+  ( main
+  ) where
+
+import Control.Arrow ((&&&))
+import Control.Monad (when)
+import Data.Version (showVersion)
+import GHC.Generics (Generic)
+import System.Environment (getArgs)
+import System.Exit (die)
+
+import Data.Aeson (FromJSON, eitherDecode, encode)
+import qualified Data.ByteString.Lazy as L
+import qualified Data.HashMap.Strict as HM
+import qualified System.Console.Docopt.NoTH as O
+import Text.InterpolatedString.Perl6 (qc)
+
+import Malodivo.Budget (manyToOne)
+import Malodivo.Types.Bill (Bill)
+import Malodivo.Types.District (District)
+
+import Paths_malodivo (version) -- from cabal
+
+usage :: String
+usage =
+  "malodivo " ++
+  showVersion version ++
+  " - budget planning tool for the Kingdom of Malodivo" ++
+  [qc|
+
+This utility reads input JSON data from standard input
+and writes output JSON data to standard output.
+
+Usage: malodivo [options] < input.json > output.json
+
+Options:
+
+  -h, --help               Show this message and exit
+
+
+|]
+
+data DistrictInfo = DistrictInfo
+  { name :: District
+  , availableFunds :: Integer
+  } deriving (Generic, FromJSON)
+
+data SimpleInput = SimpleInput
+  { bills :: [Bill]
+  , districts :: [DistrictInfo]
+  } deriving (Generic, FromJSON)
+
+process :: IO ()
+process = do
+  input <- L.getContents
+  case eitherDecode input of
+    Left err -> die err
+    Right si -> do
+      let nbills = length . bills $ si
+          funds =
+            HM.fromListWith (+) . map (name &&& availableFunds) $ districts si
+      when (nbills /= 1) $ die "We needs exactly one bill in input"
+      when (HM.null funds) $ die "We needs at least one district"
+      L.putStr . encode $ manyToOne funds (head . bills $ si)
+
+main :: IO ()
+main = do
+  doco <- O.parseUsageOrExit usage
+  args <- O.parseArgsOrExit doco =<< getArgs
+  if args `O.isPresent` O.longOption "help"
+    then putStrLn $ O.usage doco
+    else process