{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE ViewPatterns #-}

-- Features of https://microsoft.github.io/language-server-protocol/specification.html
module HsInspect.LSP.Impl where

import Control.Monad.Extra (fromMaybeM)
import Control.Monad.IO.Class (liftIO)
import Control.Monad.Trans.Except (ExceptT(..))
import Control.Monad.Trans.Except (throwE)
import Data.Cache (Cache)
import qualified Data.Cache as C
import Data.List.Extra (firstJust)
import Data.Maybe (listToMaybe)
import Data.Maybe (fromMaybe)
import Data.Text (Text)
import qualified Data.Text as T
import Debug.Trace (traceShowId)
import qualified FastString as GHC
import qualified GHC as GHC
import GHC.Paths (libdir)
import HsInspect.LSP.Context
import HsInspect.LSP.HsInspect
import qualified Lexer as GHC
import qualified SrcLoc as GHC
import qualified StringBuffer as GHC
import System.FilePath (takeDirectory)

-- TODO consider invalidation strategies, e.g. Import could use the file's
--      header, and the Context use the .ghc.* files. The index might also want
--      to hash the .ghc.flags contents plus any changes to exported symbols in
--      the current package. But beware that often it is better to have a stale
--      cache and respond with *something* than to be slow and redo the work.
data Caches = Caches
  (Cache FilePath Context) -- ^ by source root
  (Cache FilePath [Import]) -- ^ by source filename
  (Cache FilePath [Package]) -- ^ by source root

-- TODO the index could use a data structure that is faster to search

cachedContext :: Caches -> BuildTool -> FilePath -> ExceptT String IO Context
cachedContext (Caches cache _ _) tool file = do
  key <- takeDirectory <$> discoverGhcflags file
  let work = do
        ctx <- findContext file tool
        liftIO $ C.insert cache key ctx
        pure ctx
  fromMaybeM work . liftIO $ C.lookup cache key

cachedImports :: Caches -> Context -> FilePath -> ExceptT String IO [Import]
cachedImports (Caches _ cache _) ctx file =
  C.fetchWithCache cache file $ hsinspect_imports ctx

cachedIndex :: Caches -> Context -> ExceptT String IO [Package]
cachedIndex (Caches _ _ cache) ctx =
  C.fetchWithCache cache (srcdir ctx) $ \_ -> hsinspect_index ctx

-- only lookup the index, don't try to populate it
cachedIndex' :: Caches -> Context -> ExceptT String IO [Package]
cachedIndex' (Caches _ _ cache) ctx = liftIO $
  fromMaybe [] <$> C.lookup cache (srcdir ctx)

-- zero indexed
data Span = Span Int Int Int Int -- line, col, line, col

findType :: Text -> [Package] -> Maybe Text
findType qual pkgs = listToMaybe $ do
  let (T.init -> module', sym) = T.breakOnEnd "." qual
  let flatten Nothing = []
      flatten (Just as) = as
  pkg <- pkgs
  Module module'' entries <- flatten . _modules $ pkg
  if module'' /= module'
  then []
  else do
    e <- traceShowId $ flatten entries
    let matcher name typ = if name == sym && name /= typ then [typ] else []
    case e of
      Id _ name typ -> matcher name typ
      Con _ name typ -> matcher name typ
      Pat _ name typ -> matcher name typ
      TyCon _ _ _ -> []

hoverProvider :: Caches -> BuildTool -> FilePath -> (Int, Int) -> ExceptT String IO (Maybe (Span, Text))
hoverProvider caches tool file position = do
  ctx <- cachedContext caches tool file
  symbols <- cachedImports caches ctx file
  index <- cachedIndex' caches ctx
  found <- symbolAtPoint file position
  pure $ case found of
    Nothing -> Nothing
    Just (range, sym) ->
      let matcher imp = if _local imp == Just sym || _qual imp == Just sym || _full imp == sym
                        then Just $ case findType (_full imp) index of
                          Just typ -> _full imp <> " :: " <> typ
                          Nothing -> _full imp
                        else Nothing
       in (range,) <$> firstJust matcher symbols

-- c.f. haskell-tng--hsinspect-symbol-at-point
--
-- TODO consider replacing this (inefficient) ghc api usage with a regexp or
-- calling the specific lexer functions directly for the symbols we support.
symbolAtPoint :: FilePath -> (Int, Int) -> ExceptT String IO (Maybe (Span, Text))
symbolAtPoint file (line, col) = do
  buf' <- liftIO $ GHC.hGetStringBuffer file
  -- TODO for performance, and language extension reliability, find out how to
  --      start the lexer on the correct line, to avoid lexing stuff we don't
  --      care about.
  buf <- maybe (throwE "line doesn't exist") pure $ GHC.atLine 1 buf'
  let file' = GHC.mkFastString file
      startLoc = GHC.mkRealSrcLoc file' 1 1
      -- add 1 to the column because GHC.containsSpan doesn't seem to like it
      -- when the point is on the very first character of the span and if we
      -- must pick between the first or last char, we prefer the first.
      point = GHC.realSrcLocSpan $ GHC.mkRealSrcLoc file' line (col + 1)
  -- TODO construct the real dflags from the Context (remove libdir dependency)
  dflags <- liftIO . GHC.runGhc (Just libdir) $ GHC.getSessionDynFlags
  -- lexTokenStream :: StringBuffer -> RealSrcLoc -> DynFlags -> ParseResult [Located Token]
  case GHC.lexTokenStream buf startLoc dflags of
    GHC.POk _ ts  ->
      let containsPoint :: (GHC.Located GHC.Token, String) -> Maybe (Span, Text)
          containsPoint ((GHC.L (GHC.UnhelpfulSpan _) _), _) = Nothing
          containsPoint ((GHC.L (GHC.RealSrcSpan s) _), txt) =
            if GHC.containsSpan s point then Just (toSpan s, T.pack txt) else Nothing
          toSpan src = Span
            (GHC.srcSpanStartLine src - 1)
            (GHC.srcSpanStartCol src - 1)
            (GHC.srcSpanEndLine src - 1)
            (GHC.srcSpanEndCol src - 1)

       in pure . firstJust containsPoint $ GHC.addSourceToTokens startLoc buf ts

    _ -> throwE "lexer error" -- TODO getErrorMessages