{-| This module implements /fragment inlining/, which converts all fragment
spreads in a GraphQL query to inline fragments. For example, given a query like

> query {
>   users {
>     id
>     ...userFields
>   }
> }
>
> fragment userFields on User {
>   name
>   favoriteColor
> }

the fragment inliner will convert it to this:

> query {
>   users {
>     id
>     ... on User {
>       name
>       favoriteColor
>     }
>   }
> }

This is a straightforward and mechanical transformation, but it simplifies
further processing, since we catch unbound fragments and recursive fragment
definitions early in the pipeline, so parsing does not have to worry about it.
In that sense, fragment inlining is similar to the variable resolution pass
performed by "Hasura.GraphQL.Execute.Resolve", but for fragment definitions
rather than variables. -}
module Hasura.GraphQL.Execute.Inline
  ( inlineSelectionSet
  ) where

import           Hasura.Prelude

import qualified Data.HashMap.Strict.Extended  as Map
import qualified Data.HashSet                  as Set
import qualified Data.List                     as L
import qualified Data.Text                     as T

import           Control.Lens
import           Data.Text.Extended
import           Language.GraphQL.Draft.Syntax

import           Hasura.Base.Error
import           Hasura.Server.Utils

-- | Internal bookkeeping used during inlining.
data InlineEnv = InlineEnv
  { _ieFragmentDefinitions :: HashMap Name FragmentDefinition
  -- ^ All known fragment definitions.
  , _ieFragmentStack       :: [Name]
  -- ^ Fragments we’re currently inlining higher up in the call stack, used to
  -- detect fragment cycles.
  }

-- | Internal bookkeeping used during inlining.
newtype InlineState = InlineState
  { _isFragmentCache :: HashMap Name (InlineFragment NoFragments Name)
  -- ^ A cache of fragment definitions we’ve already inlined, so we don’t need
  -- to inline them again.
  }

$(makeLensesFor [("_ieFragmentStack", "ieFragmentStack")] ''InlineEnv)
$(makeLenses ''InlineState)

type MonadInline m =
  ( MonadError QErr m
  , MonadReader InlineEnv m
  , MonadState InlineState m
  )

-- | Inlines all fragment spreads in a 'SelectionSet'; see the module
-- documentation for "Hasura.GraphQL.Execute.Inline" for details.
inlineSelectionSet
  :: (MonadError QErr m, Foldable t)
  => t FragmentDefinition
  -> SelectionSet FragmentSpread Name
  -> m (SelectionSet NoFragments Name)
inlineSelectionSet fragmentDefinitions selectionSet = do
  let fragmentDefinitionMap = Map.groupOnNE _fdName fragmentDefinitions
  uniqueFragmentDefinitions <- flip Map.traverseWithKey fragmentDefinitionMap
    \fragmentName fragmentDefinitions' ->
      case fragmentDefinitions' of
        a :| [] -> return a
        _       -> throw400 ParseFailed $ "multiple definitions for fragment " <>> fragmentName
  let usedFragmentNames = Set.fromList $ fragmentsInSelectionSet selectionSet
      definedFragmentNames = Set.fromList $ Map.keys uniqueFragmentDefinitions
      -- At the time of writing, this check is disabled using
      -- a local binding because, the master branch doesn't implement this
      -- check.
      -- TODO: Do this check using a feature flag
      isFragmentValidationEnabled = False
  when (isFragmentValidationEnabled && (usedFragmentNames /= definedFragmentNames)) $
    throw400 ValidationFailed $
    "following fragment(s) have been defined, but have not been used in the query - "
    <> T.concat (L.intersperse ", "
                 $ map unName $ Set.toList $
                 Set.difference definedFragmentNames usedFragmentNames)
  traverse inlineSelection selectionSet
    & flip evalStateT InlineState{ _isFragmentCache = mempty }
    & flip runReaderT InlineEnv
    { _ieFragmentDefinitions = uniqueFragmentDefinitions
    , _ieFragmentStack = [] }
  where
    fragmentsInSelectionSet :: SelectionSet FragmentSpread Name -> [Name]
    fragmentsInSelectionSet selectionSet' = concatMap getFragFromSelection selectionSet'

    getFragFromSelection :: Selection FragmentSpread Name -> [Name]
    getFragFromSelection = \case
      SelectionField fld -> fragmentsInSelectionSet $ _fSelectionSet fld
      SelectionFragmentSpread fragmentSpread -> [_fsName fragmentSpread]
      SelectionInlineFragment inlineFragment -> fragmentsInSelectionSet $ _ifSelectionSet inlineFragment

inlineSelection
  :: MonadInline m
  => Selection FragmentSpread Name
  -> m (Selection NoFragments Name)
inlineSelection (SelectionField field@Field{ _fSelectionSet }) =
  withPathK "selectionSet" $ withPathK (unName $ _fName field) $ do
    selectionSet <- traverse inlineSelection _fSelectionSet
    pure $! SelectionField field{ _fSelectionSet = selectionSet }
inlineSelection (SelectionFragmentSpread spread) =
  withPathK "selectionSet" $
    SelectionInlineFragment <$> inlineFragmentSpread spread
inlineSelection (SelectionInlineFragment fragment@InlineFragment{ _ifSelectionSet }) = do
  selectionSet <- traverse inlineSelection _ifSelectionSet
  pure $! SelectionInlineFragment fragment{ _ifSelectionSet = selectionSet }

inlineFragmentSpread
  :: MonadInline m
  => FragmentSpread Name
  -> m (InlineFragment NoFragments Name)
inlineFragmentSpread FragmentSpread{ _fsName, _fsDirectives } = do
  InlineEnv{ _ieFragmentDefinitions, _ieFragmentStack } <- ask
  InlineState{ _isFragmentCache } <- get

  if -- If we’ve already inlined this fragment, no need to process it again.
     | Just fragment <- Map.lookup _fsName _isFragmentCache ->
       pure $! addSpreadDirectives fragment

     -- Fragment cycles are always illegal; see
     -- http://spec.graphql.org/June2018/#sec-Fragment-spreads-must-not-form-cycles
     | (fragmentCycle, _:_) <- break (== _fsName) _ieFragmentStack ->
       throw400 ValidationFailed $ "the fragment definition(s) "
         <> englishList "and" (toTxt <$> (_fsName :| reverse fragmentCycle))
         <> " form a cycle"

     -- We didn’t hit the fragment cache, so look up the definition and convert
     -- it to an inline fragment.
     | Just FragmentDefinition{ _fdTypeCondition, _fdSelectionSet }
         <- Map.lookup _fsName _ieFragmentDefinitions -> withPathK (unName _fsName) $ do

       selectionSet <- locally ieFragmentStack (_fsName :) $
         traverse inlineSelection _fdSelectionSet

       let fragment = InlineFragment
             { _ifTypeCondition = Just _fdTypeCondition
             -- As far as I can tell, the GraphQL spec says that directives
             -- on the fragment definition do NOT apply to the fields in its
             -- selection set.
             , _ifDirectives = []
             , _ifSelectionSet = selectionSet
             }
       modify' $ over isFragmentCache $ Map.insert _fsName fragment
       pure $! addSpreadDirectives fragment

     -- If we get here, the fragment name is unbound; raise an error.
     -- http://spec.graphql.org/June2018/#sec-Fragment-spread-target-defined
     | otherwise -> throw400 ValidationFailed $
       "reference to undefined fragment " <>> _fsName
  where
    addSpreadDirectives fragment =
      fragment{ _ifDirectives = _ifDirectives fragment ++ _fsDirectives }