{-# LANGUAGE TemplateHaskell #-}

-- | Representation for queries going to remote schemas. Due to the existence of
-- remote relationships from remote schemas, we can't simply reuse the GraphQL
-- document AST we define in graphql-parser-hs, and instead redefine a custom
-- structure to represent such queries.
module Hasura.RQL.IR.RemoteSchema
  ( -- AST
    SelectionSet (..),
    DeduplicatedSelectionSet (..),
    dssCommonFields,
    dssMemberSelectionSets,
    ObjectSelectionSet,
    mkInterfaceSelectionSet,
    mkUnionSelectionSet,
    Field (..),
    _FieldGraphQL,
    _FieldRemote,
    GraphQLField (..),
    fAlias,
    fName,
    fArguments,
    fDirectives,
    fSelectionSet,
    mkGraphQLField,
    -- entry points
    RemoteSchemaRootField (..),
    SchemaRemoteRelationshipSelect (..),
    RemoteFieldArgument (..),
    RemoteSchemaSelect (..),
    -- AST conversion
    convertSelectionSet,
    convertGraphQLField,
  )
where

import Control.Lens.TH (makeLenses, makePrisms)
import Data.HashMap.Strict qualified as Map
import Data.HashMap.Strict.InsOrd.Extended qualified as OMap
import Data.HashSet qualified as Set
import Data.List.Extended (longestCommonPrefix)
import Hasura.GraphQL.Parser.Constants qualified as G
import Hasura.GraphQL.Parser.Schema as G (InputValue)
import Hasura.Prelude
import Hasura.RQL.Types.Common (FieldName)
import Hasura.RQL.Types.Relationships.ToSchema
import Hasura.RQL.Types.RemoteSchema
import Hasura.RQL.Types.RemoteSchema qualified as RQL
import Hasura.RQL.Types.ResultCustomization
import Hasura.RQL.Types.ResultCustomization qualified as RQL
import Language.GraphQL.Draft.Syntax qualified as G

-------------------------------------------------------------------------------
-- Custom AST

-- | Custom representation of a selection set.
--
-- Similarly to other parts of the IR, the @r@ argument is used for remote
-- relationships.
data SelectionSet r var
  = SelectionSetObject !(ObjectSelectionSet r var)
  | SelectionSetUnion !(DeduplicatedSelectionSet r var)
  | SelectionSetInterface !(DeduplicatedSelectionSet r var)
  | SelectionSetNone
  deriving (Show, Eq, Functor, Foldable, Traversable)

-- | Representation of the normalized selection set of an interface/union type.
--
-- This representation is used to attempt to minimize the size of the GraphQL
-- query that eventually gets sent to the GraphQL server by defining as many
-- fields as possible on the abstract type.
data DeduplicatedSelectionSet r var = DeduplicatedSelectionSet
  { -- | Fields that aren't explicitly defined for member types
    _dssCommonFields :: !(Set.HashSet G.Name),
    -- | SelectionSets of individual member types
    _dssMemberSelectionSets :: !(Map.HashMap G.Name (ObjectSelectionSet r var))
  }
  deriving (Show, Eq, Functor, Foldable, Traversable, Generic)

type ObjectSelectionSet r var = OMap.InsOrdHashMap G.Name (Field r var)

-- | Constructs an 'InterfaceSelectionSet' from a set of interface fields and an
-- association list of the fields. This function ensures that @__typename@ is
-- present in the set of interface fields.
mkInterfaceSelectionSet ::
  -- | Member fields of the interface
  Set.HashSet G.Name ->
  -- | Selection sets for all the member types
  [(G.Name, ObjectSelectionSet r var)] ->
  DeduplicatedSelectionSet r var
mkInterfaceSelectionSet interfaceFields selectionSets =
  DeduplicatedSelectionSet
    (Set.insert G.___typename interfaceFields)
    (Map.fromList selectionSets)

-- | Constructs an 'UnionSelectionSet' from a list of the fields, using a
-- singleton set of @__typename@ for the set of common fields.
mkUnionSelectionSet ::
  -- | Selection sets for all the member types
  [(G.Name, ObjectSelectionSet r var)] ->
  DeduplicatedSelectionSet r var
mkUnionSelectionSet selectionSets =
  DeduplicatedSelectionSet
    (Set.singleton G.___typename)
    (Map.fromList selectionSets)

-- | Representation of one individual field.
--
-- This particular type is the reason why we need a different representation
-- from the one in 'graphql-parser-hs': we differentiate between selection
-- fields that target the actual remote schema, and fields that, instead, are
-- remote from it and need to be treated differently.
data Field r var
  = FieldGraphQL !(GraphQLField r var)
  | FieldRemote !(SchemaRemoteRelationshipSelect r)
  deriving (Show, Eq, Functor, Foldable, Traversable)

-- | Normalized representation of a GraphQL field.
--
-- This type is almost identical to 'G.Field', except for the fact that the
-- selection set is our annotated 'SelectionSet', instead of the original
-- 'G.SelectionSet'. We use this type to represent the fields of a selection
-- that do target the remote schema.
data GraphQLField r var = GraphQLField
  { _fAlias :: G.Name,
    _fName :: G.Name,
    _fArguments :: HashMap G.Name (G.Value var),
    _fDirectives :: [G.Directive var],
    _fSelectionSet :: SelectionSet r var
  }
  deriving (Show, Eq, Functor, Foldable, Traversable)

mkGraphQLField ::
  Maybe G.Name ->
  G.Name ->
  HashMap G.Name (G.Value var) ->
  [G.Directive var] ->
  SelectionSet r var ->
  GraphQLField r var
mkGraphQLField alias name =
  GraphQLField (fromMaybe name alias) name

-------------------------------------------------------------------------------
-- Remote schema entry points

-- | Root entry point for a remote schema.
data RemoteSchemaRootField r var = RemoteSchemaRootField
  { _rfRemoteSchemaInfo :: RQL.RemoteSchemaInfo,
    _rfResultCustomizer :: RQL.ResultCustomizer,
    _rfField :: GraphQLField r var
  }
  deriving (Functor, Foldable, Traversable)

-- | A remote relationship's selection and fields required for its join condition.
data SchemaRemoteRelationshipSelect r = SchemaRemoteRelationshipSelect
  { -- | The fields on the table that are required for the join condition
    -- of the remote relationship
    _srrsLHSJoinFields :: HashMap FieldName G.Name,
    -- | The field that captures the relationship
    -- r ~ (RemoteRelationshipField UnpreparedValue) when the AST is emitted by the parser.
    -- r ~ Void when an execution tree is constructed so that a backend is
    -- absolved of dealing with remote relationships.
    _srrsRelationship :: r
  }
  deriving (Eq, Show, Functor, Foldable, Traversable)

data RemoteFieldArgument = RemoteFieldArgument
  { _rfaArgument :: G.Name,
    _rfaValue :: InputValue RemoteSchemaVariable
  }
  deriving (Eq, Show)

data RemoteSchemaSelect r = RemoteSchemaSelect
  { _rselArgs :: [RemoteFieldArgument],
    _rselResultCustomizer :: ResultCustomizer,
    _rselSelection :: SelectionSet r RemoteSchemaVariable,
    _rselFieldCall :: NonEmpty FieldCall,
    _rselRemoteSchema :: RemoteSchemaInfo
  }

-------------------------------------------------------------------------------
-- Conversion back to a GraphQL document

-- | Converts a normalized selection set back into a selection set as defined in
-- GraphQL spec, in order to send it to a remote server.
--
-- This function expects a 'SelectionSet' for which @r@ is 'Void', which
-- guarantees that there is no longer any remote join field in the selection
-- set.
convertSelectionSet ::
  forall var.
  Eq var =>
  SelectionSet Void var ->
  G.SelectionSet G.NoFragments var
convertSelectionSet = \case
  SelectionSetObject s -> convertObjectSelectionSet s
  SelectionSetUnion s -> convertAbstractTypeSelectionSet s
  SelectionSetInterface s -> convertAbstractTypeSelectionSet s
  SelectionSetNone -> mempty
  where
    convertField :: Field Void var -> G.Field G.NoFragments var
    convertField = \case
      FieldGraphQL f -> convertGraphQLField f

    convertObjectSelectionSet =
      map (G.SelectionField . convertField . snd) . OMap.toList

    convertAbstractTypeSelectionSet abstractSelectionSet =
      let (base, members) = reduceAbstractTypeSelectionSet abstractSelectionSet
          commonFields = convertObjectSelectionSet base
          concreteTypeSelectionSets =
            Map.toList members <&> \(concreteType, selectionSet) ->
              G.InlineFragment
                { G._ifTypeCondition = Just concreteType,
                  G._ifDirectives = mempty,
                  G._ifSelectionSet = convertObjectSelectionSet selectionSet
                }
       in -- The base selection set first and then the more specific member
          -- selection sets. Note that the rendering strategy here should be
          -- inline with the strategy used in `mkAbstractTypeSelectionSet`
          commonFields <> map G.SelectionInlineFragment concreteTypeSelectionSets

convertGraphQLField :: Eq var => GraphQLField Void var -> G.Field G.NoFragments var
convertGraphQLField GraphQLField {..} =
  G.Field
    { -- add the alias only if it is different from the field name. This
      -- keeps the outbound request more readable
      G._fAlias = if _fAlias /= _fName then Just _fAlias else Nothing,
      G._fName = _fName,
      G._fArguments = _fArguments,
      G._fDirectives = mempty,
      G._fSelectionSet = convertSelectionSet _fSelectionSet
    }

-- | Builds the selection set for an abstract type.
--
-- Let's consider this query on starwars API:
-- The type `Node` an interface is implemented by `Film`, `Species`, `Planet`,
-- `Person`, `Starship`, `Vehicle`
--
-- query f {
--   node(id: "ZmlsbXM6MQ==") {
--     __typename
--     id
--     ... on Film {
--       title
--     }
--     ... on Species {
--       name
--     }
--   }
-- }
--
-- When we parse this, it gets normalized into this query:
--
-- query f {
--   node(id: "ZmlsbXM6MQ==") {
--     ... on Film {
--       __typename: __typename
--       id
--       title
--     }
--     ... on Species {
--       __typename: __typename
--       id
--       name
--     }
--     ... on Planet {
--       __typename: __typename
--       id
--     }
--     ... on Person {
--       __typename: __typename
--       id
--     }
--     ... on Starship {
--       __typename: __typename
--       id
--     }
--     ... on Vehicle {
--       __typename: __typename
--       id
--     }
--   }
-- }
--
-- `__typename` and `id` get pushed to each of the member types. From the above
-- normalized selection set, we want to costruct a query as close to the
-- original as possible. We do this as follows:
--
-- 1. find the longest common set of fields that each selection set starts with
--    (in the above case, they are `__typename` and `id`)
-- 2. from the above list of fields, find the first field that cannot be
--    defined on the abstract type. The fields that can be defined on the
--    abstract type are all the fields that occur before the first non abstract
--    type field (in the above case, both` __typename` and `id` can be defined
--    on the `Node` type)
-- 3. Strip the base selection set fields from all the member selection sets and
--    filter out the member type selection sets that are subsumed by the base
--    selection set
--
-- The above query now translates to this:
--
-- query f {
--   node(id: "ZmlsbXM6MQ==") {
--     __typename: __typename
--     id
--     ... on Film {
--       title
--     }
--     ... on Species {
--       name
--     }
--   }
-- }
--
-- Note that it is not always possible to get the same shape as the original
-- query and there is more than one approach to this. For example, we could
-- have picked the selection set (that can be defined on the abstract type)
-- that is common across all the member selection sets and used that as the
-- base selection.
reduceAbstractTypeSelectionSet ::
  (Eq var) =>
  DeduplicatedSelectionSet Void var ->
  (ObjectSelectionSet Void var, Map.HashMap G.Name (ObjectSelectionSet Void var))
reduceAbstractTypeSelectionSet (DeduplicatedSelectionSet baseMemberFields selectionSets) =
  (baseSelectionSet, Map.fromList memberSelectionSets)
  where
    sharedSelectionSetPrefix = longestCommonPrefix $ map (OMap.toList . snd) $ Map.toList selectionSets

    baseSelectionSet = OMap.fromList $ takeWhile (shouldAddToBase . snd) sharedSelectionSetPrefix

    shouldAddToBase = \case
      FieldGraphQL f -> Set.member (_fName f) baseMemberFields

    memberSelectionSets =
      -- remove member selection sets that are subsumed by base selection set
      filter (not . null . snd) $
        -- remove the common prefix from member selection sets
        map (second (OMap.fromList . drop (OMap.size baseSelectionSet) . OMap.toList)) $ Map.toList selectionSets

-------------------------------------------------------------------------------
-- TH lens generation

$(makePrisms ''Field)
$(makeLenses ''GraphQLField)
$(makeLenses ''DeduplicatedSelectionSet)