Module      : JSONSchema.Draft4.SchemaUnification
  Description : Unification of multiple schemas
  Copyright   : (c) Gareth Tan, 2017
  License     : MIT

  Contains a function to combine multiple JSON schemas

module JSONSchema.Draft4.SchemaUnification
  ( unifySchemas
  ) where

import           Protolude                          hiding ((<>))

import qualified Data.HashMap.Lazy                  as HM
import qualified Data.Set                           as DS
import qualified JSONSchema.Draft4                  as D4

import qualified JSONSchema.Validator.Draft4.Array  as V4Arr
import qualified JSONSchema.Validator.Draft4.Object as V4Obj

import           Data.Semigroup                     ((<>))

import qualified JSONSchema.Draft4.Internal.Utils   as Utils

{-| The primary function used to combine multiple JSON schemas.

    This relation cannot be the binary operation of a monoid because
    the empty schema will act to remove the `required` property of
    any JSON Schema it is unified with.

    This relation is also not commutative because we arbitrarily select
    from the alternatives for properties such as the schema @version@
    that cannot be unified. In addition, when one items schema is
    an array and another is an object, we simply choose arbitrarily
    because there is no sensible way of unifying the schemas while
    preserving all relevant information.
unifySchemas :: D4.Schema -> D4.Schema -> D4.Schema
unifySchemas nextSchema =
  unifyObjectConstraints nextSchema .
  unifyArrayConstraints nextSchema .

  unifyStringConstraints nextSchema .
  unifyNumericConstraints nextSchema .

  unifyAnyInstanceConstraints nextSchema .
  unifyNonvalidatingConstraints nextSchema

-- The linear unifier extracts an array of Maybes and filters them
-- to only the Just values. We then use foldr1May to fold across the
-- list.. If there were no Just values, then
-- the foldr1 fails and we get Nothing. Otherwise, the present Just
-- values are folded together using foldF.

-- |The alternative unifier applies the binary function if both are Just, returns the identity
-- if only one is Just, and Nothing if both are Nothing.
altUnifier :: (a -> a -> a) -> (b -> Maybe a) -> b -> b -> Maybe a
altUnifier binF getter next acc = applied <|> getter next <|> getter acc
  where applied = applicativeUnifier binF getter next acc

-- The applicative unifier applies the binary function but returns Nothing if either
-- is Nothing in the manner of an applicative.
applicativeUnifier :: (a -> a -> a) -> (b -> Maybe a) -> b -> b -> Maybe a
applicativeUnifier binF getter next acc = binF <$> getter next <*> getter acc

unifyNonvalidatingConstraints :: D4.Schema -> D4.Schema -> D4.Schema
unifyNonvalidatingConstraints nextSchema accSchema = accSchema {
      D4._schemaVersion = altUnifier const D4._schemaVersion nextSchema accSchema
    , D4._schemaId = altUnifier const D4._schemaId nextSchema accSchema
    , D4._schemaRef = altUnifier const D4._schemaRef nextSchema accSchema
    , D4._schemaDefinitions = altUnifier const D4._schemaDefinitions nextSchema accSchema
    , D4._schemaDependencies = altUnifier const D4._schemaDependencies nextSchema accSchema
    , D4._schemaOther = foldr HM.union HM.empty (fmap D4._schemaOther [nextSchema, accSchema])

unifyNumericConstraints :: D4.Schema -> D4.Schema -> D4.Schema
unifyNumericConstraints nextSchema accSchema =
       { D4._schemaMaximum = maxConstraint
       , D4._schemaExclusiveMaximum = emaxConstraint
       , D4._schemaMinimum = minConstraint
       , D4._schemaExclusiveMinimum = eminConstraint
       , D4._schemaMultipleOf = altUnifier const D4._schemaMultipleOf nextSchema accSchema
    schemas = [nextSchema, accSchema]
    maxes = fmap D4._schemaMaximum schemas
    emaxes = fmap D4._schemaExclusiveMaximum schemas
    mins = fmap D4._schemaMinimum schemas
    emins = fmap D4._schemaExclusiveMinimum schemas
    (maxConstraint, emaxConstraint) =
      Utils.computeMaximumConstraints maxes emaxes
    (minConstraint, eminConstraint) =
      Utils.computeMinimumConstraints mins emins

unifyStringConstraints :: D4.Schema -> D4.Schema -> D4.Schema
unifyStringConstraints nextSchema accSchema = accSchema {
      D4._schemaMaxLength = altUnifier max D4._schemaMaxLength nextSchema accSchema
    , D4._schemaMinLength = altUnifier min D4._schemaMinLength nextSchema accSchema
    , D4._schemaPattern = altUnifier const D4._schemaPattern nextSchema accSchema

unifyArrayConstraints :: D4.Schema -> D4.Schema -> D4.Schema
unifyArrayConstraints nextSchema accSchema = accSchema {
      D4._schemaItems = altUnifier unifyItems D4._schemaItems nextSchema accSchema
    , D4._schemaAdditionalItems = altUnifier uAdditional D4._schemaAdditionalItems nextSchema accSchema
    , D4._schemaMaxItems = altUnifier max D4._schemaMaxItems nextSchema accSchema
    , D4._schemaMinItems = altUnifier min D4._schemaMinItems nextSchema accSchema
    , D4._schemaUniqueItems = altUnifier (&&) D4._schemaUniqueItems nextSchema accSchema
    uAdditional :: V4Arr.AdditionalItems D4.Schema -> V4Arr.AdditionalItems D4.Schema -> V4Arr.AdditionalItems D4.Schema
    uAdditional (V4Arr.AdditionalBool b1) (V4Arr.AdditionalBool b2) =
      V4Arr.AdditionalBool $ b1 || b2
    -- allowing additional objects (True) is always at least as permissive as any schema
    -- all schemas are at least as permissive as not allowing any additional objects (False)
    uAdditional bln@(V4Arr.AdditionalBool b) obj@(V4Arr.AdditionalObject s) = if b then bln else obj
    uAdditional obj@(V4Arr.AdditionalObject s) bln@(V4Arr.AdditionalBool b) = if b then bln else obj
    uAdditional (V4Arr.AdditionalObject o1) (V4Arr.AdditionalObject o2) =
      V4Arr.AdditionalObject $ unifySchemas o1 o2

    unifyItems :: V4Arr.Items D4.Schema -> V4Arr.Items D4.Schema -> V4Arr.Items D4.Schema
    unifyItems (V4Arr.ItemsObject o1) (V4Arr.ItemsObject o2) = V4Arr.ItemsObject $ unifySchemas o1 o2
    unifyItems (V4Arr.ItemsArray xs) (V4Arr.ItemsArray ys) = V4Arr.ItemsArray $
      fmap (uncurry unifySchemas) (Utils.zipWithPadding D4.emptySchema D4.emptySchema xs ys)
    unifyItems x y = x -- possibly: merge the object schema into each of the tuple schemas? (zip (repeat o1) xs)

-- If one object does not require properties, then none of them can require
-- any properties. If the `required` array would be empty, we return Nothing
-- instead (the V4 metaschema says the required array cannot be empty)
unifyObjectConstraints :: D4.Schema -> D4.Schema -> D4.Schema
unifyObjectConstraints nextSchema accSchema = accSchema {
      D4._schemaRequired = Utils.setToMaybeSet =<< -- to avoid empty lists, which are not allowed
        applicativeUnifier DS.intersection D4._schemaRequired nextSchema accSchema
    , D4._schemaProperties =
            altUnifier (HM.unionWith unifySchemas) D4._schemaProperties nextSchema accSchema
    , D4._schemaAdditionalProperties =
           altUnifier unify D4._schemaAdditionalProperties nextSchema accSchema
    , D4._schemaMaxProperties = altUnifier max D4._schemaMaxProperties nextSchema accSchema
    , D4._schemaMinProperties = altUnifier min D4._schemaMinProperties nextSchema accSchema
    , D4._schemaPatternProperties = altUnifier (HM.unionWith unifySchemas) D4._schemaPatternProperties nextSchema accSchema
    unify :: V4Obj.AdditionalProperties D4.Schema -> V4Obj.AdditionalProperties D4.Schema -> V4Obj.AdditionalProperties D4.Schema
    unify (V4Obj.AdditionalPropertiesBool b1) (V4Obj.AdditionalPropertiesBool b2) =
      V4Obj.AdditionalPropertiesBool $ b1 || b2
    -- allowing additional objects (True) is always at least as permissive as any schema
    -- all schemas are at least as permissive as not allowing any additional objects (False)
    unify bln@(V4Obj.AdditionalPropertiesBool b) obj@(V4Obj.AdditionalPropertiesObject s) = if b then bln else obj
    unify obj@(V4Obj.AdditionalPropertiesObject s) bln@(V4Obj.AdditionalPropertiesBool b) = if b then bln else obj
    unify (V4Obj.AdditionalPropertiesObject o1) (V4Obj.AdditionalPropertiesObject o2) =
      V4Obj.AdditionalPropertiesObject $ unifySchemas o1 o2

unifyAnyInstanceConstraints :: D4.Schema -> D4.Schema -> D4.Schema
unifyAnyInstanceConstraints nextSchema accSchema = accSchema {
      D4._schemaType = altUnifier (<>) D4._schemaType nextSchema accSchema
    , D4._schemaEnum = altUnifier const D4._schemaEnum nextSchema accSchema
    , D4._schemaAllOf = altUnifier const D4._schemaAllOf nextSchema accSchema
    , D4._schemaAnyOf = altUnifier const D4._schemaAnyOf nextSchema accSchema
    , D4._schemaOneOf = altUnifier const D4._schemaOneOf nextSchema accSchema
    , D4._schemaNot = altUnifier const D4._schemaNot nextSchema accSchema