Path.hs

{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}

module Json.Path
  ( Path (..)

    -- * Encoding
  , encode
  , builderUtf8

    -- * Lookup
  , query
  , query'

    -- * Reverse
  , reverse
  ) where

import Prelude hiding (reverse)

import Data.ByteString.Short.Internal (ShortByteString (SBS))
import Data.Bytes.Builder (Builder)
import Data.Primitive (ByteArray (ByteArray))
import Data.Text.Short (ShortText)
import Json (Member (Member), Value (Array, Null, Object))

import qualified Data.Bytes.Builder as Builder
import qualified Data.Bytes.Chunks as ByteChunks
import qualified Data.Primitive as PM
import qualified Data.Text.Short.Unsafe as TS

-- | A path to an object.
data Path
  = -- | JSON path element of a key into an object, \"object.key\".
    Key {-# UNPACK #-} !ShortText !Path
  | -- | JSON path element of an index into an array, \"array[index]\".
    -- Negative numbers result in undefined behavior.
    Index {-# UNPACK #-} !Int !Path
  | Nil
  deriving (Eq, Show)

{- | Encode a path.

>>> encode (Key "foo" $ Index 5 $ Key "bar" $ Nil)
-}
encode :: Path -> ShortText
encode p = ba2st (ByteChunks.concatU (Builder.run 128 (builderUtf8 p)))

builderUtf8 :: Path -> Builder
builderUtf8 p0 = Builder.ascii '$' <> go p0
 where
  go Nil = mempty
  go (Key k p) = Builder.ascii '.' <> Builder.shortTextUtf8 k <> go p
  go (Index i p) =
    Builder.ascii '['
      <> Builder.wordDec (fromIntegral i)
      <> Builder.ascii ']'
      <> go p

{- | Search for an element at the given path. Returns 'Nothing' if
anything in the path is missing.
-}
query :: Path -> Value -> Maybe Value
query = go
 where
  go Nil v = Just v
  go (Key k p) (Object mbrs) =
    foldr
      ( \(Member key val) other ->
          if key == k
            then Just val
            else other
      )
      Nothing
      mbrs
      >>= go p
  go (Index i p) (Array vs) =
    if i < PM.sizeofSmallArray vs
      then
        let !(# e #) = PM.indexSmallArray## vs i
         in go p e
      else Nothing
  go _ _ = Nothing

{- | Variant of 'query' that returns 'Null' if anything in the path
is missing.
-}
query' :: Path -> Value -> Value
query' = go
 where
  go Nil v = v
  go (Key k p) (Object mbrs) =
    go p $
      foldr
        ( \(Member key val) other ->
            if key == k
              then val
              else other
        )
        Null
        mbrs
  go (Index i p) (Array vs) =
    if i < PM.sizeofSmallArray vs
      then
        let !(# e #) = PM.indexSmallArray## vs i
         in go p e
      else Null
  go _ _ = Null

ba2st :: ByteArray -> ShortText
ba2st (ByteArray x) = TS.fromShortByteStringUnsafe (SBS x)

reverse :: Path -> Path
reverse = go Nil
 where
  go !acc Nil = acc
  go !acc (Key k xs) = go (Key k acc) xs
  go !acc (Index i xs) = go (Index i acc) xs