-- | Add metadata tags as comments to SQL queries.

module Data.SqlCommenter
  ( sqlCommenterGoogle
  , sqlCommenterStandard
  , Attribute
  ) where

import           Hasura.Prelude

import qualified Data.List.NonEmpty as NE
import qualified Network.URI.Encode as URI

import           Data.Text.Extended (commaSeparated)

type Attribute = (Text, Text)

-- | query-tags format as defined in the Spec <https://google.github.io/sqlcommenter/spec/#sql-commenter>
sqlCommenterGoogle :: [Attribute] -> Text
sqlCommenterGoogle attributes
  | null attributes = mempty
  | otherwise = createSQLComment $ generateCommentTags (NE.fromList attributes)
  where
    createSQLComment comment = " /* " <> comment <> " */"

-- | Default 'Query Tags' format in the Hasura GraphQL Engine
-- Creates simple 'key=value' pairs of query tags. No sorting, No URL encoding.
-- If the format of query tags is not mentioned in the metadata then, query-tags
-- are formatted using hte below format
sqlCommenterStandard :: [Attribute] -> Text
sqlCommenterStandard attributes
  | null attributes = mempty
  | otherwise = createSQLComment $ generateComment (NE.fromList attributes)
  where
    generateComment attr =  commaSeparated [ k <> "=" <> v | (k, v) <- NE.toList attr ]
    createSQLComment comment = " /* " <> comment <> " */"

-- | Top-level algorithm to generate the string comment from list of
-- 'Attribute's
-- Spec <https://google.github.io/sqlcommenter/spec/#sql-commenter>
-- See Note [Ambiguous SQLCommenterGoogle Specification]
generateCommentTags :: NE.NonEmpty Attribute -> Text
generateCommentTags attributes =

  let
    -- 1. URL encode the key,value pairs. What the spec calls serialization.
    -- https://google.github.io/sqlcommenter/spec/#key-serialization-algorithm
    -- https://google.github.io/sqlcommenter/spec/#value-serialization-algorithm
    encoded = NE.map urlEncodePair attributes
    -- 2. Sort the pairs. Spec: https://google.github.io/sqlcommenter/spec/#sorting
    -- Note the 'sort' from 'Data.List' works by sorting the first element in
    -- the pair. And sorting on 'Text' is lexicographic.
    sorted = NE.sort encoded
    -- 3. Finally, serialize the pairs into a CSV. What the spec calls concatenation
    -- https://google.github.io/sqlcommenter/spec/#concatenation
    serialized = serializePairs sorted
  in serialized
  where
    urlEncodePair (k, v) = (URI.encodeText k, URI.encodeText v)
    serializePairs pairs = commaSeparated [ k <> "='" <> v <> "'" | (k, v) <- NE.toList pairs ]


{-| NOTE: [Ambiguous SQLCommenterGoogle Specification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The specification is ambiguos/unclear about the following two steps:

1. [Comment Escaping](https://google.github.io/sqlcommenter/spec/#comment-escaping)
~~~~~~~~~~~~~~~~~~~~
  The Spec states that:

    > If a comment already exists within a SQL statement, we MUST NOT mutate that statement.

  Oddly, the implementation of above rule/statement is not uniform among it's various language
  libraries. That along with the fact that the above statement is clear for the scenarios when
  we have properly formed comment, but does not state anything about what to do when the comments
  are malformed is why it was decided to skip this step.

  I have created a issue regarding this at https://github.com/google/sqlcommenter/issues/57

  When discussed the above problem with Tiru, it was decided that since in our case we do not have
  any other sql comments in the prepared statement, this step won't affect us thus it's okay to
  skip this.

2. [Escaping Meta Characters](https://google.github.io/sqlcommenter/spec/#meta-characters)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The Spec states that:

    > Meta characters such as `'` should be escaped with a slash `\`.

  And the algorithm in brief is:
    1. URL encode the Key/Value
    2. Escape the meta-characters withing the raw value; a single quote `'` becomes `\'`

  From the above statement, it could be understood that the meta-characters in context of
  sqlcommenter is `'`. And 'Network.URI.Encode.encodeText' is capable of encoding `'`
  without any need of escaping.

  ```
  Prelude Network.URI.Encode> encodeText"'"
  "%27"

  Prelude Network.URI.Encode> encodeText "\'"
  "%27"
  ```

  During the investigation, we also found that the reference implementation of the sqlcommenter
  in other languages did not even do this step:
    1. https://github.com/google/sqlcommenter/blob/master/python/sqlcommenter-python/google/cloud/sqlcommenter/__init__.py#L29
    2. https://github.com/google/sqlcommenter/blob/master/java/sqlcommenter-java/src/main/java/com/google/cloud/sqlcommenter/threadlocalstorage/State.java#L179

  The above can be summarized as:
    1. The meta-characters in the context of sqlcommenter is `'`
    2. Reference implementation of sqlcommenter does not include the step
    3. 'Network.URI.Encode.encodeText' can encode `'` without any need of escaping

  Thus, on the basis of the above three facts we decided to ignore the 'Escape Meta Characters' step.
-}