refactor: reorganize modules

* Move range logic from App to RangeQuery

* Move decoder logic to Statement modules

* Move location headers logic to functions

* Add a comment for App.hs module

src/PostgREST/RangeQuery.hs

@@ -1,6 +1,6 @@
 {-|
 Module      : PostgREST.RangeQuery
-Description : Logic regarding the `Range` header and `limit`, `offset` querystring arguments.
+Description : Logic regarding the `Range`/`Content-Range` headers and `limit`/`offset` querystring arguments.
 -}
 module PostgREST.RangeQuery (
   rangeParse
@@ -11,6 +11,8 @@ module PostgREST.RangeQuery (
 , rangeGeq
 , allRange
 , NonnegRange
+, rangeStatusHeader
+, contentRangeH
 ) where
 
 import qualified Data.ByteString.Char8 as BS
@@ -22,6 +24,7 @@ import Control.Applicative
 import Data.Ranged.Boundaries
 import Data.Ranged.Ranges
 import Network.HTTP.Types.Header
+import Network.HTTP.Types.Status
 
 import Protolude
 
@@ -70,3 +73,30 @@ allRange = rangeGeq 0
 rangeLeq :: Integer -> NonnegRange
 rangeLeq n =
   Range BoundaryBelowAll (BoundaryAbove n)
+
+rangeStatusHeader :: NonnegRange -> Int64 -> Maybe Int64 -> (Status, Header)
+rangeStatusHeader topLevelRange queryTotal tableTotal =
+  let lower = rangeOffset topLevelRange
+      upper = lower + toInteger queryTotal - 1
+      contentRange = contentRangeH lower upper (toInteger <$> tableTotal)
+      status = rangeStatus lower upper (toInteger <$> tableTotal)
+  in (status, contentRange)
+  where
+    rangeStatus :: Integer -> Integer -> Maybe Integer -> Status
+    rangeStatus _ _ Nothing = status200
+    rangeStatus lower upper (Just total)
+      | lower > total               = status416 -- 416 Range Not Satisfiable
+      | (1 + upper - lower) < total = status206 -- 206 Partial Content
+      | otherwise                   = status200 -- 200 OK
+
+contentRangeH :: (Integral a, Show a) => a -> a -> Maybe a -> Header
+contentRangeH lower upper total =
+    ("Content-Range", headerValue)
+    where
+      headerValue   = rangeString <> "/" <> totalString
+      rangeString
+        | totalNotZero && fromInRange = show lower <> "-" <> show upper
+        | otherwise = "*"
+      totalString   = maybe "*" show total
+      totalNotZero  = maybe True (0 /=) total
+      fromInRange   = lower <= upper