Add unsafeStatesOfBlock to RefinablePartition

This can be used in cases where the caller can guarantee that the
returned vector isn't referenced any more after the next mutation of
RefinablePartition.

src/Data/RefinablePartition.hs

@@ -29,6 +29,7 @@ module Data.RefinablePartition
   , groupBy
   -- * Conversion
   , freeze
+  , unsafeStatesOfBlock
   ) where
 
 import           Control.Monad (forM_, when, foldM)
@@ -247,6 +248,18 @@ statesOfBlock partition b = do
   let slice = VU.slice (block^.startOffset) len (partition^.statesByBlock)
   V.convert <$> VUU.freeze slice
 
+-- | Return a vector of all states in a given block.
+--
+-- Runtime: O(n) for n == number of states in this block
+unsafeStatesOfBlock :: RefinablePartition s -> Block -> ST s (VU.Vector State)
+unsafeStatesOfBlock partition b = do
+  block <- getBlock partition b
+
+  len <- blockSize partition b
+  let slice = VU.slice (block^.startOffset) len (partition^.statesByBlock)
+  VU.unsafeFreeze slice
+
+
 -- | Split a block into two new blocks for its marked and unmarked states.
 --
 -- Returns a tuple of (marked block, unmarked block). If there are no marked or