# Pastebin atKmxOH8
import qualified Data.HashMap.Monoidal as MH
import qualified Data.HashMap.Strict as HashMap
import Data.Semigroup (First (..))
import Prelude hiding (First)

transpose ::
  forall k1 k2 v.
  (Eq k1, Hashable k1, Eq k2, Hashable k2) =>
  HashMap k1 (HashMap k2 v) ->
  HashMap k2 (HashMap k1 v)
transpose h =
  let lists :: [(k1, [(k2, v)])]
      lists = HashMap.toList $ HashMap.toList <$> h
   in coerce . mconcat $
        [ MH.singleton k2 . MH.singleton k1 $ First v
          | (k1, m) <- lists,
            (k2, v) <- m
        ]