Refactor inner join

[mchav]

This was written in one shot so there is some repetition.

        leftIndicesToGroup = M.elems $ M.filterWithKey (\k _ -> k `elem` cs) (D.columnIndices left)
        leftRowRepresentations = VU.generate (fst (D.dimensions left)) (D.mkRowRep leftIndicesToGroup left)
        -- key -> [index0, index1]
        leftKeyCountsAndIndices   = VU.foldr (\(i, v) acc -> M.insertWith (++) v [i] acc) M.empty (VU.indexed leftRowRepresentations)
        -- key -> [index0, index1]
        rightIndicesToGroup = M.elems $ M.filterWithKey (\k _ -> k `elem` cs) (D.columnIndices right)
        rightRowRepresentations = VU.generate (fst (D.dimensions right)) (D.mkRowRep rightIndicesToGroup right)
        rightKeyCountsAndIndices  = VU.foldr (\(i, v) acc -> M.insertWith (++) v [i] acc) M.empty (VU.indexed rightRowRepresentations)
        -- key -> [(left_indexes0, right_indexes1)]
        mergedKeyCountsAndIndices = M.foldrWithKey (\k v m -> if k `M.member` rightKeyCountsAndIndices then M.insert k (VU.fromList v, VU.fromList (rightKeyCountsAndIndices M.! k)) m else m) M.empty leftKeyCountsAndIndices
        -- [(ints, ints)]
        leftAndRightIndicies = M.elems mergedKeyCountsAndIndices
        -- [(ints, ints)] (expanded to n * m)
        expandedIndices = map (\(l, r) -> (mconcat (replicate (VU.length r) l), mconcat (replicate (VU.length l) r))) leftAndRightIndicies
        expandedLeftIndicies = mconcat (map fst expandedIndices)
        expandedRightIndicies = mconcat (map snd expandedIndices)
        -- df
        expandedLeft = left { columns = VB.map (D.atIndicesStable expandedLeftIndicies) (D.columns left), dataframeDimensions = (VU.length expandedLeftIndicies, snd (D.dataframeDimensions left))}
        -- df 
        expandedRight = right { columns = VB.map (D.atIndicesStable expandedRightIndicies) (D.columns right), dataframeDimensions = (VU.length expandedRightIndicies, snd (D.dataframeDimensions right))}
        -- [string]
        leftColumns = D.columnNames left
        rightColumns = D.columnNames right

The comments are also not very informative.

This should be broken into functions and tested.

[Mastercoder0406]

Please assign me this