restrictedDamerauLevenshteinDistance'
  :: (Bits bv, Num bv) => bv -> Int -> Int -> String -> String -> Int
restrictedDamerauLevenshteinDistance' _bv_dummy m n str1 str2
  | [] <- str1 = n
  | otherwise  = extractAnswer $
                 foldl' (restrictedDamerauLevenshteinDistanceWorker
                             (matchVectors str1) top_bit_mask vector_mask)
                        (0, 0, m_ones, 0, m) str2
  where
    m_ones@vector_mask = (2 ^ m) - 1
    top_bit_mask = (1 `shiftL` (m - 1)) `asTypeOf` _bv_dummy
    extractAnswer (_, _, _, _, distance) = distance
restrictedDamerauLevenshteinDistance'
  :: (Bits bv, Num bv) => bv -> Int -> Int -> String -> String -> Int
restrictedDamerauLevenshteinDistance' _bv_dummy m n str1 str2
  | [] <- str1 = n
  | otherwise  = extractAnswer $
                 foldl' (restrictedDamerauLevenshteinDistanceWorker
                             (matchVectors str1) top_bit_mask vector_mask)
                        (0, 0, m_ones, 0, m) str2
  where
    m_ones@vector_mask = (2 ^ m) - 1
    top_bit_mask = (1 `shiftL` (m - 1)) `asTypeOf` _bv_dummy
    extractAnswer (_, _, _, _, distance) = distance
restrictedDamerauLevenshteinDistance' _bv_dummy m n str1 str2
  | [] <- str1 = n
  | otherwise  = extractAnswer $
                 foldl' (restrictedDamerauLevenshteinDistanceWorker
                             (matchVectors str1) top_bit_mask vector_mask)
                        (0, 0, m_ones, 0, m) str2
  where
    m_ones@vector_mask = (2 ^ m) - 1
    top_bit_mask = (1 `shiftL` (m - 1)) `asTypeOf` _bv_dummy
    extractAnswer (_, _, _, _, distance) = distance