+----------------+
        |"(keyi, valuei)"|
        +------+---------+
                |
                |  hasher(keyi)                   "Parallel Hash Map"
                v                                 "(with 8 submaps)"
      +--------+-------------+                   +----------------+
      | h=0x7d84ea13707f4657 |                   |  submap0       |
      +---------+------------+                   +----------------+
                |                                |  submap1       |
                | "(h ^ (h >> 3)) & 0x7"         +----------------+
                v                                |  submap2       |
           +----+----+                           +----------------+
           |"idx = 5"|                           |  submap3       |
           +----+----+                           +----------------+
                |                                |  submap4       |
                |                                +----------------+
                +------------------------------->|  submap5       |
                                                 +----------------+
                                                 |  submap6       |
                                                 +----------------+
                                                 |  submap7       |
                                                 +----------------+
      
       "parallel_hash_map with 8 submaps, each submap is an absl::flat_hash_map"