3 ms·

i feel this article is missing some detail or incorrect in reporting the actual development here. either that or i am missing something myself... hash tables a

i feel this article is missing some detail or incorrect in reporting the actual development here. either that or i am missing something myself... hash tables are constant time on average for all insertion, lookup and deletion operations, and in some special cases, which i've seen used in practice very, very often, they have very small constant run-time just like a fixed-size array (exactly equivalent in-fact). this came up in an interview question i had in 2009 where i got judged poorly for deriding the structure as "not something i've often needed", and i've seen it in much older code. i'm guessing maybe there are constraints at play here, like having to support unbounded growth, and some generic use case that i've not encountered in the wild...?

yxhuvud 2y ago
What you are missing is how the hash table behaves when it is almost full. If there is one empty spot left in the whole table, how do you find it when you insert a new entry?
- sfn42 2y ago
  Same way you find it when doing a lookup later? I know that's probably a naive answer, I honestly don't even know how a hash table works. I know how a hash map works, at least some implementations use a linked list as a bucket. So the hash gives you the bucket, then you linear search the bucket for the element. Buckets should be small so the time to search them is negligible, giving O(1) lookup and insert performance. Obviously this is different from what's being discussed here, this data structure doesn't even really get "full" but it's also the only implementation I know is in practical use. Not sure why one might use a hash table instead
  - yxhuvud 2y ago
    So using buckets with linked lists like that is neither space efficient or fast. A strategy that nowadays is more common and fast is to store conflicts in the table itself using some strategy to find a place in the table itself to put the new entry. The simplest (but not optimal) way to do this is to just take the next one that isn't used yet. This means a linear scan that once the table gets close to being full will approach O(n). To avoid this, better strategies for choosing the next place to look is used, as well as automatic resizing of the hash table at some occupancy percentage to keep the lookup chains short. Other strategies in use will also approach O(n) but will require resizing on difference occupancy percentage. What is new in this approach is that they manage to go faster than O(n) even at almost full occupancy.
    - xxs 2y ago
      >The simplest (but not optimal) way to do this is to just take the next one that isn't used yet. The linear probe is by far the most efficient way to build a hashtable on any modern hardware, nothing is near close. Everything else leads to cache trashing on misses. For the nearly full table - that's a mistake - table should not go above a specific fill factor, e.g. the notorious 75% for large tables.
      - yxhuvud 2y ago
        The problem with the linear probe is that it creates long runs of collisions, thereby forcing you to avoid that by having a lower fill factor.
        xxs 2y ago
        >that it creates long runs of collisions Yes, of course. In practice it still outperforms pretty much anything else. The lower fill factor is still cheaper (memory footprint) than having buckets and indirection.

yxhuvud 2y ago

What you are missing is how the hash table behaves when it is almost full. If there is one empty spot left in the whole table, how do you find it when you insert a new entry?

sfn42 2y ago
Same way you find it when doing a lookup later? I know that's probably a naive answer, I honestly don't even know how a hash table works. I know how a hash map works, at least some implementations use a linked list as a bucket. So the hash gives you the bucket, then you linear search the bucket for the element. Buckets should be small so the time to search them is negligible, giving O(1) lookup and insert performance. Obviously this is different from what's being discussed here, this data structure doesn't even really get "full" but it's also the only implementation I know is in practical use. Not sure why one might use a hash table instead
- yxhuvud 2y ago
  So using buckets with linked lists like that is neither space efficient or fast. A strategy that nowadays is more common and fast is to store conflicts in the table itself using some strategy to find a place in the table itself to put the new entry. The simplest (but not optimal) way to do this is to just take the next one that isn't used yet. This means a linear scan that once the table gets close to being full will approach O(n). To avoid this, better strategies for choosing the next place to look is used, as well as automatic resizing of the hash table at some occupancy percentage to keep the lookup chains short. Other strategies in use will also approach O(n) but will require resizing on difference occupancy percentage. What is new in this approach is that they manage to go faster than O(n) even at almost full occupancy.
  - xxs 2y ago
    >The simplest (but not optimal) way to do this is to just take the next one that isn't used yet. The linear probe is by far the most efficient way to build a hashtable on any modern hardware, nothing is near close. Everything else leads to cache trashing on misses. For the nearly full table - that's a mistake - table should not go above a specific fill factor, e.g. the notorious 75% for large tables.
    - yxhuvud 2y ago
      The problem with the linear probe is that it creates long runs of collisions, thereby forcing you to avoid that by having a lower fill factor.
      - xxs 2y ago
        >that it creates long runs of collisions Yes, of course. In practice it still outperforms pretty much anything else. The lower fill factor is still cheaper (memory footprint) than having buckets and indirection.