Experimental hash table implementation in just Rust (stable, no unsafe code).

This is inpired by Python 3.6's new dict implementation (which remembers the insertion order and is fast to iterate).

This implementation corresponds to their "compact hash map" idea as it is now.

Using robin hood hashing just like Rust's libstd HashMap.

• Has insert, lookup, grow
• Remove is implemented, but it perturbs the insertion order. It's the usual backwards shift deletion, but only on the index vector, so it's cheaper. Order-preserving removal would want to be implemented with tombstones, but I'm hesitant and not sure if it can be performant.

Performance:

• Iteration is very fast
• Lookup is the same-ish as libstd HashMap, possibly suffers under load more due to the index vec to entries vec indirection
• Growing the map is faster than libstd HashMap, doesn't need to move keys and values at all, only the index vec

Interesting Features:

• Insertion order is preserved (swap_remove perturbs the order, like the method name says)
• Implements .pop() -> Option<(K, V)> in O(1) time
• OrderMap::new() is empty and uses no allocation until you insert something
• No unsafe.

Where to go from here?

• It can be an indexable ordered map in the current fashion

• Ideas and PRs for how to implement insertion-order preserving remove (for example tombstones) are welcome.

• Idea for more cache efficient lookup:

  Current indices: Vec<Pos>. Pos is interpreted as (u32, u32) more or less when raw_capacity() fits in 32 bits. Pos then stores both the lower half of the hash and the entry index. This means that the hash values in Entry don't need to be accessed while scanning for an entry.

Please read the API documentation here

Recent Changes

• 0.1.1
  • Initial release