hierarchical-tags

Very much WIP

Hierarchical Tags in the style of Unreal Engine FGameplayTag. Built on top of a (hopefully) fast and cache-friendly HashedString/HString, similar to an FName.

Hashing courtesy of xxHash, with the option to use CityHash instead. Macros to choose hash size (64bit default) included.

Current State:

• HashedString creation and addition to backend map
• String retrieval from HashedString
• HashedStringMap structure resembling a Hash Table, using the Hash from HashedString as keys
• String Utils to explode hierarchical strings (strings of the form A.B.C)
• Comparison functions for HashedString, case-sensitivity selectable

To-Do List:

• HierarchicalTag/HTag structure
  • Potential integration with QuickTags instead/as well as
• Comparison functions for HashedString and HierarchicalTag, case-sensitivity selectable
• Functions to check parent and child tags for HierarchicalTag
• Companion functions/structures for HierarchicalTag to facilitate retrieving parent tags efficiently
• Companion structure to hold multiple HierarchicalTags
• Option to override default tag-separator
• Ability to load tags in bulk
• MORE & BETTER TESTS
• C++ Wrapper
• Wide-char (wchar_t) support?

Future-Concerns:

• Storing the Hash directly in HashedString could impact cache performance with 2 64bit hashes per structure. (though this is only an issue if case-insensitive checks are enabled)
  • FNames are implemented as some packed integer, partially an index, partially some other data to help with sorting?
  • Storing the hash index rather than the hash would compact the size of the HString (who needs 2+ billion strings anyway?) and would not impact comparisons, but would make string retrieval more indirect.
  • Switching to indexes may negate the need for the map? Or the map pivots from storing hash->string to hash->index. (Maybe try to implement this as an IndexedString)
• HashedStringMap is not thread-safe in the slightest
• FNames support some form of "lexical" less-than/greater-than functions, I assume to allow for basic list sorting? Do we care about that?
• FGameplayTag can achieve efficient network transfer with "fast gameplay tag replication" because all tags are supposed to be known at start-up and therefore have some shared index on both client and server. The ability to block tags from being created at runtime could be useful in support of a similar system.