Brief overview of hash functions supported by Ruby TDB

Ruby TDB supports several alternative hash functions in addition to the defaults supported by TDB upstream. Hash functions behave and perform differently depending on the key and type of keys you use. We support several popular hash functions (and will accept patches to support more).

Changing hash functions on an already-created database will cause corruption, so don’t do it.

TDB Upstream Defaults

• the default hash use by TDB is based on the hash algorithm from gdbm. You may specify this by passing explicitly to TDB.new: :hash => :default

• the new default (available via TDB::INCOMPATIBLE_HASH) is the Jenkins lookup3 hash. :hash => :jenkins_lookup3

Murmur family

The Murmur family of hashes are supported by Ruby TDB. MurmurHash3 will be supported as soon as it becomes finalized. Most of these are not endian-neutral so databases are no compatible between machines of different endianness and were designed with x86 and x86_64 in mind (they may crash or not work on other architectures).

• :murmur2 - the simple and fast implementation

• :murmur2a - words of the author:

  This is a variant of MurmurHash2 modified to use the
  Merkle-Damgard construction. Bulk speed should be identical to
  Murmur2, small-key speed will be 10%-20% slower due to the added
  overhead at the end of the hash.
  
  This variant fixes a minor issue where null keys were more likely to
  collide with each other than expected, and also makes the algorithm
  more amenable to incremental implementations. All other caveats from
  MurmurHash2 still apply.
  

• :murmur2_aligned - a safer, but slower variant of :murmur2 designed for platforms where unaligned 4-byte reads can crash the machine.

• :murmur2_neutral - endian/alignment-neutral version of the simple implementation, half as fast according to the author.

• :murmur1 - simple and fast historical version

• :murmur1_aligned - according to the author, the performance of this one should be as good or better than the simple version.

FNV family

• :fnv1a - the recommended variant of the popular Fowler-Noll-Vo hash function

Bernstein hashes

• :djb3 - The hash currently favored by Bernstein. See www.cse.yorku.ca/~oz/hash.html

• :djb2 - See www.cse.yorku.ca/~oz/hash.html

Originally generated with the Darkfish Rdoc Generator 2, modified by wrongdoc.