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TDB ERR MT

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

SipHash

en.wikipedia.org/wiki/Sip_Hash 131002.net/siphash/

Currently (as of Ruby 2.1) the favored hash in Ruby for hash-flooding protection.

:siphash24 - the reference implementation

Murmur family

The Murmur family of hashes are supported by Ruby TDB. 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).

:murmur3a - The latest 32-bit version optimized for x86
```
https://code.google.com/p/smhasher/wiki/MurmurHash3
```
: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 - See www.cse.yorku.ca/~oz/hash.html
:djb2 - See www.cse.yorku.ca/~oz/hash.html

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