`folly/GroupVarint.h`
---------------------
`folly/GroupVarint.h` is an implementation of variable-length encoding for 32-
and 64-bit integers using the Group Varint encoding scheme as described in
Jeff Dean's [WSDM 2009 talk][wsdm] and in [Information Retrieval: Implementing
and Evaluating Search Engines][irbook].
[wsdm]: http://research.google.com/people/jeff/WSDM09-keynote.pdf
[irbook]: http://www.ir.uwaterloo.ca/book/addenda-06-index-compression.html
Briefly, a group of four 32-bit integers is encoded as a sequence of variable
length, between 5 and 17 bytes; the first byte encodes the length (in bytes)
of each integer in the group. A group of five 64-bit integers is encoded as a
sequence of variable length, between 7 and 42 bytes; the first two bytes
encode the length (in bytes) of each integer in the group.
`GroupVarint.h` defines a few classes:
* `GroupVarint<T>`, where `T` is `uint32_t` or `uint64_t`:
Basic encoding / decoding interface, mainly aimed at encoding / decoding
one group at a time.
* `GroupVarintEncoder<T, Output>`, where `T` is `uint32_t` or `uint64_t`,
and `Output` is a functor that accepts `StringPiece` objects as arguments:
Streaming encoder: add values one at a time, and they will be
flushed to the output one group at a time. Handles the case where
the last group is incomplete (the number of integers to encode isn't
a multiple of the group size)
* `GroupVarintDecoder<T>`, where `T` is `uint32_t` or `uint64_t`:
Streaming decoder: extract values one at a time. Handles the case where
the last group is incomplete.
The 32-bit implementation is significantly faster than the 64-bit
implementation; on platforms supporting the SSSE3 instruction set, we
use the PSHUFB instruction to speed up lookup, as described in [SIMD-Based
Decoding of Posting Lists][cikmpaper] (CIKM 2011).
[cikmpaper]: http://www.stepanovpapers.com/CIKM_2011.pdf
For more details, see the header file `folly/GroupVarint.h` and the
associated test file `folly/test/GroupVarintTest.cpp`.
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