mirror of
https://codeberg.org/forgejo/forgejo.git
synced 2024-12-14 10:30:36 +03:00
68 lines
2.9 KiB
Markdown
68 lines
2.9 KiB
Markdown
|
# Govarint
|
||
|
|
|
||
|
|
This project aims to provide a simple API for the performant encoding and decoding of 32 and 64 bit integers using a variety of algorithms.
|
||
|
|
|
||
|
|
[](https://www.flickr.com/photos/tsevis/8648521649/)
|
||
|
|
|
||
|
|
## Usage
|
||
|
|
|
||
|
|
Each integer encoding algorithm conforms to an encoding and decoding interface.
|
||
|
|
The interfaces also specify the size of the unsigned integer, either 32 or 64 bits, and will be referred to as XX below.
|
||
|
|
To create an encoder:
|
||
|
|
|
||
|
|
NewU32Base128Encoder(w io.Writer)
|
||
|
|
NewU64Base128Encoder(w io.Writer)
|
||
|
|
NewU32GroupVarintEncoder(w io.Writer)
|
||
|
|
|
||
|
|
For encoders, the only two commands are `PutUXX` and `Close`.
|
||
|
|
`Close` must be called as some integer encoding algorithms write in multiples.
|
||
|
|
|
||
|
|
var buf bytes.Buffer
|
||
|
|
enc := NewU32Base128Encoder(&buf)
|
||
|
|
enc.PutU32(117)
|
||
|
|
enc.PutU32(343)
|
||
|
|
enc.Close()
|
||
|
|
|
||
|
|
To create a decoder:
|
||
|
|
|
||
|
|
NewU32Base128Decoder(r io.ByteReader)
|
||
|
|
NewU64Base128Decoder(r io.ByteReader)
|
||
|
|
NewU32GroupVarintDecoder(r io.ByteReader)
|
||
|
|
|
||
|
|
For decoders, the only command is `GetUXX`.
|
||
|
|
`GetUXX` returns the value and any potential errors.
|
||
|
|
When reading is complete, `GetUXX` will return an `EOF` (End Of File).
|
||
|
|
|
||
|
|
dec := NewU32Base128Decoder(&buf)
|
||
|
|
x, err := dec.GetU32()
|
||
|
|
|
||
|
|
## Use Cases
|
||
|
|
|
||
|
|
Using fixed width integers, such as uint32 and uint64, usually waste large amounts of space, especially when encoding small values.
|
||
|
|
Optimally, smaller numbers should take less space to represent.
|
||
|
|
|
||
|
|
Using integer encoding algorithms is especially common in specific applications, such as storing edge lists or indexes for search engines.
|
||
|
|
In these situations, you have a sorted list of numbers that you want to keep as compactly as possible in memory.
|
||
|
|
Additionally, by storing only the difference between the given number and the previous (delta encoding), the numbers are quite small, and thus compress well.
|
||
|
|
|
||
|
|
For an explicit example, the Web Data Commons Hyperlink Graph contains 128 billion edges linking page A to page B, where each page is represented by a 32 bit integer.
|
||
|
|
By converting all these edges to 64 bit integers (32 | 32), sorting them, and then using delta encoding, memory usage can be reduced from 64 bits per edge down to only 9 bits per edge using the Base128 integer encoding algorithm.
|
||
|
|
This figure improves even further if compressed using conventional compression algorithms (3 bits per edge).
|
||
|
|
|
||
|
|
## Encodings supported
|
||
|
|
|
||
|
|
`govarint` supports:
|
||
|
|
|
||
|
|
+ Base128 [32, 64] - each byte uses 7 bits for encoding the integer and 1 bit for indicating if the integer requires another byte
|
||
|
|
+ Group Varint [32] - integers are encoded in blocks of four - one byte encodes the size of the following four integers, then the values of the four integers follows
|
||
|
|
|
||
|
|
Group Varint consistently beats Base128 in decompression speed but Base128 may offer improved compression ratios depending on the distribution of the supplied integers.
|
||
|
|
|
||
|
|
## Tests
|
||
|
|
|
||
|
|
go test -v -bench=.
|
||
|
|
|
||
|
|
## License
|
||
|
|
|
||
|
|
MIT License, as per `LICENSE`
|