This is an incomplete implementation of the memory format of Facebook's Gorilla database in Rust, as described in Gorilla: A Fast, Scalable, In-Memory Time Series Database.

Example

There is an example at examples/csv_to_packed.rs. Run as follows:

cargo run --release --example csv_to_packed

This will read the file examples/test_data.csv and compress it in memory. It's not a very interesting file, but replacing it with your favorite data will show compression ratio and speed differences between compressed and uncompressed reads.

There are also examples in the test code in the modules.

Implementation details

The Gorilla Paper leaves some details out:

• The number of significant bits when compressing doubles are stored in a 6 bits, giving a max value of 63. The key thing to notice is that only 63 values are actually needed: 1 through 64. I solve this by storing M - 1, where M is the number of significant bits ([MEANING64] in code). Another implementation stores it by storing M & 63 and resolving it at read time. Either solution is fine. The former optimizes for read speed and the latter for write speed.
• The number of leading zeros is stored in 5 bits, which gives a maximum of 31 leading zeros. There is nothing preventing significant bits from having leading zeros, though, so we just use 31 if it's 31 or higher. ([LEADING31] in code)
• Leading number in previous XOR. Are we storing that or the XOR itself? If the former, the window will keep the same if we reuse it, if not it might shrink as new data comes in. Unsure about the best solution. ([XORORLEADING] in code)
• IntStream writes the number plus a bias so that the resulting number is always a non-negative number. This makes it fast to encode and decode without branching or being dependent on hardware representation of numbers. The initial version was not as smart and took about twice as long to decode.

Further work

• Measure and optimize performance
• Resolve open questions in Implementation details
• Implement the rest of the paper
• Investigate whether Rust's Write and Read traits could be used instead of hand rolled traits
• Better naming:
  • Stream can now mean both bit-stream and compressed stream
  • Writer can refer both to the Writer trait and its impls or to a "compressor"