% divANS Module
Overview
The divANS
crate is meant to be used for generic data compression.
The algorithm has been tuned to significantly favor gains in compression ratio
over performance, operating at line speeds of 150 Mbit/s.
The name originates from "divided-ANS" since the intermediate representation is divided from the ANS codec
More information at https://blogs.dropbox.com/tech/2018/06/building-better-compression-together-with-divans/
Divans should primarily be considered for cold storage and compression research. The compression algorithm is highly modular and new algorithms only need to be written a single time since generic trait specialization constructs optimized variants of the codec for both compression and decompression at compile time.
Rust Usage
Decompression
extern crate divans;
fn main() {
use std::io;
let stdin = &mut io::stdin();
{
use std::io::{Read, Write};
let mut reader = divans::DivansDecompressorReader::new(
stdin,
4096, // buffer size
);
io::copy(&mut reader, &mut io::stdout()).unwrap();
}
}
Compression
extern crate divans;
fn main() {
use std::io;
let stdout = &mut io::stdout();
{
use std::io::{Read, Write};
let mut writer = divans::DivansBrotliHybridCompressorWriter::new(
stdout,
divans::DivansCompressorOptions{
literal_adaptation:None, // should we override how fast the cdfs converge for literals?
window_size:Some(22), // log 2 of the window size
lgblock:None, // should we override how often metablocks are created in brotli
quality:Some(11), // the quality of brotli commands
dynamic_context_mixing:Some(2), // if we want to mix together the stride prediction and the context map
use_brotli:divans::BrotliCompressionSetting::default(), // ignored
use_context_map:true, // whether we should use the brotli context map in addition to the last 8 bits of each byte as a prior
force_stride_value: divans::StrideSelection::UseBrotliRec, // if we should use brotli to decide on the stride
},
4096, // internal buffer size
);
io::copy(&mut io::stdin(), &mut writer).unwrap();
writer.flush().unwrap();
}
}
C usage
The C api is a standard compression API like the one that zlib provides. Despite being rust code, no allocations are made unless the CAllocator struct is passed in with the custom_malloc field set to NULL. This means that any user of the divans library may provide their own allocation system and all allocations will go through that allocation system. The pointers returned by custom_malloc must be 32-byte aligned.
Compression
#include "divans/ffi.h"
// compress to stdout
DivansResult compress(const unsigned char *data, size_t len) {
unsigned char buf[4096];
struct CAllocator alloc = {custom_malloc, custom_free, custom_alloc_opaque}; // set all 3 to NULL to use rust allocators
struct DivansCompressorState *state = divans_new_compressor_with_custom_alloc(alloc);
divans_set_option(state, DIVANS_OPTION_USE_CONTEXT_MAP, 1);
divans_set_option(state, DIVANS_OPTION_DYNAMIC_CONTEXT_MIXING, 2);
divans_set_option(state, DIVANS_OPTION_QUALITY, 11);
while (len) {
size_t read_offset = 0;
size_t buf_offset = 0;
DivansResult res = divans_encode(state,
data, len, &read_offset,
buf, sizeof(buf), &buf_offset);
if (res == DIVANS_FAILURE) {
divans_free_compressor(state);
return res;
}
data += read_offset;
len -= read_offset;
fwrite(buf, buf_offset, 1, stdout);
}
DivansResult res;
do {
size_t buf_offset = 0;
res = divans_encode_flush(state,
buf, sizeof(buf), &buf_offset);
if (res == DIVANS_FAILURE) {
divans_free_compressor(state);
return res;
}
fwrite(buf, buf_offset, 1, stdout);
} while(res != DIVANS_SUCCESS);
divans_free_compressor(state);
return DIVANS_SUCCESS;
}
Decompression
#include "divans/ffi.h"
//decompress to stdout
DivansResult decompress(const unsigned char *data, size_t len) {
unsigned char buf[4096];
struct CAllocator alloc = {custom_malloc, custom_free, custom_alloc_opaque}; // set all 3 to NULL for using rust allocators
struct DivansDecompressorState *state = divans_new_decompressor_with_custom_alloc(alloc);
DivansResult res;
do {
size_t read_offset = 0;
size_t buf_offset = 0;
res = divans_decode(state,
data, len, &read_offset,
buf, sizeof(buf), &buf_offset);
if (res == DIVANS_FAILURE || (res == DIVANS_NEEDS_MORE_INPUT && len == 0)) {
divans_free_decompressor(state);
return res;
}
data += read_offset;
len -= read_offset;
fwrite(buf, buf_offset, 1, stdout);
} while (res != DIVANS_SUCCESS);
divans_free_decompressor(state);
return DIVANS_SUCCESS;
}
Structure of the divANS codebase
Top Level Modules
Module | Purpose |
---|---|
probability | Optimized implementations of 16-wide 4-bit CDF's that support online training and renormalization |
codec/interface | CrossCommandState tracks data to be kept between brotli commands. Examples include CDF's, the previous few bytes, the ring buffer for copies, etc |
codec/dict | Encode/decode parts of the file that may arise from the included brotli dictionary |
codec/copy | Encode/decode parts of the file that have already been seen before and are still in the ring buffer |
codec/block_type | Encode/decode markers in the file which divans can use as a prior for literals, distances or even command type |
codec/context_map | Encode/decode the brotli context_map which remaps the previous 6 bits and literal_block_type to a prior between 0 and 255 |
codec/literal | Encode/decode new raw data that appears in the file. This can use a number of strategies or combinations of strategies to encode each nibble |
codec/priors | Structs defining the size of the tables that contain dynamically-trained CDF holding statistics about past-data. |
codec/weights | struct that blend between multiple CDFs based on prior efficacy |
codec/specializations | Optimization system to generate separate codepaths for currently-running nibble-decode or encode path, based on which priors were selected |
codec | Encode/decode the overall commands themselves and track the state of the compression of the overall file and if it is complete |
divans_decompressor | Implementation of Decompressor trait that parses divans headers and translates the ANS stream into commands and into raw data |
brotli_ir_gen | Implementation of Compressor trait that calls into the brotli codec and extracts the command array per metablock to be encoded |
divans_compressor | Alternate implementation of Compressor trait that calls into raw_to_cmd instead of brotli to get the command array per metablock |
divans_to_raw | DecoderSpecialization for the codec to assume default input commands and incrementally populate them |
cmd_to_divans | EncoderSpecialization for the codec to take input commands and produce divans |
raw_to_cmd | Future: a substitute for the Brotli compressor to generate commands |
cmd_to_raw | Interpret a list of Brotli commands and produce the uncompressed file |
arithmetic_coder | Define EntropyEncoder and EntropyDecoder arithmetic coder traits |
ans | Fast implementation of EntropyEncoder and EntropyDecoder interfaces |
billing | Plugin to add attribution to an ArithmeticEncoderOrDecoder by providing the same interface and wrapping the en/decoder |
alloc_util | Allocator that reuses a single slice of memory over many allocations |
slice_util | A mechanism to borrow and reference an existing slice that can be frozen, unborrowing the slice, when divans returns to the caller to request more input or output space |
resizable_buffer | Simple resizing byte buffer that can hold the raw input and output streams being processed |
reader | Read implementation for both encoding and decoding of divans |
writer | Write implementation for both encoding and decoding of divans |
Overall flow
To Encode a file,
- a
writer::DivansBrotliHybridCompressorWriter
instantiates abrotli_ir_gen::BrotliDivansHybridCompressor
- The compressor has both a
brotli::BrotliEncoderStateStruct
from the brotli crate as well as acodec::DivansCodec<ANSEncoder, EncodeSpecialization>
. - Using
brotli_ir_gen::BrotliDivansHybridCompressor::encode
, the compressor feeds input data into thebrotli::BrotliEncoderStateStruct
- by calling
brotli::BrotliEncoderCompressStream
- by calling
brotli::BrotliEncoderCompressStream
can trigger a callback intobrotli_ir_gen::BrotliDivansHybridCompressor::divans_encode_commands
- The callback will consist of a slice of
brotli::interface::Command
items - These items are fed into the
codec::DivansCodec<ANSEncoder, EncodeSpecialization>::encode_or_decode
, which encodes them into divans format.codec::DivansCodec<ANSEncoder, cmd_to_divans::EncoderSpecialization>::encode_or_decode
accomplishes this by using theEncoderSpecialization
to pull input commands as the source of truth- unfortunately brotli can pass as much data as it wishes to the caller, up to the maximum metablock size of 16 megs.
- this means the caller has to buffer this data in a
resizable_buffer::ResizableBuffer
- this means the caller has to buffer this data in a
- The callback will consist of a slice of
- When all the callbacks have completed,
brotli_ir_gen::BrotliDivansHybridCompressor::encode_stream
does its best to flush the raw buffer - Eventually when the user calls
brotli_ir_gen::BrotliDivansHybridCompressor::flush
a similar procedure is followed but with finish flags set
To Decode a file,
- a
reader::DivansDecompressorReader
instantiates adivans_decompressor::DivansDecompressor
- The decompressor is an enum that switches from
HeaderParser
mode intoDecode
mode after the 16 byte raw header has been parsed divans_decompressor::DivansDecompressor::Decode
has acodec::DivansCodec<ANSDecoder, DecoderSpecialization>
within.- Using
brotli_ir_gen::DivansDeompressor::decode
, the decompressor feeds input data directly intocodec::DivansCodec<ANSDecoder, DecoderSpecialization>
- The codec.encode_or_decode is designed to receive commands as input when encoding, so the
divans_to_raw::DecoderSpecialization
simply makes placeholder commands for each type of command so that the same codepath can encode and decode commands
- The codec.encode_or_decode is designed to receive commands as input when encoding, so the
- when a final state is reached, a checksum is written and success is returned
The codec state machine
codec::DivansCodec
has three members, cross_command_state
, which tracks the probability models, the state
, to track which kind of command is being decoded, and codec_traits
, used as a repository of compiler constant values that happen to be set that way during this decode or encode phase based on the header and command data.
The state
value is an enumerant that can either carry command-specific information or can mark that the ring buffer must be populated, etc.
Overview of the available codec states
Begin
: This state means that the decoder is not in the middle of coding a particular command, so the next step will be to decode what the next command isLiteral(literal::LiteralState
: the coder is in the process of coding raw literals to be injected into the fileDict(dict::DictState)
: the coder is in the process of coding a word that appears in the brotli dictionaryCopy(copy::CopyState)
: the coder is in the process of coding a reference to pull data from the ring bufferBlockSwitchLiteral(block_type::LiteralBlockTypeState)
: The coder was instructed to serialize an arbitrary value that will affect how the predictor models future literalsBlockSwitchCommand(block_type::BlockTypeState)
: The coder was instructed to serialize an arbitrary value that will affect how the predictor models nothing (TODO)BlockSwitchDistance(block_type::BlockTypeState)
: The coder was instructed to serialize an arbitrary value that will affect how the predictor models distances to copy from and dictionary values.PredictionMode(context_map::PredictionModeState)
: The coder was instructed to serialize out a context map that remaps the BlockSwitchLiteral value plus the last 6 bits into a value in [0, 255] that is used as an index into the array of CDFs to be trainedPopulateRingBuffer(Command<AllocatedMemoryPrefix<u8, AllocU8>>)
When Literal, Dict, or Copy states reach their termination state, those states are moved into the PopulateRingBuffer state.PopulateRingBuffer
uses thecmd_to_raw::DivansRecoderState
stored inDivanCodec::CrossCommandState
to populate the ring buffer- If DecoderSpecialization is selected,
cmd_to_raw::DivansRecoderState
copies the data to the output bytes, returning and requesting NeedBytes until all bytes have been serialized - Otherwise the EncoderSpecialization avoids serializing those bytes.
- If DecoderSpecialization is selected,
- After all necessary bytes were serialized and the ring buffer populated, then the last_8_literals are saved to be used as future priors
WriteChecksum(usize)
This state happens if an end command (0xf) is encountered during a decode or acode::DivansCodec::flush
happens on encode- Currently checksum support is not active, but 8 bytes are simply serialized
DivansSuccess
This state is reached when WriteChecksum is complete on the decoder or when the final command is reached on the encoderEncodedShutdownNode
|ShutdownCoder
|CoderBufferDrain
appear only in teh encoder during flush/close after the EOF node type as flushed
Acknowledgements
Special thanks to Jaroslaw (Jarek) Duda and Fabian Giesen for genius work and their detailed and thoughtful presentation of the ANS algorithm.