Puffin: A deterministic deflate re-compressor (for patching purposes)

Table of Contents

Puffin

Puffin is a deterministic deflate recompressor. It is mainly used for patching deflate compressed images (zip, gzip, etc.) because patches created from deflate files/streams are usually large (deflate has a bit-aligned format, hence, changing one byte in the raw data can cause the entire deflate stream to change drastically.)

Puffin has two tools (operations): puffdiff and puffpatch (shown here.) The purpose of puffdiff operation is to create a patch between a source and a target file with one or both of them having some deflate streams. This patch is used in the puffpatch operation to generate the target file from the source file deterministically. The patch itself is created by bsdiff library (but can be replaced by any other diffing mechanism). But, before it uses bsdiff to create the patch, we have to transform both the source and target files into a specific format so bsdiff can produce smaller patches. We define puff operation to perform such a transformation. The resulting stream is called a puff stream. The reverse transformation (from puff stream to deflate stream) is called a huff operation. huff is used in the client to transform the puff stream back into its original deflate stream deterministically.

For information about deflate format see RFC 1951.

puff and huff

puff is an operation that decompresses only the Huffman part of the deflate stream and keeps the structure of the LZ77 coding unchanged. This is roughly equivalent of decompressing ‘half way’.

huff is the exact opposite of puff and it deterministically converts the puff stream back to its original deflate stream. This deterministic conversion is based on two facts:

• There is no need to perform LZ77 algorithm. This means the deflate stream could be built by any LZ77 algorithm.
• The dynamic Huffman tables can be recreated uniquely from the code length array stored inside the puff stream. This means the deflate stream can be reconstructed deterministically using the Huffman tables.

The inclusion of Huffman tables in the puff stream has minuscule space burden (on average maximum 320 bytes for each block. There is roughly one block per 32KB of uncompressed data).

bsdiff of two puffed streams has much smaller patch in comparison to their deflate streams, but it is larger than uncompressed streams.

The major benefits

• Size of the patch is smaller than deflate stream’s patch.
• puff and huff are deterministic operations.
• huff is in order of 10X faster than full recompression. It is even faster than Huffman algorithm because it already has the Huffman tables and does not need to reconstruct it.
• Both algorithms have very low memory footprint.
• The underlying LZ77 algorithm can be anything (as long as it is deflate compatible). This includes google’s zopfli

The drawbacks

• The need to define a file format for the puffed stream and stay with this format forever. If format needs to be changed in the future, then some versioning mechanism should be there to handle it and backward compatibility should be maintained.
• The need to define a payload format and stay with it forever. Similarly needs to be versioned if required later change.
• Does not reduces the patch size as full recompression.

puffdiff and puffpatch

A deflate compressed file (gzip, zip, etc.) contains multiple deflate streams at different locations. When this file is puffed, the resulting puff stream contains both puffs and the raw data that existed between the deflates in the compressed file. When performing huff operation, the location of puffs in the puff stream and deflates in the deflate stream should be passed upon. This is necessary as huff operation has to know exactly where the locations of both puffs and deflates are. (See the following image)

Similarly puffpatch requires deflates and puffs locations in both the source and target streams. These location information is saved using Protobufs in the patch generated by bsdiff. One requirement for these two operations are that puffpatch has to be efficient in the client. Client devices have limited memory and CPU bandwidth and it is necessary that each of these operations are performed with the most efficiency available. In order to achieve this efficiency a new operation can be added to bspatch, that reads and writes into a puff streams using special interfaces for puffing and huffing on the fly.

Puffin Patch Format

• Magic (4 bytes) - The string literal “PUF1”.
• Header Length (4 bytes) - The size of the header (length of the generated Protobuf).
• Header - Lengths and locations of deflate and puffs streams in source and target files in a Protobuf format. See puffin.proto.
• Patch - This is a binary array directly generated by the bsdiff program.

Puffin Stream Format

• Block Header (3+ bytes) - Defines the type of the block.
• Data - A mix of literals list and copy length/distances.
• End of Block (2 bytes) - The end of block symbol. Similar to the symbols for copy length/distance but without the distance bytes. The actual copy length value is 259 (0x81FF).

Block Header Format

• Length (2 Bytes) - The size of the block header excluding the two Length bytes itself - 1.
• Final Block (1 Bit) - Whether the block is the last one or not.
  • 1 - Final block
  • 0 - Middle block
• Type (2 Bits)
  • 0 - Uncompressed. Immediately after the header, zero or one literals list is present which defines the content of the uncompressed blocks.
  • 1 - Fixed Huffman table. The list of literals and length/distances comes immediately after the header. Fixed Huffman table is defined the deflate specification and will not change.
  • 2 - Dynamic Huffman table. The dynamic Huffman table comes at the end of the block header.
  • 3 - Undefined. This is an error.
• Skip Bits (5 Bits) - Used only for uncompressed blocks (For other types its value is zero). In an uncompressed block, the RFC 1951 skips any bits after reading final block and type bits until the byte boundary in the input. However, it does not define what the value of these bits should be. Most implementations assume 0, but some implementations may use any value. This segment contains those bits as a five-bits integer. When writing the block header back to the deflate format, the actual number of bits which where skipped will be calculated easily.
• Huffman Table - It only comes for dynamic Huffman table.

Dynamic Huffman Table Format

Depending on the scheme for storing the Huffman tables, the payload size can change. We discovered that the following scheme does not produce the smallest payload, but it is the most deterministic one. In a deflate stream, Huffman tables for literals/length and distances are themselves Huffman coded. In this format, we also puff the Huffman tables into the puff stream instead of completely decompressing them.

There are three tables stored in this structure very similar to the one defined in RFC 1951. A Huffman table can be defined as an array of unsigned integer code length values. Three Puffed Huffman tables appear like the following scheme. The first table (codes) is the Huffman table used to decode the next two Huffman tables. The second Huffman table is used to decode literals and lengths, and the third Huffman table is used to decode distances.

• Literal/Length Count (1 byte) - Number of alphabets used for literal/length Huffman codes - 257.
• Distance Count (1 byte) - Number of alphabets used for distance Huffman codes - 1.
• Alphabet Count (1 byte) - Number of alphabets for coding two previous Huffman tables - 4.
• Code Lengths ((Alphabet Count + 1) / 2 bytes) - A list of codes for reading the next two Huffman tables. Each byte contains two codes. If the number of codes is odd, the last four Bits will be zero.
• Literal/Length/Distance Code Lengths - List of code lengths for encoding literals/lengths/distance followed The encoding is as follows:
  • [0..15] - Represent code [0..15]
  • [16..19] - Copy the last code length [3..6] times.
  • [20..27] - Repeat code length of 0 for [3..10] times.
  • [28..155] - Repeat code length of 0 for [11..138] times.

Literals List

Literals lists are constructed by a “length” value followed by “length” bytes of literals. The Puffer should try to merge adjacent literals lists as much as possible into one literals list in the puff stream. This Is a length value followed by length bytes of literals (Even if there is only one literal.)

• Tag (1 Bit) - The value is 0 indicating that this is a list of literals (not a copy length/distance).
• Length - The number of literals that would follow in the list.
  • (7 Bits) Length = [1 .. 127], The value is: Length - 1
  • (7 Bits + 2 Bytes) Length = [128 .. 65663], The values are: 127, Length - 127. Conserves size by using only one byte if the number of upcoming literals is smaller or equal to 127 (About 99% of literals length in a normal deflate stream fit into this category.) We should never have zero length literals. Otherwise it will use three bytes.
• Literals (Length Bytes) - A sequence of Length number of literals.

Copy Length/Distance

This Is a Length value followed by a Distance value.

• Tag (1 Bit) - The value is 1 indicating that this is a copy length/distance field.
• Length - Conserves size by using only one byte if the length value is smaller or equal to 129. Otherwise two bytes are used.
  • (7 Bits) - Length = [3 .. 129], The value is: Length - 3
  • (7 Bites + 1 Byte) Length = [130 .. 258], The value is: 127, Length - 130:
• Distance (2 Bytes) - Distance = [1 .. 32768], The value is: Distance - 1. The distance value as an unsigned integer.

Building Puffin

Currently Puffin is being used in both Android and Chrome OS and is built differently in each of them. There is also a Makefile build, but it is not comprehensive.

Usage

Puffin builds an executable puffin which can be used to perform diffing and patching algorithms using Puffin format. To get the list of options available run:

puffin --help

It can also be used as a library (currently used by update_engine) that provides different APIs.

Compute Patch

To compute the diff between our current state and the original changes: git diff 4180a65119ef2c333c4d33c9e39869da89a8faea -- .

References