--- /dev/null
+
+LZO stream format as understood by Linux's LZO decompressor
+===========================================================
+
+Introduction
+
+ This is not a specification. No specification seems to be publicly available
+ for the LZO stream format. This document describes what input format the LZO
+ decompressor as implemented in the Linux kernel understands. The file subject
+ of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on
+ the compressor nor on any other implementations though it seems likely that
+ the format matches the standard one. The purpose of this document is to
+ better understand what the code does in order to propose more efficient fixes
+ for future bug reports.
+
+Description
+
+ The stream is composed of a series of instructions, operands, and data. The
+ instructions consist in a few bits representing an opcode, and bits forming
+ the operands for the instruction, whose size and position depend on the
+ opcode and on the number of literals copied by previous instruction. The
+ operands are used to indicate :
+
+ - a distance when copying data from the dictionary (past output buffer)
+ - a length (number of bytes to copy from dictionary)
+ - the number of literals to copy, which is retained in variable "state"
+ as a piece of information for next instructions.
+
+ Optionally depending on the opcode and operands, extra data may follow. These
+ extra data can be a complement for the operand (eg: a length or a distance
+ encoded on larger values), or a literal to be copied to the output buffer.
+
+ The first byte of the block follows a different encoding from other bytes, it
+ seems to be optimized for literal use only, since there is no dictionary yet
+ prior to that byte.
+
+ Lengths are always encoded on a variable size starting with a small number
+ of bits in the operand. If the number of bits isn't enough to represent the
+ length, up to 255 may be added in increments by consuming more bytes with a
+ rate of at most 255 per extra byte (thus the compression ratio cannot exceed
+ around 255:1). The variable length encoding using #bits is always the same :
+
+ length = byte & ((1 << #bits) - 1)
+ if (!length) {
+ length = ((1 << #bits) - 1)
+ length += 255*(number of zero bytes)
+ length += first-non-zero-byte
+ }
+ length += constant (generally 2 or 3)
+
+ For references to the dictionary, distances are relative to the output
+ pointer. Distances are encoded using very few bits belonging to certain
+ ranges, resulting in multiple copy instructions using different encodings.
+ Certain encodings involve one extra byte, others involve two extra bytes
+ forming a little-endian 16-bit quantity (marked LE16 below).
+
+ After any instruction except the large literal copy, 0, 1, 2 or 3 literals
+ are copied before starting the next instruction. The number of literals that
+ were copied may change the meaning and behaviour of the next instruction. In
+ practice, only one instruction needs to know whether 0, less than 4, or more
+ literals were copied. This is the information stored in the <state> variable
+ in this implementation. This number of immediate literals to be copied is
+ generally encoded in the last two bits of the instruction but may also be
+ taken from the last two bits of an extra operand (eg: distance).
+
+ End of stream is declared when a block copy of distance 0 is seen. Only one
+ instruction may encode this distance (0001HLLL), it takes one LE16 operand
+ for the distance, thus requiring 3 bytes.
+
+ IMPORTANT NOTE : in the code some length checks are missing because certain
+ instructions are called under the assumption that a certain number of bytes
+ follow because it has already been garanteed before parsing the instructions.
+ They just have to "refill" this credit if they consume extra bytes. This is
+ an implementation design choice independant on the algorithm or encoding.
+
+Byte sequences
+
+ First byte encoding :
+
+ 0..17 : follow regular instruction encoding, see below. It is worth
+ noting that codes 16 and 17 will represent a block copy from
+ the dictionary which is empty, and that they will always be
+ invalid at this place.
+
+ 18..21 : copy 0..3 literals
+ state = (byte - 17) = 0..3 [ copy <state> literals ]
+ skip byte
+
+ 22..255 : copy literal string
+ length = (byte - 17) = 4..238
+ state = 4 [ don't copy extra literals ]
+ skip byte
+
+ Instruction encoding :
+
+ 0 0 0 0 X X X X (0..15)
+ Depends on the number of literals copied by the last instruction.
+ If last instruction did not copy any literal (state == 0), this
+ encoding will be a copy of 4 or more literal, and must be interpreted
+ like this :
+
+ 0 0 0 0 L L L L (0..15) : copy long literal string
+ length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte)
+ state = 4 (no extra literals are copied)
+
+ If last instruction used to copy between 1 to 3 literals (encoded in
+ the instruction's opcode or distance), the instruction is a copy of a
+ 2-byte block from the dictionary within a 1kB distance. It is worth
+ noting that this instruction provides little savings since it uses 2
+ bytes to encode a copy of 2 other bytes but it encodes the number of
+ following literals for free. It must be interpreted like this :
+
+ 0 0 0 0 D D S S (0..15) : copy 2 bytes from <= 1kB distance
+ length = 2
+ state = S (copy S literals after this block)
+ Always followed by exactly one byte : H H H H H H H H
+ distance = (H << 2) + D + 1
+
+ If last instruction used to copy 4 or more literals (as detected by
+ state == 4), the instruction becomes a copy of a 3-byte block from the
+ dictionary from a 2..3kB distance, and must be interpreted like this :
+
+ 0 0 0 0 D D S S (0..15) : copy 3 bytes from 2..3 kB distance
+ length = 3
+ state = S (copy S literals after this block)
+ Always followed by exactly one byte : H H H H H H H H
+ distance = (H << 2) + D + 2049
+
+ 0 0 0 1 H L L L (16..31)
+ Copy of a block within 16..48kB distance (preferably less than 10B)
+ length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte)
+ Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S
+ distance = 16384 + (H << 14) + D
+ state = S (copy S literals after this block)
+ End of stream is reached if distance == 16384
+
+ 0 0 1 L L L L L (32..63)
+ Copy of small block within 16kB distance (preferably less than 34B)
+ length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)
+ Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S
+ distance = D + 1
+ state = S (copy S literals after this block)
+
+ 0 1 L D D D S S (64..127)
+ Copy 3-4 bytes from block within 2kB distance
+ state = S (copy S literals after this block)
+ length = 3 + L
+ Always followed by exactly one byte : H H H H H H H H
+ distance = (H << 3) + D + 1
+
+ 1 L L D D D S S (128..255)
+ Copy 5-8 bytes from block within 2kB distance
+ state = S (copy S literals after this block)
+ length = 5 + L
+ Always followed by exactly one byte : H H H H H H H H
+ distance = (H << 3) + D + 1
+
+Authors
+
+ This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
+ analysis of the decompression code available in Linux 3.16-rc5. The code is
+ tricky, it is possible that this document contains mistakes or that a few
+ corner cases were overlooked. In any case, please report any doubt, fix, or
+ proposed updates to the author(s) so that the document can be updated.
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