linux/Documentation/lzo.txt
<<
>>
Prefs
   1
   2LZO stream format as understood by Linux's LZO decompressor
   3===========================================================
   4
   5Introduction
   6
   7  This is not a specification. No specification seems to be publicly available
   8  for the LZO stream format. This document describes what input format the LZO
   9  decompressor as implemented in the Linux kernel understands. The file subject
  10  of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on
  11  the compressor nor on any other implementations though it seems likely that
  12  the format matches the standard one. The purpose of this document is to
  13  better understand what the code does in order to propose more efficient fixes
  14  for future bug reports.
  15
  16Description
  17
  18  The stream is composed of a series of instructions, operands, and data. The
  19  instructions consist in a few bits representing an opcode, and bits forming
  20  the operands for the instruction, whose size and position depend on the
  21  opcode and on the number of literals copied by previous instruction. The
  22  operands are used to indicate :
  23
  24    - a distance when copying data from the dictionary (past output buffer)
  25    - a length (number of bytes to copy from dictionary)
  26    - the number of literals to copy, which is retained in variable "state"
  27      as a piece of information for next instructions.
  28
  29  Optionally depending on the opcode and operands, extra data may follow. These
  30  extra data can be a complement for the operand (eg: a length or a distance
  31  encoded on larger values), or a literal to be copied to the output buffer.
  32
  33  The first byte of the block follows a different encoding from other bytes, it
  34  seems to be optimized for literal use only, since there is no dictionary yet
  35  prior to that byte.
  36
  37  Lengths are always encoded on a variable size starting with a small number
  38  of bits in the operand. If the number of bits isn't enough to represent the
  39  length, up to 255 may be added in increments by consuming more bytes with a
  40  rate of at most 255 per extra byte (thus the compression ratio cannot exceed
  41  around 255:1). The variable length encoding using #bits is always the same :
  42
  43       length = byte & ((1 << #bits) - 1)
  44       if (!length) {
  45               length = ((1 << #bits) - 1)
  46               length += 255*(number of zero bytes)
  47               length += first-non-zero-byte
  48       }
  49       length += constant (generally 2 or 3)
  50
  51  For references to the dictionary, distances are relative to the output
  52  pointer. Distances are encoded using very few bits belonging to certain
  53  ranges, resulting in multiple copy instructions using different encodings.
  54  Certain encodings involve one extra byte, others involve two extra bytes
  55  forming a little-endian 16-bit quantity (marked LE16 below).
  56
  57  After any instruction except the large literal copy, 0, 1, 2 or 3 literals
  58  are copied before starting the next instruction. The number of literals that
  59  were copied may change the meaning and behaviour of the next instruction. In
  60  practice, only one instruction needs to know whether 0, less than 4, or more
  61  literals were copied. This is the information stored in the <state> variable
  62  in this implementation. This number of immediate literals to be copied is
  63  generally encoded in the last two bits of the instruction but may also be
  64  taken from the last two bits of an extra operand (eg: distance).
  65
  66  End of stream is declared when a block copy of distance 0 is seen. Only one
  67  instruction may encode this distance (0001HLLL), it takes one LE16 operand
  68  for the distance, thus requiring 3 bytes.
  69
  70  IMPORTANT NOTE : in the code some length checks are missing because certain
  71  instructions are called under the assumption that a certain number of bytes
  72  follow because it has already been guaranteed before parsing the instructions.
  73  They just have to "refill" this credit if they consume extra bytes. This is
  74  an implementation design choice independent on the algorithm or encoding.
  75
  76Byte sequences
  77
  78  First byte encoding :
  79
  80      0..17   : follow regular instruction encoding, see below. It is worth
  81                noting that codes 16 and 17 will represent a block copy from
  82                the dictionary which is empty, and that they will always be
  83                invalid at this place.
  84
  85      18..21  : copy 0..3 literals
  86                state = (byte - 17) = 0..3  [ copy <state> literals ]
  87                skip byte
  88
  89      22..255 : copy literal string
  90                length = (byte - 17) = 4..238
  91                state = 4 [ don't copy extra literals ]
  92                skip byte
  93
  94  Instruction encoding :
  95
  96      0 0 0 0 X X X X  (0..15)
  97        Depends on the number of literals copied by the last instruction.
  98        If last instruction did not copy any literal (state == 0), this
  99        encoding will be a copy of 4 or more literal, and must be interpreted
 100        like this :
 101
 102           0 0 0 0 L L L L  (0..15)  : copy long literal string
 103           length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte)
 104           state = 4  (no extra literals are copied)
 105
 106        If last instruction used to copy between 1 to 3 literals (encoded in
 107        the instruction's opcode or distance), the instruction is a copy of a
 108        2-byte block from the dictionary within a 1kB distance. It is worth
 109        noting that this instruction provides little savings since it uses 2
 110        bytes to encode a copy of 2 other bytes but it encodes the number of
 111        following literals for free. It must be interpreted like this :
 112
 113           0 0 0 0 D D S S  (0..15)  : copy 2 bytes from <= 1kB distance
 114           length = 2
 115           state = S (copy S literals after this block)
 116         Always followed by exactly one byte : H H H H H H H H
 117           distance = (H << 2) + D + 1
 118
 119        If last instruction used to copy 4 or more literals (as detected by
 120        state == 4), the instruction becomes a copy of a 3-byte block from the
 121        dictionary from a 2..3kB distance, and must be interpreted like this :
 122
 123           0 0 0 0 D D S S  (0..15)  : copy 3 bytes from 2..3 kB distance
 124           length = 3
 125           state = S (copy S literals after this block)
 126         Always followed by exactly one byte : H H H H H H H H
 127           distance = (H << 2) + D + 2049
 128
 129      0 0 0 1 H L L L  (16..31)
 130           Copy of a block within 16..48kB distance (preferably less than 10B)
 131           length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte)
 132        Always followed by exactly one LE16 :  D D D D D D D D : D D D D D D S S
 133           distance = 16384 + (H << 14) + D
 134           state = S (copy S literals after this block)
 135           End of stream is reached if distance == 16384
 136
 137      0 0 1 L L L L L  (32..63)
 138           Copy of small block within 16kB distance (preferably less than 34B)
 139           length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)
 140        Always followed by exactly one LE16 :  D D D D D D D D : D D D D D D S S
 141           distance = D + 1
 142           state = S (copy S literals after this block)
 143
 144      0 1 L D D D S S  (64..127)
 145           Copy 3-4 bytes from block within 2kB distance
 146           state = S (copy S literals after this block)
 147           length = 3 + L
 148         Always followed by exactly one byte : H H H H H H H H
 149           distance = (H << 3) + D + 1
 150
 151      1 L L D D D S S  (128..255)
 152           Copy 5-8 bytes from block within 2kB distance
 153           state = S (copy S literals after this block)
 154           length = 5 + L
 155         Always followed by exactly one byte : H H H H H H H H
 156           distance = (H << 3) + D + 1
 157
 158Authors
 159
 160  This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
 161  analysis of the decompression code available in Linux 3.16-rc5. The code is
 162  tricky, it is possible that this document contains mistakes or that a few
 163  corner cases were overlooked. In any case, please report any doubt, fix, or
 164  proposed updates to the author(s) so that the document can be updated.
 165