zlib

参考
- rfc1950 ZLIB Compressed Data Format Specification version 3.3
- rfc1951 DEFLATE Compressed Data Format Specification version 1.3
- https://en.wikipedia.org/wiki/DEFLATE

CMF
- 0-3 CM Compression method
  - 8 -> deflate
- 4-7 CINFO Compression info
  - CM=8 LZ77 window size, CINFO=7 -> 32K window size
FLG
- 0-4 - FCHECK (check bits for CMF and FLG)
- 5 - FDICT (preset dictionary)
- 6-7 - FLEVEL (compression level)
  - 0 - fastest
  - 1 - fast
  - 2 - default
  - 3 - maximum compression, slowest algorithm

file /tmp/data
# /tmp/data: zlib compressed data
printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" | cat - /tmp/data | gzip -dc > /tmp/out
cat /tmp/out

# pigz 并行压缩
brew install pigz
pigz -z test
pigz -d -z test.zz

# openssl zlib -d -in /tmp/data

# qpdf
zlib-flate -uncompress < FILE

Header

L	ZLIB	GZIP	Notes
1	78 01	1F 8B	No Compression/low
2	78 5E	1F 8B	Fast Compression
3	78 5E	1F 8B
4	78 5E	1F 8B
5	78 5E	1F 8B
6	78 9C	1F 8B	Default Compression
7	78 DA	1F 8B	Best Compression
8	78 DA	1F 8B
9	78 DA	1F 8B

CMF -Compression Method and flags

bits 0 to 3  CM     Compression method
bits 4 to 7  CINFO  Compression info

CM = 8
- Deflate
- window size up to 32K
- used by gzip and PNG
- CINFO - base-2 logarithm of the LZ77 window size, minus eight
  - CINFO=7 indicates a 32K window size
  - 因此大多前缀都是 78
CM = 15
- reserved

bits 0 to 4  FCHECK  (check bits for CMF and FLG)
bit  5       FDICT   (preset dictionary)
bits 6 to 7  FLEVEL  (compression level)

 +---+---+
 |CMF|FLG| (2 bytes - Defines the compression mode - More details below)
 +---+---+
 +---+---+---+---+
 |     DICTID    | (4 bytes. Present only when FLG.FDICT is set.) - Mostly not set
 +---+---+---+---+
 +=====================+
 |...compressed data...| (variable size of data)
 +=====================+
 +---+---+---+---+
 |     ADLER32   |  (4 bytes of checksum)
 +---+---+---+---+