PKWARE DCL Implode
PKWARE DCL Implode is a compressed data format associated with PKWARE Data Compression Library (DCL), a commercial data compression software product. It is not the only format supported by DCL, but it is possibly the most notable one.
The terms "blast" and "explode" are sometimes used in association with this format. When used as a file format, it may be identified as TTCOMP.
DCL Implode is not the same format as ZIP's old "implode" compression method (#6). It is unfortunate that there are two formats named "implode" associated with the same company. It can be used in ZIP, as compression method #10, but support for this is limited.
DCL Implode is based on LZ77 with Huffman coding. It uses only predefined Huffman codebooks, so it is simpler than most compression methods in this family.
A DCL Implode datastream starts with a two-byte header. The first byte indicates the type of compression:
0x00: binary compression
0x01: ASCII compression (a method optimized for compressing text)
The second byte specifies the size of the dictionary:
0x04: 1024 bytes
0x05: 2048 bytes
0x06: 4096 bytes
Most common is
0x00 0x06, indicating binary compression and a 4K dictionary.
Since the header bytes aren't very discriminating, it may be worth noting that the last 3 bytes of the datastream will match one of these 8 bit-patterns (assuming padding bits are 0, and there are no padding bytes):
... xxxxxxxx 0000000x 11111111 ... xxxxxxxx 10000000 01111111 ... 0xxxxxxx 11000000 00111111 ... 00xxxxxx 11100000 00011111 ... 000xxxxx 11110000 00001111 ... 0000xxxx 11111000 00000111 ... 00000xxx 11111100 00000011 ... 000000xx 11111110 00000001
Formats that may use DCL Implode compression include:
- CMZ (archive format)
- CP Shrink
- InstallShield Z
- PACKER (ImagiSOFT)
- ZAR (Zip-Archiv)
- ZIP (compression method #10)
- Reverse-engineered information by Ben Rudiak-Gould (Usenet post - comp.compression, 2001)
- zlib - Does not support DCL Implode directly, but its source code includes a decoder: contrib/blast.
- Lawine (a Starcraft/Brood War Library)
- https://github.com/JoshVarga/blast - An encoder/decoder written in Go
- PKWare Library, at old-dos.ru
- PKZIP Command Line v4.00 (Windows) - Supports DCLImplode compression
- PKWARE DCL
See also TTCOMP#Software.