DICT Chunk

See more on Chunks in general.

This is a data used to seed a dictionary-based compression algorithm. A dictionary-based compression engine sucb as Deflation may be pre-loaded with data that is representative of the content being compressed. If multiple files refer to the same dictionary, you save space. Using a pre-defined dictionary really saves space, since it is not included explicitly in the archive at all.

DICT Chunk structure

Flags

Flagusage
a (correlated) Cleared.
b (subtype) Set.
r (range of instances) all used in the general manner.
p (multi-part) & c
y (payload specification)
i (instance sizes) Set when r is set, Clear otherwise.
n (pointer) Cleared.
If set, see the DICT-n or DICT-nd definitions instead.
d (redundant)

Although the y (payload specification) is allowed, it may not refer to itself!

Subtype

Subtype is set to 65 (hex 41).

Instance Number

The Instance Number is matched against a parameter in a COMP chunk that is documented to refer to a DICT instance.

Reserved and Predefined Instance Numbers

Instance numbers from hex 40 through hex FFF (64 through 4095) are reserved. An archive shall not contain instances in this range.

Pre-defined DICT definitions
InstanceMeaning
64C/C++ Source Code
65Perl Source Code
∴To be determined

The binary definition of these pre-defined dictionaries are found in the pre-defined chunk files.

DICT Chunk Payload

The Payload contains

Usage Notes

Examples

DICT-nd Chunk

A part number, as with DATA.

DICT-n Chunk

A URL.


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