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Add some explanation of the Pletter format

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Sylvain Glaize 2024-02-21 22:43:47 +01:00
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@ -3,3 +3,54 @@ Unpletter is a C and C++ version of the unpacker for the Pletter compression sch
By Sylvain Glaize, 2024. By Sylvain Glaize, 2024.
Distributed as the same license as the Pletter packer. See license.txt on the root folder. Distributed as the same license as the Pletter packer. See license.txt on the root folder.
# Pletter format explained
Pletter is a compression scheme for the Z80 CPU, similar to ZX0 but with
different choices in terms of compression and a slightly different format.
The format intertwines two streams of data:
- the byte bitstream, which present only complete bytes to read (read 8 bit by 8 bit),
- the variable bitstream, which contains numbers in a variable number of bit format.
Thus, the algorithm maintain a pair of pointers : one for the byte bitstream, and one
for the variable bitstream (the pointer to the bitstream is actually itself two pointers,
one for the pointed byte and one for the bit inside the byte).
When reading a byte from the byte bitstream, the algorithm will get it and advance the
pointer to the next byte.
When reading a number from the variable bitstream, the algorithm with read the necessary
bits, and when it depletes the current byte bits, it will jump to the next byte pointed
by the byte stream pointer, and adjust this other pointer accordingly (move it one byte
further).
Numbers in the variable bitstream are encoded with an interlaced elias gamma code.
Elias gamma code is described here: https://en.wikipedia.org/wiki/Elias_gamma_coding.
The interlaced way of encoding it is by interleaving the bits of the first part (the
unary encoded power of 2) with the bits of the second part (the binary encoded number
of the rest). Also, in Pletter, the bits are inverted compared to what is described
on the Wikipedia page for the unary part.
Example:
- 5 encoding is `00 1 01` (2 to the power of 2, to which is added 1)
- Inverted, it is `11 0 01` (the bits are reversed)
- interlaced, it is `10 11 0`. This way, the algorithm can read the bit by pairs
and stops when it encounters a 0 for the first bit of the pair.
In Pletter, the variable stream contains the information on the next part to decode (0
means the next byte is to be copied as is ; 1 means it is a back reference block).
It also encodes the distance of the back reference block (the length is taken from the
byte stream).
# Change by the version found in the dsk2rom
In this version, the three bits normally written at the start of the algorithm to encoded the
additional bit number (q_value) for the back reference distance are ommitted.
The algorithm fixes this value to 2, and thus the alrigthm starts by copiing
the first byte as literal.