56 lines
2.4 KiB
Markdown
56 lines
2.4 KiB
Markdown
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Simple Uxn LZ Format
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====================
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Goals:
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* Anyone can implement it
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* Small source code size
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* Easy to implement from Uxn
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* Mildly better than RLE
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Non-goals:
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* High compression ratio
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* High compression speed
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Format
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------
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It's a stream of commands. The first byte encodes the first command. Read the commands from the input until there's no more input.
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There are two commands. Literal and dictionary.
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```
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Byte 1 Byte 2+n
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┌─────────────────┐ ┌─────
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Literal │ 0 x x x x x x x │ │ ....
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(Always 1 byte) └─────────────────┘ └─────
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Length of literal Bytes to copy to output
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(Adjust by adding 1)
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Byte 1 Byte 2
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Dictionary ┌─────────────────┐ ┌─────────────────┐
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(2 bytes version)│ 1 0 x x x x x x │ │ x x x x x x x x │
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└─────────────────┘ └─────────────────┘
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Length of Offset into
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dictionary match dictionary
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(Adjust by adding 4) (Adjust by adding 1)
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Byte 1 Byte 2 Byte 3
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Dictionary ┌─────────────────┬─────────────────┐ ┌─────────────────┐
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(3 bytes version)│ 1 1 x x x x x x │ x x x x x x x x │ │ x x x x x x x x │
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└─────────────────┴─────────────────┘ └─────────────────┘
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Length of dictionary match Offset into
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(Adjust by adding 4) dictionary
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(Adjust by adding 1)
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```
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* The maximum dictionary history size is 256 bytes.
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* Dictionary offsets should be treated as the distance from the end of last byte that was output.
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* Example: an offset of 0 means go back by 1 bytes into the history.
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* `a b c d e f|g`
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* Example: an offset of 5 means go back by 6 bytes into the history.
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* `a|b c d e f g`
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