libaaruformat/docs/spec/appendixes/compression.adoc

[appendix]
== 🗃️ Compression Types

This apprendix lists all supported compression algorithms used within AaruFormat images.

NOTE: Compression method definitions may evolve over time.
For the latest and most accurate listing, refer to the `libaaruformat` source.

[cols="1,5",options="header"]
|===
|Value
|Algorithm

|0 |None
|1 |LZMA — 5-byte properties prefix followed by compressed stream
|2 |FLAC
|3 |LZMA after Claunia Subchannel Transform (see Appendix D) — 5-byte properties prefix followed by compressed stream
|4 |Zstandard (zstd) — standard zstd frame, no prefix
|5 |Zstandard after Claunia Subchannel Transform (see Appendix D) — standard zstd frame, no prefix
|===

=== 📝 Notes on LZMA Properties Prefix (IDs 1 and 3)

For LZMA-compressed blocks, the on-disk payload immediately following the block header is:

----
[ 5-byte LZMA properties ] [ compressed stream ]
----

The `cmpLength` field in the block header covers the **entire payload** including the 5-byte properties prefix:

----
cmpLength = 5 + compressed_stream_size
----

The `cmpCrc64` checksum covers only the **compressed stream**, excluding the properties prefix:

----
cmpCrc64 = CRC64(payload[5 .. cmpLength - 1])
----

To decompress:

1. Read `cmpLength` bytes from the file position after the block header.
2. The first 5 bytes are LZMA properties (see LZMA SDK specification).
3. The remaining `cmpLength - 5` bytes are the compressed stream.
4. Decompress into `length` bytes using the properties and the compressed stream.

=== 📝 Notes on Zstandard (IDs 4 and 5)

Unlike LZMA (IDs 1 and 3), Zstandard compressed payloads are stored as self-contained zstd frames.
The `cmpLength` field in the block header covers the entire compressed payload with no additional prefix bytes.

ID 5 (`ZstdCst`) applies the Claunia Subchannel Transform before compression and is restricted to blocks with data type `CdSubchannel`.
This mirrors the constraint on ID 3 (`LzmaCst`).