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spec: clarify LZMA properties prefix in cmpLength and cmpCrc64 scope

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Kevin Bortis
2026-03-29 15:28:08 +02:00
parent 3af5f1ec41
commit 2cbd88a15b
5 changed files with 37 additions and 10 deletions
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31
docs/spec/appendixes/compression.adoc
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@@ -12,13 +12,40 @@ For the latest and most accurate listing, refer to the `libaaruformat` source.
|Algorithm
|0 |None
|1 |LZMA — stream prepended by 5 bytes of parameters
|1 |LZMA — 5-byte properties prefix followed by compressed stream
|2 |FLAC
|3 |LZMA after Claunia Subchannel Transform (see Appendix D) — stream prepended by 5 bytes of parameters
|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.

4
docs/spec/blocks/data.adoc
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@@ -56,7 +56,7 @@ typedef struct BlockHeader
|uint32_t
|4 bytes
|cmpLength
|The size in bytes of the compressed data that follows this header.
|The total size in bytes of the compressed payload that follows this header. For LZMA (IDs 1, 3) this includes the 5-byte properties prefix; see Appendix C for details.
|uint32_t
|4 bytes
@@ -66,7 +66,7 @@ typedef struct BlockHeader
|uint64_t
|8 bytes
|cmpCrc64
|The CRC64-ECMA checksum of the compressed data that follows this header.
|CRC64-ECMA of the compressed payload. For LZMA (IDs 1, 3) this covers the compressed stream only, excluding the 5-byte properties prefix; see Appendix C for details.
|uint64_t
|8 bytes

4
docs/spec/blocks/datastream_payload.adoc
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@@ -54,7 +54,7 @@ typedef struct DataStreamPayloadHeader
|uint32_t
|4 bytes
|cmpLength
|Compressed length in bytes (includes LZMA properties if compression = Lzma)
|Total compressed payload size in bytes. For LZMA this includes the 5-byte properties prefix; see Appendix C.
|uint32_t
|4 bytes
@@ -64,7 +64,7 @@ typedef struct DataStreamPayloadHeader
|uint64_t
|8 bytes
|cmpCrc64
|CRC64-ECMA checksum of the compressed payload data
|CRC64-ECMA of the compressed payload. For LZMA this covers the compressed stream only, excluding the 5-byte properties prefix; see Appendix C.
|uint64_t
|8 bytes

4
docs/spec/blocks/ddt.adoc
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@@ -61,7 +61,7 @@ typedef struct DdtHeader
|uint64_t
|8 bytes
|cmpLength
|The size in bytes of the compressed table that follows this header.
|The total size in bytes of the compressed payload that follows this header. For LZMA this includes the 5-byte properties prefix; see Appendix C.
|uint64_t
|8 bytes
@@ -71,7 +71,7 @@ typedef struct DdtHeader
|uint64_t
|8 bytes
|cmpCrc64
|The CRC64-ECMA checksum of the compressed table that follows this header.
|CRC64-ECMA of the compressed payload. For LZMA this covers the compressed stream only, excluding the 5-byte properties prefix; see Appendix C.
|uint64_t
|8 bytes

4
docs/spec/blocks/ddt2.adoc
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@@ -114,7 +114,7 @@ Writers *MUST NOT* produce this identifier in new images.
|uint64_t
|8 bytes
|cmpLength
|The size in bytes of the compressed table that follows this header.
|The total size in bytes of the compressed payload that follows this header. For LZMA this includes the 5-byte properties prefix; see Appendix C.
|uint64_t
|8 bytes
@@ -124,7 +124,7 @@ Writers *MUST NOT* produce this identifier in new images.
|uint64_t
|8 bytes
|cmpCrc64
|The CRC64-ECMA checksum of the compressed table that follows this header.
|CRC64-ECMA of the compressed payload. For LZMA this covers the compressed stream only, excluding the 5-byte properties prefix; see Appendix C.
|uint64_t
|8 bytes