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docs: amend 7z ranged design for encoded (LZMA) headers

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Live spike showed start+end sparse reconstruction is insufficient for
encoded-header 7z; fetch the mid-file packed header region as a 3rd
region (parse PackInfo). Adds Task 4b.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
admin
2026-07-27 11:49:19 +02:00
co-authored by Claude Opus 4.8
parent abdfa437d9
commit 3595f6f097
2 changed files with 34 additions and 7 deletions
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docs/superpowers/specs/2026-07-27-ranged-archive-listing-design.md
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@@ -82,14 +82,29 @@ the end; the signature header stores the end header's location.
1. Ranged-read `[0, 32)`; validate magic `37 7A BC AF 27 1C`. Read LE `uint64`
`NextHeaderOffset` (byte 12) and `NextHeaderSize` (byte 20). End header is at absolute offset
`32 + NextHeaderOffset`, length `NextHeaderSize`.
2. Ranged-read `[32 + NextHeaderOffset, NextHeaderSize)`.
3. `listFromSparse` with regions `{0: sigHeader}` and `{32+NextHeaderOffset: endHeader}`, total
= file size, runner = `7z l`. `parse7zOutput` yields names+sizes (`crc32: null`, as today).
4. Return `null` on bad magic / read failure / CLI error.
2. Ranged-read `[32 + NextHeaderOffset, NextHeaderSize)` — the "next header".
3. Branch on the next header's first byte (a 7z property id):
- **`0x01` (kHeader, plain/uncompressed header):** two regions suffice —
`{0: sigHeader}` and `{32+NextHeaderOffset: endHeader}`.
- **`0x17` (kEncodedHeader, LZMA-compressed header):** the next header is only a *descriptor*
whose `PackInfo` points at a packed header stream stored **in the middle** of the file (not
at EOF). Parse the descriptor's `StreamsInfo → kPackInfo (0x06)` to read `PackPos` and the
`PackSize`s (7z variable-length "numbers"; sum them). Ranged-read the contiguous packed
region `[32 + PackPos, Σ PackSize)` and add it as a **third** sparse region. `7z l` then
decodes the header from that region.
- **anything else:** return `null` (→ fallback).
4. `listFromSparse` with the 2 or 3 regions, runner = `7z l`. `parse7zOutput` yields
names+sizes (`crc32: null`, as today). Return `null` on bad magic / read failure / CLI error.
`7z l` seeks to the end header (incl. decoding an LZMA-encoded header via the real binary) and
never reads the packed-stream gap, so the sparse holes are untouched. All 7z placeholders are
single-part.
**Why the third region is required (spike finding, 2026-07-27):** the original two-region
(start+end) reconstruction was proven insufficient in a live test — `7z l` rejected it with
"Cannot open the file as [7z] archive" because these archives use an *encoded* header whose
compressed bytes live in a packed stream in the file body (a sparse hole), not at EOF. The
`0x17` branch fetches exactly that packed region. `7z l` still never touches the file-data
packed streams (it only lists), so those gaps stay sparse. The `read7zNumber` reader (7z's
base-128-ish variable-length integer with a first-byte length mask) and the minimal
`kPackInfo` walk are the only new 7z binary parsing; `7z l` still does the actual file listing.
All 7z placeholders are single-part.
### RAR ranged listing