Fix LZMA dictionary size parsing in 7z archives (read 4 bytes instead of 3)

[analytical-engines]

Report: LZMA dictionary size incorrectly parsed (only 3 bytes read instead of 4)

Summary

In 7zFolder.swift, the LZMA dictionary size is read using only 3 bytes instead of 4, causing decompression to fail with LZMAError.notEnoughToRepeat for archives with dictionary sizes >= 16MB.

Environment

• SWCompression version: 4.8.6
• Platform: macOS 15.2 (Darwin 25.1.0)
• Swift version: 5.x

Steps to Reproduce

1. Create or obtain a 7z archive compressed with LZMA and dictionary size 16MB or larger (e.g., LZMA:24 = 2^24 = 16MB)
2. Attempt to decompress using SevenZipContainer.open(container:)
3. Decompression fails with LZMAError.notEnoughToRepeat

Expected Behavior

Archive should decompress successfully, as it does with the official 7z command-line tool and other applications.

Actual Behavior

Decompression fails with LZMAError.notEnoughToRepeat.

Root Cause

In Sources/7-Zip/7zFolder.swift, lines 174-176:

  var dictionarySize = 0
  for i in 1..<4 {
      dictionarySize |= properties[i].toInt() << (8 * (i - 1))
  }

The loop 1..<4 only iterates over indices [1, 2, 3], reading 3 bytes. However, the LZMA dictionary size is stored as a 4-byte little-endian integer in properties[1...4].

For a dictionary size of 16MB (0x01000000 in little-endian):

• properties[1] = 0x00

• properties[2] = 0x00

• properties[3] = 0x00

• properties[4] = 0x01 ← Not read!

  Result: dictionarySize is computed as 0, then adjusted to the minimum 4096 bytes by LZMAProperties. The decoder allocates a 4KB dictionary instead of the required 16MB, causing notEnoughToRepeat when the data references bytes beyond this undersized window.

Proposed Fix

Change 1..<4 to 1..<5:

  var dictionarySize = 0
  for i in 1..<5 {
      dictionarySize |= properties[i].toInt() << (8 * (i - 1))
  }

Verification

• Test archive info from 7z t:

• Method = LZMA:24

• Solid = +

• Physical Size = 86250112

  The archive is valid and decompresses correctly with the official 7-Zip implementation.

Related

This may be related to issue #40, which was fixed in 4.8.5 for LZMA2. However, this bug is in the LZMA (not LZMA2) code path in 7zFolder.swift.