Quantum Compression

Purpose

This guide explains Quantum compression, a legacy statistical compression algorithm used in older CAB files.

References

Getting Started Guide
CAB Format

Concepts

What is Quantum?

Quantum is a statistical compression algorithm developed by David Stafford for Microsoft. It uses probability modeling and arithmetic coding to achieve good compression ratios.

Key Features

Statistical compression - Uses probability models
Configurable window - 10 to 21 bits (1 KB to 2 MB)
Legacy algorithm - Rarely used in new applications
CAB only - Exclusive to CAB format

Status

Quantum is considered experimental in Cabriolet. It’s supported for compatibility with legacy CAB files but not recommended for new applications.

Performance characteristics

Metric	Value
Compression ratio	45-65% for text, 35-55% for executables
Compression speed	Medium (5-15 MB/s)
Decompression speed	Medium (10-50 MB/s)
Memory usage	Medium to High (window-dependent)
Window size	Configurable: 2^10 to 2^21 bytes (1 KB to 2 MB)

Metric

Value

Compression ratio

45-65% for text, 35-55% for executables

Compression speed

Medium (5-15 MB/s)

Decompression speed

Medium (10-50 MB/s)

Memory usage

Medium to High (window-dependent)

Window size

Configurable: 2^10 to 2^21 bytes (1 KB to 2 MB)

Usage

CAB Files Only

compressor = Cabriolet::CAB::Compressor.new(compression: :quantum)
compressor.add_file('data.txt')
compressor.write('archive.cab')

Command line:

cabriolet create --compression=quantum archive.cab files/

Quantum is only available in CAB format. Other formats do not support it.

Window Sizes

Bits	Window Size	Notes
10	1 KB	Minimum size
15	32 KB	Typical small files
16	64 KB	Default
18	256 KB	Large files
21	2 MB	Maximum size

Bits

Window Size

Notes

10

1 KB

Minimum size

15

32 KB

Typical small files

16

64 KB

Default

18

256 KB

Large files

21

2 MB

Maximum size

Algorithm Details

Compression Process

Model building - Create probability model of input
Symbol encoding - Encode symbols based on probabilities
Arithmetic coding - Compress using statistical encoding
Window sliding - Maintain context window

Characteristics

Context-based - Uses previous data to predict next
Adaptive - Model updates as compression proceeds
Complex - More sophisticated than dictionary methods

Comparison

Algorithm	Ratio	Speed	Recommended
LZSS	30-50%	Fast	Yes (for SZDD/HLP)
MSZIP	40-60%	Medium	Yes (default)
LZX	50-70%	Slow	Yes (for distribution)
Quantum	45-65%	Medium	No (legacy only)

Algorithm

Ratio

Speed

Recommended

LZSS

30-50%

Fast

Yes (for SZDD/HLP)

MSZIP

40-60%

Medium

Yes (default)

LZX

50-70%

Slow

Yes (for distribution)

Quantum

45-65%

Medium

No (legacy only)

When to Use Quantum

Use Quantum only when:

Compatibility required - Legacy CAB files use Quantum
Extracting old files - Decompressing existing Quantum CABs
Historical reasons - Maintaining old systems

Do NOT use Quantum for:

New applications - Use MSZIP or LZX instead
Maximum compression - LZX is better
Maximum speed - LZSS or None is faster
Non-CAB formats - Not supported

Recommended Alternatives

Instead of Quantum, use:

Use Case	Recommended	Why
General purpose	MSZIP	Better support, similar ratio
Maximum compression	LZX	Higher ratio, wider support
Maximum speed	LZSS or None	Faster compression/decompression
Legacy compatibility	Quantum	Only valid use case

Use Case

Recommended

Why

General purpose

MSZIP

Better support, similar ratio

Maximum compression

LZX

Higher ratio, wider support

Maximum speed

LZSS or None

Faster compression/decompression

Legacy compatibility

Quantum

Only valid use case

Examples

Extract Legacy CAB

require 'cabriolet'

# Decompress old Quantum CAB
decompressor = Cabriolet::CAB::Decompressor.new
cabinet = decompressor.open('old-archive.cab')

cabinet.files.each do |file|
  puts "#{file.filename} - Quantum compressed"
end

decompressor.extract_all(cabinet, 'output/')

Convert to Modern Format

require 'cabriolet'

# Extract Quantum CAB
old_decompressor = Cabriolet::CAB::Decompressor.new
old_cabinet = old_decompressor.open('quantum.cab')
old_decompressor.extract_all(old_cabinet, 'temp/')

# Re-compress with MSZIP
new_compressor = Cabriolet::CAB::Compressor.new(compression: :mszip)

Dir.glob('temp/**/*').each do |file|
  next if File.directory?(file)
  new_compressor.add_file(file)
end

new_compressor.write('mszip.cab')

puts "Converted Quantum → MSZIP"

Identify Compression Type

require 'cabriolet'

decompressor = Cabriolet::CAB::Decompressor.new
cabinet = decompressor.open('archive.cab')

cabinet.folders.each_with_index do |folder, i|
  case folder.compression
  when :quantum
    puts "Folder #{i}: Quantum (legacy)"
  when :mszip
    puts "Folder #{i}: MSZIP (recommended)"
  when :lzx
    puts "Folder #{i}: LZX (maximum compression)"
  when :none
    puts "Folder #{i}: Uncompressed"
  end
end

Troubleshooting

"Unsupported Quantum"

If you see experimental/unsupported warnings:

Quantum support is limited
Use for extraction only
Don’t create new Quantum CABs

Poor Performance

Quantum may be slower than expected:

Use MSZIP for better balance
Use LZX for better compression
Use LZSS for better speed

Compatibility Issues

Quantum CABs may not work everywhere:

Some tools don’t support Quantum
MSZIP is more widely compatible
LZX is better supported

Migration Guide

To migrate from Quantum:

Extract existing CABs ruby decompressor = Cabriolet::CAB::Decompressor.new decompressor.open('quantum.cab') decompressor.extract_all(cabinet, 'extracted/')
Re-compress with MSZIP or LZX ruby compressor = Cabriolet::CAB::Compressor.new(compression: :mszip) # or compressor = Cabriolet::CAB::Compressor.new(compression: :lzx)
Test thoroughly bash cabriolet test new-archive.cab

Best practices

Avoid for new files - Use MSZIP or LZX instead
Extract only - Don’t create new Quantum CABs
Convert old files - Migrate to modern algorithms
Document usage - Note why Quantum is used if required
Test compatibility - Verify decompression works