You can freely build on How Computers Compress Data Using Dictionary Building

What this patent covers

This patent details a system that shrinks data, like text or files, making them take up less space. It works by reading through the data and building a 'dictionary' of frequently occurring sequences of characters, called strings. When it finds a string in the data that's already in its dictionary, it replaces that string with a short code. If it finds a string that's not quite in the dictionary but is close, it adds a new, longer string to the dictionary. This new string is made up of the longest matching string it found and the very next character in the data. The system uses a clever 'hashing' technique to quickly search its dictionary for matches. To get the original data back, a separate 'decompressor' uses the same dictionary-building logic to reconstruct the data from the codes.

What is now free to use

All 1 claims of US 4558302 are in the public domain. Specifically:

The 1 dependent claim add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• Data compression that doesn't involve building a dictionary of previously seen strings.

• Compression methods that don't use a 'longest match' strategy to find patterns.

• Systems that don't use a hashing function to speed up dictionary lookups.

• Decompression methods that don't reconstruct the dictionary in a similar way to the compressor.

• Compression that doesn't extend a matched string with the next input character to create a new dictionary entry.

Who is building on this today

The principles described in this patent are fundamental to countless software applications and operating systems today. Major tech companies like Microsoft (for ZIP support in Windows), Apple (for file archiving), and Google (for data handling in Android and ChromeOS) implement variations of these compression techniques. Many open-source libraries also build upon these foundational concepts.

Products built on expired version of this technology

How to cite this patent in your documentation

Sperry Corp. US Patent 4558302. High speed data compression and decompression apparatus and method. Granted 1985, expired 2003. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.