Even if developers could afford patent searches, these are not a reliable method of avoiding the use of patented techniques. This is because patent searches do not reveal pending patent applications (which are kept confidential by the Patent Office). Since it takes several years on the average for a software patent to be granted, this is a serious problem: a developer could begin designing a large program after a patent has been applied for, and release the program before the patent is approved. Only later will the developer learn that distribution of the program is prohibited.
For example, the implementors of the widely-used public domain data
compression program compress followed an algorithm obtained from
the journal IEEE Computer. (This algorithm is also used in
several popular programs for microcomputers, including PKZIP.)
They and the user community were surprised to learn later that patent
number 4,558,302 had been issued to one of the authors of the article.
Now Unisys is demanding royalties for using this algorithm. Although
the program compress is still in the public domain, using it
means risking a lawsuit.
The Patent Office does not have a workable scheme for classifying software patents. Patents are most frequently classified by end results, such as "converting iron to steel;" but many patents cover algorithms whose use in a program is entirely independent of the purpose of the program. For example, a program to analyze human speech might infringe the patent on a speedup in the Fast Fourier Transform; so might a program to perform symbolic algebra (in multiplying large numbers); but the category to search for such a patent would be hard to predict.
You might think it would be easy to keep a list of the patented software techniques, or even simply remember them. However, managing such a list is nearly impossible. A list compiled in 1989 by lawyers specializing in the field omitted some of the patents mentioned in this paper.