Puffer train (CA)

In a cellular automaton a puffer train or simply puffer is a finite pattern that moves itself across the universe, leaving debris behind. Thus a pattern consisting of only a puffer will grow arbitrarily large over time. Puffers differ from spaceships in that spaceships do not leave debris behind. Nonetheless, puffers are considered to have periods and speeds in the same way that spaceships have. (See the article on spaceships for more information.)

When considering the period of a puffer, sometimes there are two periods to consider. One is the period of the puffer. The other is the apparent period of the pattern of debris produced. This is often the same as the period of the puffer, but sometimes is a factor of the period. A puffer for which the apparent period deduced from the debris is smaller than the period of the engine is a pseudo period puffer. Such puffers are typically produced by artificial means. A true period puffer is one in which the period of the debris matches that of the puffer.

Puffers are divided into two classes, dirty puffers and clean puffers. While there is no precise distinction, a dirty puffer is one in which there is little order in the debris. (Of course, the debris will still eventually be periodic.) A clean puffer has a small amount of debris that appears much more organized. A dirty puffer can sometimes be turned into a clean puffer by adding spaceships of the same velocity as the puffer that affect what debris results.

A puffer whose debris consists entirely of spaceships is called a rake .