:shocked_pikachu:

Renegadry aside, for those who are more interested in the Information Theory perspective on this:

Kolmogorov complexity is a good teaching tool, but hardly used in engineering practice because it contains serious foot-guns.

One example of defining K complexity(S, M) is the length of the shortest initial tape contents P for a given abstract machine M such that, when M is started on this tape, the machine halts with final tape contents P+S. Obviously, one must be very careful to define things like “initial state”, “input”, “halt”, and “length”, since not all universal machines look like Turing machines at first glance, and the alphabet size must either be consistent for all strings or else appear as an explicit log factor.

Mike’s intuitive understanding was incorrect in two subtle ways:

1. Without specifying the abstract machine M, the K complexity of a string S is not meaningful. For instance, given any S, one may define an abstract machine with a single instruction that prints S, plus other instructions to make M Turing complete. That is, for any string S, there is an M_S such that complexity(S, M_S) = 1 bit. Alternatively, it would be possible to define an abstract machine M_FS that supports filesystem operations. Then the complexity using Patrick’s solution could be made well-defined by measuring the length of the concatenation of the decompressor P with a string describing the initial filesystem state.

2. Even without adversarial examples, and with a particular M specified, uniform random strings’ K complexity is only _tightly concentrated around_ the strings’ length plus a machine-dependent constant. As Patrick points out, for any given string length, some individual string exemplars may have much smaller K complexity; for instance, due to repetition.