What do you mean, "analog computer"? Wouldn't it need to be digital? Remember, "digital" does not have to be computerized. A mechanical adding machine can be digital.
Well, the Purple code is like the Enigma. It consists of a bunch of interlocked letter wheels. The interlocking relationship changes after each letter is typed. In effect, it is exactly the same technology as an odometer, which is certainly analog.
A digital computer works on numbers. An analog computer works on mechanical relationships. I would be interested in hearing of any mechanical adding machine that was digital. An analog computer, such as an odometer, is usally single purpose while a digital computer is multipurpose. Ortolan88
Sure thing -- I have used (obsolete) mechanical digital adding machines to add, subtract, multiply, and, with enormous effort, take square roots. The numbers were represented as digits using a series of wheels, one for each power of ten. Babbage's Analytical Engine was another example of a mechanical digital computer.
I'd also disagree with you about an odometer being analog. If the number is stored as a set of digits, then the representation of the number is digital, even if the representation of each digit is analog. Note also that the digits (except the lowest) move in discrete steps.
Here are four examples, showing the full gamut:
- Mechanical / digital: Babbage's Analytical Engine
- Mechanical / analog: Mechanical rotating-wheel integrators, slide rules
- Electronic / digital: nearly all modern computers, pocket calculators
- Electronic / analog: Op-amp based 1960s and 1970s analog computers
Note also that there is nothing requiring a digital computer to be programmable, or Turing-complete (even though nearly all are). For example, FPGA codebreaking engines and galactic physics simulators are special-purpose electronic digital computers.