Automata-based programming

Automata-based programming is a programming paradigm in which the program or part of it is thought of as a model of a finite state machine (FSM) or any other (often more complicated) formal automaton (see automata theory). Sometimes a potentially infinite set of possible states is introduced, and such a set can have a complicated structure, not just an enumeration.

FSM-based programming is generally the same, but, formally speaking, doesn't cover all possible variants, as FSM stands for finite state machine, and automata-based programming doesn't necessarily employ FSMs in the strict sense.

The following properties are key indicators for automata-based programming:

The whole execution of the automata-based code is a (possibly explicit) cycle of the automaton's steps.

Another reason for using the notion of automata-based programming is that the programmer's style of thinking about the program in this technique is very similar to the style of thinking used to solve mathematical tasks using Turing machines, Markov algorithms, etc.

Consider a program in C that reads a text from standard input stream, line by line, and prints the first word of each line. It is clear we need first to read and skip the leading spaces, if any, then read characters of the first word and print them until the word ends, and then read and skip all the remaining characters until the end-of-line character is encountered. Upon reaching the end of line character (regardless of the stage), we restart the algorithm from the beginning, and upon encountering the end of file condition (regardless of the stage), we terminate the program.

The program which solves the example task in traditional (imperative) style can look something like this:

The same task can be solved by thinking in terms of finite state machines. Note that line parsing has three stages: skipping the leading spaces, printing the word and skipping the trailing characters. Let's call them states before, inside and after. The program may now look like this:

Although the code now looks longer, it has at least one significant advantage: there's only one reading (that is, call to the getchar() function) instruction in the program. Besides that, there's only one loop instead of the four the previous versions had.

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