This is a small Turing machine simulator that I programmed to check one of my exercises on the topic.
From Wikipedia:
A Turing machine is a mathematical model of computation that defines an abstract machine that manipulates symbols on a strip of tape according to a table of rules. Despite the model's simplicity, given any computer algorithm, a Turing machine capable of implementing that algorithm's logic can be constructed.
Read more about this here.
The exercise asked me to create a Turing machine that checked whether a given string, made from the alphabet composed of the characters a, b and c, contained the same number of a's and b's or the same number of a's and c's.
While the authors solved the exercise through a 4-tape Turing machine, I found that a 3-tape Turing machine would be more efficient, and went ahead and solved it through that kind of machine.
To test my solution, I decided to make this program in C.
A Turing machine is essentially composed of infinite tapes (in this case, four), and a Finite-State Automaton as the processing component of the machine.
I recreated the four tapes, one being the input tape and the other three the memory tapes, through character arrays. For memory usage, I decided to make the tapes 100 character long at most, but there technically is no limit to the length of the tape.
Then I programmed the logic of the FSA with if-else statements and a switch statement: the machine reads the input tape one character at a time and writes one A in the first tape for each a in the input tape, one B in the second tape for each b and one C in the third tape for each c.
Once the input string ends (which is when the machine reads blank space, in this case represented by the null character \0), the machine stops and reverses the three memory tapes, deleting one symbol from each, until at least one of the tapes returns to the beginning, which is marked by a special symbol: Z.
If all three of the tapes or if the A tape and one of the other two tapes return to Z at the same time, then the string meets the criteria imposed by the exercise, and the machine accepts it.
The program uses a struct to represent the machine's configuration, which includes the tapes' indexes, the state in which the machine currently is (in my original design, the machine has three states: q0, q1 and q2) and whether it will accept the input string or not.
To do that, I used the debugger in VS Code and put breakpoints in the main loops of the program. Just watch the four tapes get updated through the Variables window.
I might update this program in the future adding a visualiser for the tapes.