Build a Self-Replicating Program (Quine)

Have you ever wondered if a program could write itself? Welcome to the fascinating world of quines!

The Challenge

The rules are deceptively simple:

1. Create a program that outputs its own source code.
2. No cheating allowed–opening and reading the source file is strictly forbidden.
3. The program must be self-contained–receiving any kind of input invalidates the quine.

Sounds impossible? Let's dive in and see how it's done!

The Kleene's Theorem: A Theoretical Foundation

Before we get our hands dirty with code, let's explore the theoretical basis for quines. Enter Kleene's theorem, a concept that predates modern computers but lays the groundwork for our self-replicating programs.

First Form—The Fixed-Point Theorem

Kleene's Fixed-Point Theorem is a fundamental concept in computability theory. Let's break it down:

For every conditional program e, there exists a program that satisfies:

ϕe(x) = f(e,x)

Where:

• e represents the program logic (the algorithm or function)
• ϕe represents its syntax (how the program is written)
• x is any input
• f is a computable function that takes the program and input as arguments

In simpler terms, this theorem guarantees the existence of a program whose execution results in its own source code. It's saying that for any computable function f, we can find a program e that, when given to f along with some input x, produces the same result as running e on x.

This is mind-bending because it suggests that programs can reference themselves in a meaningful way!

Second Form—The Quine Theorem

Building on the first form, we can deduce a more specific and powerful statement:

For every computable function f, there exists a program p such that:

p = f(p)

In other words, there's always a program p that, when given as input to f, produces itself as output. This is the essence of a quine!

This form directly implies that for any programming language powerful enough to represent all computable functions, we can write a program that prints its own source code.

Let's Build a Quine

Now that we understand the theoretical foundation, let's create our first quine. Here are the requirements, explained:

1. Print the source code without opening the file.
   • This ensures true self-replication without external dependencies.
2. Include at least two comments.
   • Comments demonstrate that even non-executable parts of the code can be replicated.
3. Use at least two functions.
   • This shows that quines can work with more complex program structures.
4. Ensure that diff <(./quine) quine.c returns nothing (indicating perfect replication).
   • This Unix command compares the output of running the quine with its source file.
   • If they're identical, diff will produce no output, confirming successful self-replication.

A C Solution

Here's an example of a quine in C that meets our criteria:

#include <stdio.h>

/*
   Hey, this is an outside comment
*/

char *get_str()
{
    return "#include <stdio.h>%1$c%1$c/*%1$c   Hey, this is an outside comment%1$c*/%1$c%1$cchar *get_str()%1$c{%1$c%2$creturn %3$c%4$s%3$c;%1$c}%1$c%1$cint main(void)%1$c{%1$c%2$c/*%1$c%2$c   Hey, this is an inside comment%1$c%2$c*/%1$c%2$cchar *str = get_str();%1$c%2$cprintf(str, 10, 9, 34, str);%1$c}%1$c";
}

int main(void)
{
    /*
       Hey, this is an inside comment
    */
    char *str = get_str();
    printf(str, 10, 9, 34, str);
}

Take a moment to analyze this code. Can you see how it manages to print itself?

The Name "Quine"

You might be wondering about the origin of the term "quine." It's named after Willard Van Orman Quine, a philosopher who explored the concept of indirect self-reference. His work in logic and philosophy laid the groundwork for understanding these self-referential programs.

Want to Explore Further?

If you've caught the quine bug (pun intended), there's a whole world of self-replicating programs to discover! During my time at 42 Paris School, I worked on numerous quine-related exercises. Feel free to check out my repository for more challenging quine implementations. Who knows? You might just become a quine master yourself! 😊

Remember, the journey of a thousand quines begins with a single self-replicating step. Happy coding!