BrainFuck part 7 - Quine (+ some non-BF quine theory)

Welcome!

Welcome to this new playground about the BF language. Make sure you already read

• Getting started with BrainFuck
• BrainFuck part 2 - Working with arrays
• BrainFuck part 3 - Write a BF interpreter in BF
• BrainFuck part 4 - Advanced maths
• BrainFuck part 5 - Math sequences
• BrainFuck part 6 - 16-bit integers

playgrounds if you didn't already !

The goal of this playground is to create our own quine using BF.

Wait... what?

• A quine.
• Who's this guy ?

Actually, it can be both who, and what.

• Willard Van Orman Quine was an american logician
• A quine is a kind of computer program that prints its own source code, named after Willard Van Orman Quine

Ok I'm sure I can create my own quine

• Be my guest
• Ok let me try using Java

public class Quine {

    public static void main(String[] args) {
        System.out.println("public class Quine {");
        System.out.println();
        System.out.println("public static void main(String[] args) {");
        System.out.println("System.out.println(\"public class Quine {\");");
        
    }
}

• DAMNED !!!

Yes, actually there is a trick

No, this thing is just not possible !

Actually it is. It can even be proved that any Turing-complete language has quines.

There is a "yet another theorem named Fixed-Point theorem" that states that, for any computable function F applied on computable functions, there is a program P so that P (the program) and F(P) (the transformed program) are doing the same thing.

Based on this theorem, let's consider a function PrintCode(...) that prints code of any computable function. Previous theorem states for at least one computable program P, PrintCode(P) and P are doing the same job. PrintCode(P) prints P source code, so P as well prints its own source code. Q.E.D.

So, any Turing-complete language has a quine ?

Yes, as long as any computable function can be implemented. But obviously, there is a trick...

public class Quine {
    public static void main(String[] args) {
        char[] code = new char[]{/*** to be replaced ***/};
        boolean comma = false;
        System.out.println("public class Quine {");
        System.out.println("    public static void main(String[] args) {");
        System.out.print("        char[] code = new char[]{");
        for(char c : code){if(comma)System.out.print(','); comma = true; System.out.print("'" + (c=='\''||c=='\\' ? "\\"+c : c == '\n' ? "\\n": c) +"'");}
        for(char c : code)System.out.print(c);
}}

Ok, so this program:

• Declares an array, and a boolean value
• Prints some lines (from our source code)
• Then (this is the trick) prints array contents as comma-separated chars
• And finally (this is the second part of the trick) prints array contents as chars

Now, let's suppose our array contains chars from the end of the comment to the end of the program. Our program will then

• first, print the source code beginning up to the array definition
• then prints array contents from array itself
• then prints end of source code, using array as well

Okay....

Note: we handle special chars like quotes, backslashes or carriage returns differently in the code

So, now, let's create an array of chars from '];' on third line up to the end of the program (and escape quotes, backslashes and carriage returns...).

public class Quine {
    public static void main(String[] args) {
        char[] code = new char[]{'}',';','\n',' ',' ',' ',' ',' ',' ',' ',' ','b','o','o','l','e','a','n',' ','c','o','m','m','a',' ','=',' ','f','a','l','s','e',';','\n',' ',' ',' ',' ',' ',' ',' ',' ','S','y','s','t','e','m','.','o','u','t','.','p','r','i','n','t','l','n','(','"','p','u','b','l','i','c',' ','c','l','a','s','s',' ','Q','u','i','n','e',' ','{','"',')',';','\n',' ',' ',' ',' ',' ',' ',' ',' ','S','y','s','t','e','m','.','o','u','t','.','p','r','i','n','t','l','n','(','"',' ',' ',' ',' ','p','u','b','l','i','c',' ','s','t','a','t','i','c',' ','v','o','i','d',' ','m','a','i','n','(','S','t','r','i','n','g','[',']',' ','a','r','g','s',')',' ','{','"',')',';','\n',' ',' ',' ',' ',' ',' ',' ',' ','S','y','s','t','e','m','.','o','u','t','.','p','r','i','n','t','(','"',' ',' ',' ',' ',' ',' ',' ',' ','c','h','a','r','[',']',' ','c','o','d','e',' ','=',' ','n','e','w',' ','c','h','a','r','[',']','{','"',')',';','\n',' ',' ',' ',' ',' ',' ',' ',' ','f','o','r','(','c','h','a','r',' ','c',' ',':',' ','c','o','d','e',')','{','i','f','(','c','o','m','m','a',')','S','y','s','t','e','m','.','o','u','t','.','p','r','i','n','t','(','\'',',','\'',')',';',' ','c','o','m','m','a',' ','=',' ','t','r','u','e',';',' ','S','y','s','t','e','m','.','o','u','t','.','p','r','i','n','t','(','"','\'','"',' ','+',' ','(','c','=','=','\'','\\','\'','\'','|','|','c','=','=','\'','\\','\\','\'',' ','?',' ','"','\\','\\','"','+','c',' ',':',' ','c',' ','=','=',' ','\'','\\','n','\'',' ','?',' ','"','\\','\\','n','"',':',' ','c',')',' ','+','"','\'','"',')',';','}','\n',' ',' ',' ',' ',' ',' ',' ',' ','f','o','r','(','c','h','a','r',' ','c',' ',':',' ','c','o','d','e',')','S','y','s','t','e','m','.','o','u','t','.','p','r','i','n','t','(','c',')',';','\n','}','}','\n'};
        boolean comma = false;
        System.out.println("public class Quine {");
        System.out.println("    public static void main(String[] args) {");
        System.out.print("        char[] code = new char[]{");
        for(char c : code){if(comma)System.out.print(','); comma = true; System.out.print("'" + (c=='\''||c=='\\' ? "\\"+c : c == '\n' ? "\\n": c) +"'");}
        for(char c : code)System.out.print(c);
}}

Tadaaaaa

A quine, in Java. Now, let's talk about BF :)

Note : to avoid complex char escapes, we may have stored our data into integers instead of chars.