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# Quine

Quine is a nonempty program that upon execution prints its own source code. It takes no input, just prints out the source code, and that without "cheating" such as reading the source from a file. A self-replicating program of a sort, quine performs a task similar to that of a robot capable of creating copies of itself (after all we humans are such robots). The name quine refers to the philosopher Willard Quine and his paradox that exhibits a structure similar to self-replicating programs. Quine belongs to the class of standard/fun/interesting programs with which we like to entertain ourselves and that are used for comparing programming languages, it's a cousin of such superstars as hello world, compiler bombs, 99 bottles of beer or fizzbuzz, but out of these quine is of the greatest interest to mathematicians.

From mathematical viewpoint quine is a fixed point of a function (not to be confused with fixed_point arithmetic) represented by the programming language. That is once we say the programming language is a function f(x), where x is source code and the function's output is the program's output, quine is such x that f(x) = x. A quine can be written in any Turing complete language, the proof comes from the fixed point theorem (which says a function satisfying certain conditions always have a fixed point).

The difficulty in constructing a quine lies indeed in the self reference which makes the solution "run away" as we're chasing it: as we are working on the solution (changing the source code), the goal itself (source code we have to output) keeps changing, and so we cannot just single-line print a string literal containing the source we currently have at hand because the string would have to contain itself, making it infinite in length. Challenges mathematically similar by this aspect -- i.e. complicated by self reference -- include for example constructing self matching regular expressions (but for this to be non-trivial additional constraints must be given: let's say that the regex must be enclosed between `/`s) or creating images that spell out their own hash.

The idea commonly used to create a quine is following:

1. On first line start a definition of string S, later copy-paste to it the string on the second line.
2. On second line put a command that prints the first line, assigning to S the string in S itself, and then prints S (the second line itself).

Yet a stronger quine is so called radiation hardened quine, a quine that remains quine even after any one character from the program has been deleted (found here in Ruby: https://github.com/mame/radiation-hardened-quine). A polyglot quine is quine which is a valid quine in several languages at once. Other plays on the theme of quine include e.g. a program that produces a bigger program which will again produce yet bigger program etc.

Another extension of a quine is multiquine -- this is NOT a polyglot quine! Multiquine is a quine written in some programming language L0; under normal circumstances this program behaves like a normal quine, but it has an extra feature: when passed a parameter N (e.g. through CLI flag or through standard input), it will print a program in another language, LN, which itself is this multiquine (so it can again be used to get back the program in L0 and so on). I.e. a multiquine is a quine which can switch between several languages.

In the Text esoteric programming language every program is a quine (and so also a radiation hardened one).

## List Of Quines

Brainfuck: not short, has over 2100 characters.

C:

    #include <stdio.h>
    char s[] = "#include <stdio.h>%cchar s[] = %c%s%c;%cint main(void) { printf(s,10,34,s,34,10,10); return 0; }";
    int main(void) { printf(s,10,34,s,34,10,10); return 0; }

comun:

    0 46 32 34 S 34 32 58 83 S --> S: "0 46 32 34 S 34 32 58 83 S --> " .

Python:

    s="print(str().join([chr(115),chr(61),chr(34)]) + s + str().join([chr(34),chr(10)]) + s)"
    print(str().join([chr(115),chr(61),chr(34)]) + s + str().join([chr(34),chr(10)]) + s)

text:

    This is a quine in text.

TODO: more, make biquine of C and comun

## See Also

- polyglot

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