4.4 KiB
Quine
Quine is a nonempty program that when executed prints its own source code. It takes no input, just prints out the source code, without "cheating" such as reading the source code 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 belong to the class of standard/fun/interesting programs with which we like to compare 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 functions satisfying certain conditions always have a fixed point).
Attempt mathematically similar to making quines include e.g. making self matching regular expressions (for this task to be non-trivial the regex has to be for example enclosed between /s). Yet another similar challenge is a polyglot program -- one that is a valid program in several languages -- some programs can be quines and polyglots at the same time, i.e. polyglot quines.
The challenging part of writing quines lies indeed in the self reference and the fact that as we are working on the solution, we keep changing the goal -- normally we cannot just single-line print a string literal containing the source because that string literal would have to contain itself, making it infinite in length. The idea commonly used to solve the problem is following:
- On first line start a definition of string S, later copy-paste to it the string on the second line.
- 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). 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; }
0 46 32 34 S 34 32 58 83 S --> S: "0 46 32 34 S 34 32 58 83 S --> " .
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