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Reversible-2D is a 2D programming language which can be reversed. It is unique because it still has variables. The short for Reversible-2D is R2D. For version 1, it is R2D1. When version 2 comes, some ppl will lol. It is made by TehZ.


The current commands:

 \ and /  Mirrors.
 P        Swaps the value of the current thread and the next cell in the source code.
 O        Outputs the value of the current thread.
 I        If the value to the right of this command is not space, throws an error. If it is space, this will read a byte from stdin and put it to the cell to the right.
 +        Adds 1 to the value of the current thread.
 -        Opposite of +.
 ¤        Ends the current thread. After a thread is terminated, it counts how many steps it has been terminated.
 N        Prints a newline character to stdout.
 Other    Nothing.

Commands are reversed by:

 \ and /  Moving the other way through.
 P        Repeating the process.
 O        Removing last printed character from console/file.
 I        Taking the value of the cell to the right of the command and putting it into the threads value.
 +        Doing - instead.
 -        Doing + instead.
 ¤        Starting the thread and turning it around (after the amount of steps it has been terminated :D).
 N        Same as O.
 Other    Nothing.

Commands are CaSe-SeNsItIvE!


A thread in R2D is not a normal thread, because all threads execute in the same process. They are placed in the source with the following commands:

 V     Down
 >     Right
 ^     Up
 <     Left

A thread holds a 16 bit value (because java char's is 16 bit) which can be changed through the commands.


Threads will execute in a fixed and well defined order:

 Y is first priority      The lower Y coordinate at start, the sooner a thread will run.
 X is second priority     The lower X coordinate at start, the sooner a thread will run.

Note on Threads

If a thread collides with a thread starting point in the same direction as the thread, there will come undefined behavior. If it was allowed, Reversible-2D would not be reversible.


There is two places that variables can be:

 Source      Each cell is a variable.
 Threads     Each thread is a variable.


Hello World:


The above program will write "Hello, World!" followed by a newline.

Cat (ignoring eof):

 / > \
 \O I/

The above program will copy all input to output. *NOTE* It will actually produce undefined behavior, but it works in the standard interpreter.



The above program will write "HHeil!" followed by a newline, followed by "lo, World!" followed by a newline.

Turing Completeness

Reversible-2D is not Turing-Complete, because any program can either always terminate or always loop forever (but a forever loop gives undefined behavior).


The official implementation.