Befunge
Befunge is a two-dimensional esoteric programming language invented in 1993 by Chris Pressey with the goal of being as difficult to compile as possible.
Contents |
[edit] History
Befunge is believed to be the first two-dimensional, ASCII-based, general-purpose (in the sense of "you could plausibly write Hunt the Wumpus in it" [1]) programming language. Its form was influenced in part by the multimedia scripting application AmigaVision, and in part by Forth.
The practice of multiprogramming was probably first developed in Befunge. The wire-crossing problem was also possibly first considered in the context of Befunge.
The original Befunge (known as "Befunge-93" to distinguish it from others) has spawned many descendants and remote cousins. The closest relative, and most direct extension, of Befunge-93 is Befunge-98 of the Funge-98 family of languages. Each Funge extends the central concepts of Befunge to a given number of dimensions (for example, Unefunge is one-dimensional, Trefunge is three-dimensional, Nefunge is n-dimensional, etc.).
[edit] Etymology
The word "Befunge" started life as a typographical error for the word "before", typed by Curtis Coleman at 4AM on a BBS chat system.
It was then reverse-endowed with a fictional morphology where it took on the meaning Be- (a corruption of the prefix bi-, for "two") + funge (fictional root of the word fungible, i.e. "interchangeable"). That is, to interchange (program codes with data) in two (dimensions).
The name is pronounced /bee-FUNJ/.
[edit] Language overview
A Befunge program consists of a two-dimensional playfield of fixed size. The playfield is initially loaded with the instructions of the program. It also doubles as an updateable storage unit.
Execution proceeds by the means of a program counter (-93) or instruction pointer (-98). The instruction pointer begins at a set location (the upper-left corner of the playfield) and is initially travelling in a set direction (right). As it encounters instructions, they are executed. The instructions may have an effect on the instruction pointer's direction and position (-98 only). The following, for example, is literally an infinite loop:
>v ^<
Instructions may also affect the contents of a stack in the manner of Forth.
[edit] Instructions
Befunge-93 has the following commands:
| Cmd | Description |
|---|---|
+
|
Add two top stack values |
-
|
Subtract two top stack values |
*
|
Multiply two top stack values |
/
|
Division |
%
|
Modulo division |
!
|
Logical NOT |
`
|
Greater Than |
>
|
PC direction right |
<
|
PC direction left |
^
|
PC direction up |
v
|
PC direction down |
?
|
Random PC direction |
_
|
Horizontal IF |
|
|
Vertical IF |
"
|
Toggle stringmode |
:
|
Duplicate top stack value |
\
|
Swap top stack values |
$
|
Pop (remove) top stack value |
.
|
Output integer |
,
|
Output ASCII |
#
|
Bridge: jump over next command |
g
|
Get value from code |
p
|
Put value at code |
&
|
Input integer |
~
|
Input character |
@
|
End program |
0 – 9
|
Push corresponding value onto the stack |
[edit] Computational class
Because Befunge-93 programs are given an explicit limit of 80x25 cells on the size of their playfield, but are also given a working stack, any Befunge-93 program should be simulatable by a push-down automaton.
However, the converse is not true; there surely exist some push-down automata which cannot be simulated by any Befunge-93 program (because they contain more states than can be encoded in the 80x25 playfield).
Befunge-98 removes the fixed-size restriction on the playfield, and thus should be Turing-complete.
[edit] Compilation
As stated, the design goal for Befunge was to create a language which was difficult to compile. This was realized by two main features:
- self-modifying – the
pinstruction can write new instructions into the playfield; and - multi-dimensional – the same instruction can be executed in four different contexts (in a left-to-right series of instructions, or right-to-left, or upward or downward.)
Nevertheless, these obstacles have been overcome to some degree, and Befunge compilers have been written, using appropriate techniques.
The bf2c compiler included with the standard Befunge-93 distribution uses threaded code: each instruction is compiled to a snippet of C code, and control flows through the snippets just as it does in a Befunge interpreter (that is, conditionally on the value of some 'direction' register.) This does not result in a significant advantage over a good interpreter. Note that the bf2c compiler is not correct since it does not handle p correctly, but it would not be impossible to make it do so (although the C language might not be well-suited for this.)
The Betty compiler, for example, treats every possible straight line of instructions as a subprogram, and if a p instruction alters that subprogram, that subprogram is recompiled. This is an interesting variation on just-in-time compilation, and it results in a much better advantage over an interpreter, since many instructions can be executed in native code without making intervening decisions on the 'direction' register.
There are also programs which combine a Befunge interpreter and a copy of a given Befunge program into a single executable which runs the Befunge program when started. For Befunge-93 this can easily be done by having a preallocated 80×25 cell storage space in the interpreter, and filling it in with the chosen Befunge program. This might be considered a sort of pathological version of threaded code, and while it produces a similar effect to a compiler, that is, it generates a "native executable", it is not really considered the same thing. Sometimes such a tool is called a pseudo-compiler or linker. TBC (Tim's Befunge Compiler), and BFC (BeFunge Compiler) written by Uranium-239, are examples of such tools.
[edit] Examples
[edit] Hello, world!
0"!dlroW ,olleH">:#,_@
[edit] Cat program
~:1+!#@_,
[edit] Factorial
0&>:1-:v v *_$.@ ^ _$>\:^
[edit] Sieve of Eratosthenes
2>:3g" "-!v\ g30 < |!`"O":+1_:.:03p>03g+:"O"`| @ ^ p3\" ":< 2 234567890123456789012345678901234567890123456789012345678901234567890123456789
[edit] Quine
01->1# +# :# 0# g# ,# :# 5# 8# *# 4# +# -# _@
[edit] Simple game ("Less or More")
vv < <
2
^ v<
v1<?>3v4
^ ^
> >?> ?>5^
v v
v9<?>7v6
v v<
8
> > ^
vv < <
2
^ v<
v1<?>3v4
^ ^
> >?> ?>5^
v v v ,*25 <<
v9<?>7v6 ,,
v v< ""
8 ><
> > ^ ""v
>*:.>0"!rebmun tupnI">:#,_$25*,:&:99p`|^< _0"!niw uoY">:#,_$25*,@
^ < >:99g01-*+^
[edit] Interpreters
[edit] Vb.Net
It will fail if you use the 'g' or 'p' function outside the 80x20 playfield. However, you can use them outside this range if the size of your code is greater.
Module Module1
Sub Main()
Console.WriteLine("Befunge Inteper By BlackCap")
Console.Write("filepath> ")
Dim code = FixLines(IO.File.ReadAllText(Console.ReadLine, Text.Encoding.Default))
Dim Stack As New Stack(Of Integer)
Dim StringMode, SkipNext As Boolean
Dim xCur, yCur, dir As Integer
Dim g = Function(i%) If(Stack.Count <= i, 0, Stack(i))
Dim pop = Function() If(Stack.Count > 0, Stack.Pop, 0)
Do
If xCur < 0 OrElse yCur < 0 OrElse xCur >= code(0).Length OrElse yCur >= code.Length Then
Select Case dir
Case Is = 0 : xCur = 0
Case Is = 1 : yCur = 0
Case Is = 2 : xCur = code(0).Length - 1
Case Is = 3 : yCur = code.Length - 1
End Select
End If
Dim c As Char = code(yCur)(xCur)
If SkipNext Then
SkipNext = False
Else
If StringMode Then
If c = Chr(34) Then StringMode = False Else Stack.Push(AscW(c))
Else
Select Case c
Case Is = "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" : Stack.Push(Val(c)) ' Diggits
Case Is = "@" : Exit Do ' Terminate
Case Is = "$" : pop() ' Pop
Case Is = ":":Stack.Push(g(0)) ' Dupe
Case Is = "\" ' Swap
Dim a = pop(), b = pop()
Stack.Push(a) : Stack.Push(b)
Case Is = "+", "-", "*", "/", "%" ' Math
Dim a = pop(), b = pop()
Stack.Push(If(c = "+", b + a, If(c = "-", b - a, If(c = "*", b * a, If(c = "/", b / a, b Mod a)))))
Case Is = Chr(34) : StringMode = True ' StringMode
Case Is = "!" : Stack.Push(If(pop() = 0, 1, 0)) ' Not
Case Is = "`" ' Greater
Dim a = pop(), b = pop()
Stack.Push(If(b > a, 1, 0))
Case Is = "." : Console.Write(pop()) ' Output int
Case Is = "," : Console.Write(ChrW(pop())) ' Output ASCII
Case Is = "~" ' Read single char
Console.Write(vbCrLf & vbCrLf & "> ")
Stack.Push(AscW(Console.ReadKey.KeyChar))
Case Is = "&" ' Read a line
Console.Write(vbCrLf & vbCrLf & "=> ")
Array.ForEach(Console.ReadLine.ToArray, Sub(x As Char) Stack.Push(AscW(x)))
Case Is = "#" : SkipNext = True ' Skip
Case Is = ">", "v", "<", "^" : dir = ">v<^".IndexOf(c) ' Change direction
Case Is = "_" : dir = (pop() <> 0) * -2 ' Hor if
Case Is = "|" : dir = 1 + (pop() <> 0) * -2 ' Ver if
Case Is = "?" ' Random direction
Static R As New Random
dir = R.Next(0, 4)
Case Is = "g" ' Get
Dim y = pop(), x = pop()
Stack.Push(AscW(code(y)(x)))
Case Is = "p" ' Put
Dim y = pop(), x = pop(), val = pop()
code(y) = code(y).Remove(x) & ChrW(val) & code(y).Substring(x + 1)
End Select
End If
End If
Select Case dir
Case Is = 0 : xCur += 1 ' Right
Case Is = 1 : yCur += 1 ' Down
Case Is = 2 : xCur -= 1 ' Left
Case Is = 3 : yCur -= 1 ' Up
End Select
Loop
Console.WriteLine(vbCrLf & vbCrLf & "The program was terminated" & vbCrLf & "Press any key to exit")
Console.ReadKey()
End Sub
Function FixLines(str As String) As String()
Dim lines = Split(str, vbCrLf).ToList
Dim l% = Math.Max((From a In lines Order By a.Length).Last.Length, 80)
For i = lines.Count To 20
lines.Add("")
Next
For i = 0 To lines.Count - 1
For i2 = lines(i).Length + 1 To l
lines(i) &= " "
Next
Next
Return lines.ToArray
End Function
End Module
[edit] Related languages
Befunge was preceded in 1991 by a similar but less featureful language Biota, which was designed for experiments in self-reproduction. It was followed soon after, in 1994, by another similar language, Orthagonal, the design of which was spurred by a discussion on alt.folklore.computers. Each of these three languages originated (as far as anyone can tell) completely independently of the other two.
Befunge has also provided inspiration to the design of subsequent languages, the most similar of these are known as fungeoids. Most of the languages are not similar enough to be called direct descendants, but often the author mentions the influence of Befunge in the accompanying commentary. Such languages include Wierd, a two-dimensional Turing tarpit; Befreak, a reversible language; and PATH, which combines in elements of Brainfuck.
[edit] External resources
Befunge-93
- Befunge-93 documentation.
- Befunge-93 Online Compilation.
- JavaScript Befunge-93 interpreter.
- befungee, a Befunge-93 interpreter written in python, with a debugger and a concurrent mode.
- Bephunge, an implementation of Befunge-93 in PHP (command-line) (mirror in the Esoteric File Archive).
- Befunge programs in the Esoteric File Archive.
- YaBI93 - multiplatform Befunge93 interpreter (with IDE) for Java 1.5.
- Marsh/Bejit A fast interpreter and a quasi JIT.
Befunge-98 and beyond
- Funge-98 specification.
- vsync's Funge stuff.
- Fungus (from the Wayback Machine; retrieved on 22 March 2007) - a nice Befunge-98 IDE for Win32. Warning: its interperter is not fully standards compliant.
- mooz' Befunge page (from the Wayback Machine; retrieved on 25 September 2006) - contains a Javascript interpreter and several interesting Befunge programs.
- BeQunge A cross-platform Funge-98 interpreter, code editor, and debugger. Works in any number of dimensions.
- J^4: Befunge Jeffrey Lee's Befunge site, features plenty of interesting programs.
- Mycology and CCBI A complete Befunge-98 test suite based on the specification, and an interpreter which passes all the tests.
- cfunge - Small fast Befunge-98 interpreter in C, standard compliant.
- Sponge - a compiler (in Common Lisp) from a tiny subset of Scheme to Befunge 98.
- PyFunge - A Befunge-93/Funge-98 interpreter in Python. Its goal is a fully functional, compliant and optimizing implementation of Funge-98.
- Fungi - A standards compliant Funge-98 interpreter and debugger written in Haskell.
- Closidrium - Author retrospective of creating a Befunge-98 Interpreter in Clojure.
