Brainf****d

I got brainf****d on a Saturday afternoon. No i didn't get admitted for psychological reasons. Brainf*** is an esoteric programming language. Its a turing complete language with as few as 8 instructions:

>	Increment the pointer
<	Decrement the pointer
+	Increment the byte at the pointer
-	Decrement the byte at the pointer
.	Output the byte at the pointer
,	Input a byte and store it in the byte at the pointer
[	Jump forward past the matching ] if the byte at the pointer is zero
]	Jump backward to the matching [ unless the byte at the pointer is zero

Click here for a brief description of the language. The wikipedia entry is more detailed spoiling the fun in discovering the quirks of the language.

I started writing an interpreter for BF in Ruby, which was a simple switch. But the interesting part is actually writing code in BF. Its much more challenging than Assembly language because of the instruction set and using a tape like memory without any registers.

Here is the BF interpreter in Ruby:

if ARGV.length != 1
 puts "Usage: ruby brainfuck.rb filename"
 exit
end
mem = Array.new(30000,0x00)
loopstack = Array.new
p = 0

code  = File.open(ARGV[0]).readlines.join("")

i=0
while i < code.length
 case code[i]
 when '>'
   p += 1
 when '<'
   p -= 1
 when '+'
   mem[p] += 1
 when '-'
   mem[p] -= 1
 when '.'
   print mem[p].chr
 when ','
   mem[p] = STDIN.getc
 when '['
   if mem[p].zero?
     i = code.index(']',i)
   else
     loopstack.push i
   end
 when ']'
   if(mem[p].zero?)
     loopstack.pop
   else
     i = loopstack.last
   end
 end
 i+=1
end

And here is the traditional HelloWorld program in BF:

#H
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++
.
>
#e
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
+
.
>
#l
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++
.
>
#l
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++
.
>
#o
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
+++++++++++
.
>
#SPACE
++++++++++++++++++++++++++++++++
.
>
#w
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
+++++++++++++++++++
.
>
#o
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
+++++++++++
.
>
#r
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++
.
>
#l
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++
.
>
#d
++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++++
.

So long for HelloWorld? This would probably be the worst language for beginners!

The only work in the Interpreter was managing loops expressed using [ and ]. I ended up using a stack to manage loops similar to the way stacks are used in procedural languages to transfer control to methods and return back.

Here is a program to iterate from 0 to 9 in BF:

#ASCII value of 0
++++++++++++++++++++++++++++++++++++++++++++++++
>
#Loop counter variable
++++++++++
[
< . + >
-
]