From: Matthew Moss <matt@...>
Date: 2008-11-25T07:01:30+09:00
Subject: Re: [QUIZ] Befunge (#184)

Hopefully the quiz isn't intimidating... It's a fairly simple language  
to implement. It took me about 30 minutes to get a simple,  
straightforward version working (my submission, shown below), and I  
may do some revisions later to try shrinking the code and making it  
more Ruby-ish. (Also to handle some conditions not currently handled  
well...) I mostly want to revise the run and step methods to be less  
case-y.

Feel free to steal this and make it better, or just to look at to get  
an idea of how to implement your own.


#!/usr/bin/env ruby

class Befunge93

   def self.load_and_run(filename)
     self.new(File.read(filename)).run
   end

   def initialize(text)
     rows = text.split(/[\r\n]+/)                              # split  
input into rows
     @hgt = rows.size                                          # find  
program height
     @wid = rows.max { |a, b| a.size <=> b.size }.size         # find  
program width
     @code = rows.map { |row| row + " " * (@wid - row.size) }  # pad  
rows, store code (r, c)
     @pc, @dir = [0, 0], :east
     @stack = []
     @stringmode = false
   end

   def run
     loop do
       if @stringmode
         case curr
         when ?"
           @stringmode = false
         else
           push curr
         end
       else
         case curr
         when ?0..?9
           push (curr - ?0)
         when ?+, ?-, ?*, ?/, ?%
           b, a = pop, pop
           push a.send(curr.to_sym, b)
         when ?!
           push (pop.zero? ? 1 : 0)
         when ?`
           b, a = pop, pop
           push (a > b ? 1 : 0)
         when ?.
           puts pop
         when ?>
           @dir = :east
         when ?<
           @dir = :west
         when ?^
           @dir = :north
         when ?v
           @dir = :south
         when ??
           @dir = [:east, :west, :north, :south][rand(4)]
         when ?_
           @dir = pop.zero? ? :east : :west
         when ?|
           @dir = pop.zero? ? :south : :north
         when ?"
           @stringmode = true
         when ?:
           push top
         when ?\\
           a = pop     # what about underflow?
           b = pop
           push a
           push b
         when ?$
           pop
         when ?.
           print pop
         when ?,
           print pop.chr
         when ?#
           step
         when ?g
           r, c = pop, pop
           push @code[r][c]
         when ?p
           r, c, v = pop, pop, pop
           @code[r][c] = v
         when ?&
           print "int?> "
           push gets.to_i
         when ?~
           print "chr?> "
           push gets[0]    # Ruby 1.8, maybe not 1.9?
         when ?@
           break
         end
       end
       step    # move program counter
     end
   end

   private

   def curr
     @code[ @pc[0] ][ @pc[1] ]
   end

   def step
     case @dir
     when :north
       @pc[0] = (@pc[0] - 1 + @hgt) % @hgt
     when :south
       @pc[0] = (@pc[0] + 1) % @hgt
     when :east
       @pc[1] = (@pc[1] + 1) % @wid
     when :west
       @pc[1] = (@pc[1] - 1 + @wid) % @wid
     end
   end

   def push(val)
     @stack.push(val)
   end

   def pop
     @stack.pop || 0
   end

   def top
     @stack.last || 0
   end
end

if __FILE__ == $0
   Befunge93.load_and_run(ARGV.shift)
end