befunge.rb

class Befunge
  attr_accessor :x, :y, :stack, :d, :ans, :code
  def initialize code
    @x, @y, @stack, @d, @ans, @code = 0, 0, [], '>', '', code.lines.map(&:chomp)
  end
  def c
    @code[y][x]
  end
  def m
    case d
    when '>' then @x = (x + 1) % @code[y].length
    when '<' then @x = (x - 1) % @code[y].length
    when '^' then @y = (y - 1) % @code.size
    when 'v' then @y = (y + 1) % @code.size
    end
  end
  def interpret!
    while c != '@'
      # puts "#{ans} | #{c} | #{stack.join(' ')}"
      case c
      when '0'..'9' then @stack.push c.to_i
      when '+', '-', '*', '/', '%' then @stack.push _ = @stack.pop(2).inject(c) rescue 0
      when '!' then @stack.push @stack.pop.zero? ? 1 : 0
      when '`' then @stack.push @stack.pop(2).inject(:>) ? 1 : 0
      when '^', 'v', '<', '>' then @d = c
      when '?' then @d = ['^', 'v', '<', '>'].sample
      when '_' then @d = @stack.pop.zero? ? '>' : '<'
      when '|' then @d = @stack.pop.zero? ? 'v' : '^'
      when '"' then m; (@stack.push c.ord; m) while c != '"'
      when ':' then @stack.push @stack.last || 0
      when '\\' then @stack.size == 1 ? @stack.push(0) : @stack.concat(@stack.pop(2).reverse)
      when '$' then @stack.pop
      when '.' then @ans.concat @stack.pop.to_s
      when ',' then @ans.concat @stack.pop.chr
      when '#' then m
      when '&' then @stack.push gets.to_i
      when 'p' then @stack.pop(3).tap { |v, x, y| @code[y][x] = v.chr }
      when 'g' then @stack.pop(2).tap { |x, y| @stack.push @code[y][x].ord }
      end
      m
    end
    self
  end
  def self.interpret code
    new(code).interpret!.ans
  end
end
def interpret(code)
  Befunge.interpret(code)
end

puts interpret <<-EOF
2>:3g" "-!v\\  g30          <
 |!`"O":+1_:.:03p>03g+:"O"`|
 @               ^  p3\\" ":<
2 234567890123456789012345678901234567890123456789012345678901234567890123456789
EOF