The beginning of a Befunge interpreter for RC pairing interview

befunge.rb

# A Befunge interpreter written in Ruby
# Language specification: https://github.com/catseye/Befunge-93/blob/master/doc/Befunge-93.markdown

class ProgramCounter
    attr_accessor :x, :y, :dir

    def initialize x, y, dir
        @x, @y, @dir = x, y, dir
    end
end

class Befunge
    def initialize text
        @pc = ProgramCounter.new(0, 0, :right)

        @playfield = text.split("\n").map do |line|
            symbols = line.split("")
        end

        @height = @playfield.length
        @width = @playfield.map(&:length).max

        # fill up rows to maximal length
        @playfield.each do |row|
            while row.length < @width
                row << " "
            end
        end
    end

    def run
        loop do
            print_state
            if interpret_command == :quit
                return
            end
            move_program_counter
            sleep 0.5
        end
    end

    def print_state
        output = ""
        @playfield.each_with_index do |row, y|
            row.each_with_index do |symbol, x|
                if @pc.x == x and @pc.y == y
                    output << "("
                elsif @pc.x == x-1 and @pc.y == y
                    output << ")"
                else
                    output << " "
                end
                output << symbol
            end
            if @pc.x == @width-1 and @pc.y == y
                output << ")"
            else
                output << " "
            end
            output << "\n"
        end
        output << "-"*@width*2
        puts output
        STDOUT.flush
    end

    def interpret_command
        command = @playfield[@pc.y][@pc.x]

        case command
        when " "
            # do nothing
        when "<"
            @pc.dir = :left
        when ">"
            @pc.dir = :right
        when "^"
            @pc.dir = :up
        when "v"
            @pc.dir = :down
        when "@"
            :quit
        end
    end

    def move_program_counter
        case @pc.dir
        when :left
            @pc.x -= 1
        when :right
            @pc.x += 1
        when :up
            @pc.y -= 1
        when :down
            @pc.y += 1
        end

        # wrap around, if necessary
        @pc.x = @pc.x % @width
        @pc.y = @pc.y % @height
    end
end

# ^, >, v, and < change PC direction
# @ ends program

maze = <<HERE
 >v
 v>
 <  ^
 @  >
HERE

Befunge.new(maze).run

# digits are pushed on the stack
# . outputs top stack value as int, followed by a space

# ? moves PC in a random direction

# '#' skips over next command

# +, -, *, /, and % do calculations on top two stack values, bottom value first

# ~ input ASCII value of next char

# & input int until next space

# $ pops top stack value

# : duplicates top stack value

# \ swaps top two stack values

# _ horizontal if: left if top element is non-zero, else right

# | vertical if: up if top element is non-zero, else down

# ! negates top value

# ` replaces two stack values with 1, if bottom > top, 0 otherwise

# " toggles stringmode: all character cells will have their ASCII value pushed onto the stack until another " is encountered
# , outputs top element as char

# g gets value at (x, y), determined by top two stack values

# p writes third-top stack value to (x, y), determined by top two stack values

# final challenge :D

life = <<HERE
v>>31g> ::51gg:2v++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 
9p BXY|-+<v3*89<%+ *                                                      *   + 
21 >98 *7^>+\-0|<+ *                                                     *    + 
*5 ^:+ 1pg15\,:< + *                                                     ***  + 
10^  <>$25*,51g1v+                                                            + 
-^ p<| -*46p15:+<+                                                            + 
> 31^> 151p>92*4v+                                                            + 
 ^_ ".",   ^ vp1<+                                                            + 
>v >41p      >0 v+                                                            + 
:5! vg-1g15-1g14<+                                                            + 
+1-+>+41g1-51gg+v+                                                            + 
1p-1vg+1g15-1g14<+                                                            + 
g61g>+41g51g1-g+v+                                                            + 
14*1v4+g+1g15g14<+                           * *                              + 
5>^4>1g1+51g1-g+v+                           * *                              + 
^ _^v4+gg15+1g14<+                           ***                              + 
>v! >1g1+51g1+g+v+                                                            + 
g8-v14/*25-*4*88<+                                                            + 
19+>g51gg" "- v  +                                                            + 
4*5  v<   v-2:_3v+                                                            + 
 >^   |!-3_$  v<-+                                                            + 
^    < <      <|<+                                                         ***+ 
>g51gp ^ >51gp^>v+                                                            + 
^14"+"<  ^g14"!"<++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
HERE