Brainf*ck interpreter

bf.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

MEMORY_SIZE = 30000
ASCII_SIZE = 256

def parse(code)
  memory = []
  MEMORY_SIZE.times do
    memory << 0
  end

  ptr = 0
  idx = 0
  step = 0
  output = ""

  while idx < code.length
    c = code[idx]
    step += 1

    case c
    when '>' # increment pointer
      ptr += 1

      if ptr >= MEMORY_SIZE
        puts "pointer out of range (too big) <#{idx}>"
        exit 1
      end

    when '<' # decrement pointer
      ptr -= 1

      if ptr < 0
        puts "pointer out of range (too small) <#{idx}>"
        exit 1
      end

    when '+' # increment memory num
      memory[ptr] += 1

      if memory[ptr] >= ASCII_SIZE
        puts "memory[#{ptr}] out of range (too big) <#{idx}>"
        exit 1
      end

    when '-' # decrement memory num
      memory[ptr] -= 1

      if memory[ptr] < 0
        puts "memory[#{ptr}] out of range (too small) <#{idx}>"
        exit 1
      end

    when '.' # output the character at pointer
      output += memory[ptr].chr()

    when ',' # input the character
      puts "Sorry, but input operator(,) is NOT supported..."
      exit 1

    when '['
      if memory[ptr] == 0
        depth = 0

        loop do
          idx += 1
          step += 1

          if idx >= code.length
            puts "index out of range <#{idx}>"
            exit 1
          end

          if code[idx] == '['
            depth += 1
          elsif code[idx] == ']'
            if depth == 0
              break
            else
              depth -= 1
            end
          end
        end
      end

    when ']'
      if memory[ptr] > 0
        depth = 0

        loop do
          idx -= 1
          step += 1

          if idx < 0
            puts "index out of range (#{i})"
            exit 1
          end

          if code[idx] == ']'
            depth += 1
          elsif code[idx] == '['
            if depth == 0
              break
            else
              depth -= 1
            end
          end
        end
      end

    else
      puts "Invalid operator: #{c}"
      exit 1
    end

    idx += 1
  end

  puts "output: #{output}"
  puts "step  : #{step}"
end


# ----- main ----- #

if ARGV.length < 0
  exit 1
end

# remove except operator
code = ARGV.join.gsub(/[^><+-.,\[\]]/, "")
puts code
parse(code)