bf/ruby/bf.rb

def scan(source)
  source
    .chars
    .filter { |c| "+-<>[].,".include?(c) }
    .map(&:to_sym)
end

def parse(tokens)
  position = 0
  tree = Array.new

  parse_node = Proc.new do |token|
    case token
    when :+, :-, :>, :<, :'.', :'.', :','
      position += 1
      token
    when :"["
      position += 1
      body = Array.new

      while position < tokens.size && tokens[position] != :"]"
        body << parse_node.call(tokens[position])
      end

      raise "Unexpected EOF; expected ]" if position == tokens.size
      position += 1 if tokens[position] == :"]"

      body
    else
      raise "Unrecognized token #{token}"
    end
  end

  while position < tokens.size
    tree << parse_node.call(tokens[position])
  end

  tree
end

def interpret(tree)
  stack = Array.new(100) { 0 }
  position = 0
  pointer = 0

  interpret_node = Proc.new do |node|
    case node
    when :+
      stack[pointer] += 1
    when :-
      stack[pointer] -= 1
    when :>
      raise "Out of bounds" if pointer >= 30_000
      pointer += 1
    when :<
      raise "Below zero" if pointer <= 0
      pointer -= 1
    when :'.'
      STDOUT.write stack[pointer].chr
    when :','
      char = STDIN.getc
      stack[pointer] = char.ord
    when Array

      while stack[pointer] > 0
        node.each { |subnode| interpret_node.call(subnode) }
      end

    else
      raise "Unexpected node #{node}"
    end
  end

  while position < tree.length
    interpret_node.call(tree[position])
    position += 1
  end

  STDOUT.print("\n")
end

interpret(parse(scan(ARGV.first))).inspect