output_safety.rb

require 'erb'
require 'active_support/core_ext/kernel/singleton_class'

class ERB
  module Util
    HTML_ESCAPE = { '&' => '&amp;',  '>' => '&gt;',   '<' => '&lt;', '"' => '&quot;', "'" => '&#39;' }
    JSON_ESCAPE = { '&' => '\u0026', '>' => '\u003E', '<' => '\u003C' }
    HTML_ESCAPE_ONCE_REGEXP = /["><']|&(?!([a-zA-Z]+|(#\d+));)/
    JSON_ESCAPE_REGEXP = /[&"><]/

    # A utility method for escaping HTML tag characters.
    # This method is also aliased as <tt>h</tt>.
    #
    # In your ERB templates, use this method to escape any unsafe content. For example:
    #   <%=h @person.name %>
    #
    #   puts html_escape('is a > 0 & a < 10?')
    #   # => is a &gt; 0 &amp; a &lt; 10?
    def html_escape(s)
      s = s.to_s
      if s.html_safe?
        s
      else
        s.gsub(/[&"'><]/, HTML_ESCAPE).html_safe
      end
    end

    # Aliasing twice issues a warning "discarding old...". Remove first to avoid it.
    remove_method(:h)
    alias h html_escape

    module_function :h

    singleton_class.send(:remove_method, :html_escape)
    module_function :html_escape

    # A utility method for escaping HTML without affecting existing escaped entities.
    #
    #   html_escape_once('1 < 2 &amp; 3')
    #   # => "1 &lt; 2 &amp; 3"
    #
    #   html_escape_once('&lt;&lt; Accept & Checkout')
    #   # => "&lt;&lt; Accept &amp; Checkout"
    def html_escape_once(s)
      result = s.to_s.gsub(HTML_ESCAPE_ONCE_REGEXP, HTML_ESCAPE)
      s.html_safe? ? result.html_safe : result
    end

    module_function :html_escape_once

    # A utility method for escaping HTML entities in JSON strings
    # using \uXXXX JavaScript escape sequences for string literals:
    #
    #   json_escape('is a > 0 & a < 10?')
    #   # => is a \u003E 0 \u0026 a \u003C 10?
    #
    # Note that after this operation is performed the output is not
    # valid JSON. In particular double quotes are removed:
    #
    #   json_escape('{"name":"john","created_at":"2010-04-28T01:39:31Z","id":1}')
    #   # => {name:john,created_at:2010-04-28T01:39:31Z,id:1}
    def json_escape(s)
      result = s.to_s.gsub(JSON_ESCAPE_REGEXP, JSON_ESCAPE)
      s.html_safe? ? result.html_safe : result
    end

    module_function :json_escape
  end
end

class Object
  def html_safe?
    false
  end
end

class Numeric
  def html_safe?
    true
  end
end

module ActiveSupport #:nodoc:
  class SafeBuffer < String
    UNSAFE_STRING_METHODS = %w(
      capitalize chomp chop delete downcase gsub lstrip next reverse rstrip
      slice squeeze strip sub succ swapcase tr tr_s upcase prepend
    )

    alias_method :original_concat, :concat
    private :original_concat

    class SafeConcatError < StandardError
      def initialize
        super 'Could not concatenate to the buffer because it is not html safe.'
      end
    end

    def [](*args)
      if args.size < 2
        super
      else
        if html_safe?
          new_safe_buffer = super
          new_safe_buffer.instance_eval { @html_safe = true }
          new_safe_buffer
        else
          to_str[*args]
        end
      end
    end

    def safe_concat(value)
      raise SafeConcatError unless html_safe?
      original_concat(value)
    end

    def initialize(*)
      @html_safe = true
      super
    end

    def initialize_copy(other)
      super
      @html_safe = other.html_safe?
    end

    def clone_empty
      self[0, 0]
    end

    def concat(value)
      if !html_safe? || value.html_safe?
        super(value)
      else
        super(ERB::Util.h(value))
      end
    end
    alias << concat

    def +(other)
      dup.concat(other)
    end

    def %(args)
      args = Array(args).map do |arg|
        if !html_safe? || arg.html_safe?
          arg
        else
          ERB::Util.h(arg)
        end
      end

      self.class.new(super(args))
    end

    def html_safe?
      defined?(@html_safe) && @html_safe
    end

    def to_s
      self
    end

    def to_param
      to_str
    end

    def encode_with(coder)
      coder.represent_scalar nil, to_str
    end

    UNSAFE_STRING_METHODS.each do |unsafe_method|
      if String.new.respond_to?(unsafe_method) 
        define_method(unsafe_method) do |*args, &block|
          to_str.send(unsafe_method, *args, &block)
        end
        
        define_method("#{unsafe_method}!") do |*args|
          @html_safe = false
          super(*args)
        end
      end
    end
  end
end

class String
  def html_safe
    ActiveSupport::SafeBuffer.new(self)
  end
end