I needed a thread-safe JSMin library for compressing javascripts on the fly on UploadBooth, so I took an existing ruby implementation and made it thread safe. I don’t think there was license defined when I got it, so I am re-releasing it as-is.

require ‘monitor’

class JSMin
  EOF = 1
  include MonitorMixin

  # jsmin — Copy the input to the output, deleting the characters which are
  # insignificant to JavaScript. Comments will be removed. Tabs will be
  # replaced with spaces. Carriage returns will be replaced with linefeeds.
  # Most spaces and linefeeds will be removed.
  # thread safe
  def minimize(jstext)
    synchronize do
      @theA = ""
      @theB = ""
      @current = 0
      @output = ""

      @text = jstext
      @theA = "\n"
      action(3)
      while (@theA != JSMin::EOF)
          case @theA
          when " "
              if (isAlphanum(@theB))
                  action(1)
              else
                  action(2)
              end
          when "\n"
              case (@theB)
              when "{","[","(","+","-"
                  action(1)
              when " "
                  action(3)
              else
                  if (isAlphanum(@theB))
                      action(1)
                  else
                      action(2)
                  end
              end
          else
              case (@theB)
              when " "
                  if (isAlphanum(@theA))
                      action(1)
                  else
                      action(3)
                  end
              when "\n"
                  case (@theA)
                  when "}","]",")","+","-","\"","\\", "’", ‘"’
                      action(1)
                  else
                      if (isAlphanum(@theA))
                          action(1)
                      else
                          action(3)
                      end
                  end
              else
                  action(1)
              end
          end
      end
      @output
    end
  end
 
  private
  # isAlphanum — return true if the character is a letter, digit, underscore,
  # dollar sign, or non-ASCII character
  def isAlphanum(c)
     return false if !c || c == JSMin::EOF
     return ((c >= ‘a’ && c <= ‘z’) || (c >= ‘0’ && c <= ‘9’) ||
             (c >= ‘A’ && c <= ‘Z’) || c == ‘_’ || c == ‘$’ ||
             c == \’ || c[0] > 126)
  end

  # get — return the next character from stdin. Watch out for lookahead. If
  # the character is a control character, translate it to a space or linefeed.
  # thread safe
  def get
    return JSMin::EOF if @current>(@text.length-1)
    c = @text[@current]
    @current += 1
    c = c.chr
    return c if (c >= " " || c == "\n" || c.unpack("c") == JSMin::EOF)
    return "\n" if (c == "\r")
    return " "
  end

  # Get the next character without getting it.
  def peek
      lookaheadChar = @text[@current]
      return lookaheadChar.chr
  end

  # mynext — get the next character, excluding comments.
  # peek() is used to see if a ‘/‘ is followed by a ‘/‘ or ‘*‘.
  def mynext
      c = get
      if (c == "/")
          if(peek == "/")
              while(true)
                  c = get
                  if (c <= "\n")
                  return c
                  end
              end
          end
          if(peek == "*")
              get
              while(true)
                  case get
                  when "*"
                     if (peek == "/")
                          get
                          return " "
                      end
                  when JSMin::EOF
                      raise "Unterminated comment"
                  end
              end
          end
      end
      return c
  end

  # action — do something! What you do is determined by the argument: 1
  # Output A. Copy B to A. Get the next B. 2 Copy B to A. Get the next B.
  # (Delete A). 3 Get the next B. (Delete B). action treats a string as a
  # single character. Wow! action recognizes a regular expression if it is
  # preceded by ( or , or =.
  def action(a)
      if(a==1)
          @output << @theA
      end
      if(a==1 || a==2)
          @theA = @theB
          if (@theA == "\’" || @theA == "\"")
              while (true)
                  @output << @theA
                  @theA = get
                  break if (@theA == @theB)
                  raise "Unterminated string literal" if (@theA <= "\n")
                  if (@theA == "\\")
                      @output << @theA
                      @theA = get
                  end
              end
          end
      end
      if(a==1 || a==2 || a==3)
          @theB = mynext
          if (@theB == "/" && (@theA == "(" || @theA == "," || @theA == "=" ||
                               @theA == ":" || @theA == "[" || @theA == "!" ||
                               @theA == "&" || @theA == "|" || @theA == "?" ||
                               @theA == "{" || @theA == "}" || @theA == ";" ||
                               @theA == "\n"))
              @output << @theA
              @output << @theB
              while (true)
                  @theA = get
                  if (@theA == "/")
                      break
                  elsif (@theA == "\\")
                      @output << @theA
                      @theA = get
                  elsif (@theA <= "\n")
                      raise "Unterminated RegExp Literal" + @output
                  end
                  @output << @theA
              end
              @theB = mynext
          end
      end
  end
end