gistfile1.rb

class Interpolate
    
  DQ = "\""
  SQ = "'"
  ESCAPE = "\\"
  CODE = "#"
  DEBUG = false
  
  def quote? char
    char == DQ || char == SQ
  end
  
  def escape? char
    char == ESCAPE  
  end
  
  def transist state, opts = {}
    old_state = @state
    self.send("#{@state}_to_#{state}", opts )
    
    @state = state    
    @stream << [old_state, @buffer] if @buffer
    @buffer = ""
  end


  #### HOOKS START
  def init_to_code opts
  end
    
  def code_to_string opts
    @quote = opts[:quote]
  end
  
  def string_to_code opts
    @quote = nil
  end
  
  def string_to_interpolation opts
    @braces = 1
    @buffer = @buffer.slice(0,@buffer.length - 1)
  end

  def interpolation_to_string opts; end
  
  def string_to_interpolation_simple opts
    @buffer = @buffer.slice(0,@buffer.length - 1)
  end  

  def interpolation_simple_to_string opts; end
  
  
  def interpolation_to_string_in_interpolation opts
    @interp_quote = opts[:quote]
  
  end  
  
  def string_in_interpolation_to_interpolation opts
    @interp_quote = nil
    @buffer = @buffer.slice(1, @buffer.length)
  end
  ####### HOOKES END
  
  def build_stream s
    
    @input = s
    
    history = []
    history[0] = nil
    @stream = []
    @state = :init
    transist :code
    @braces = 0
    
    @input.split("").each do |char|
      @char = char
      case @state
      when :code
        if quote?(char) && !escape?(history[0])

          transist(:string, :quote => char)
        else
          @buffer += char
        end
      when :string
        if history[1] != ESCAPE && history[0] == CODE && char.match(/[a-zA-Z_\$\{]/)          
          if char == "{"
            transist :interpolation
          else
            transist :interpolation_simple
            @buffer += char
          end       
        elsif char == @quote && !escape?(history[0])
          transist :code
        else
          @buffer += char
        end
      when :interpolation
        
        if @interpolation_quote
          if char == @interpolation_quote && history[0] != ESCAPE
            @interpolation_quote = nil
          end
        else          
          if char == "{"
            @braces += 1
          elsif char == "}"
            @braces -= 1
          elsif quote?(char) && history[0] != ESCAPE
            @interpolation_quote = char            
          end
        end
        
        if @braces == 0
          transist :string
        else
          @buffer += char
        end
      when :interpolation_simple
        if char.match(/[a-zA-Z_\$0-9]/)
          @buffer += char
        else
          transist :string
          if char == @quote            
            transist :code
          else
            @buffer += char
          end
        end
      end
      history.unshift(char).slice(0,5)      

    end
    
    @stream
  end
  

  def stringify_token back, token, fwd
    
    type = token[0]
    data = token[1]
    
    case type
    when :init
      ""
    when :code
      data
    when :string
      "'#{data}'"
    when :interpolation
      s = ""
      s += " + " #if back[1].length > 0
      s += data.match(/[^A-Za-z\$_0-9"' ]/) ? "(#{data})" : data
      s += " + " #if fwd[1].length > 0
      s
    when :interpolation_simple
      s = ""
      s += " + " #if back[1].length > 0 #|| back[0] != :string
      s += data
      s += " + " #if fwd[1].length > 0 #|| fwd[0] != :string
      s
    when :string_in_interpolation
      data
    else
      type
    end
    

  end
  def stringify stream
    ret = ""
    
    stream.each_with_index do |token, i|
      ret += stringify_token(stream[i-1] || [], stream[i], stream[i+1] || [])
    end
    return ret
  end
  
  def expand input
    build_stream input
    debug "tokens:   " + @stream.inspect
    stringify @stream
  end
  
  def run input
    string = input
    debug "---------------------------"
    debug   "input:    " + input
    while true
      expanded = expand string


      if expanded == string
        debug "output:   " + string
        return expanded 
      end

      debug "expanded: " + expanded
      string = expanded
    end
  end
  
  def debug x
    puts x if DEBUG
  end
end


coffee =<<EOF
just_some(code)
"1 + 2"
a: "1 + 2" + '1 + 2'
b: "$x$y$z"
b: "$x" + '${x}' + '${x}'
c: "abc $y def"
c: "${x ? 1 : 2}"
c: " ${x ? 1 : 2} "
c: " ${x ? 1 : "hello"} "
c: " ${x ? 1 : "hello ${xx} "} "
c: " ${x ? 1 : "hello ${fn('}')} "} "
c: " ${"nested ${"nested ${"nested ${"nested ${"nested"}"}"}"}"} "
"""
 heredoc!
"""
EOF
coffee.gsub!("$", "#")

# per line:
coffee.split("\n").each do |line|
  puts Interpolate.new.run(line)
end

# whole lot:
puts Interpolate.new.run(coffee)