class Regexp::Lexer

def self.lex(input, syntax = "ruby/#{RUBY_VERSION}", &block)
  new.lex(input, syntax, &block)
end

def ascend(type, token)
  case type
  when :group, :assertion
    self.nesting = nesting - 1 if CLOSING_TOKENS.include?(token)
  when :set
    self.set_nesting = set_nesting - 1 if token == :close
  when :conditional
    self.conditional_nesting = conditional_nesting - 1 if token == :close
  end
end

def break_literal(token)
  text = token.text
  if text.scan(/./mu).length > 1
    lead = text.sub(/.\z/mu, "")
    last = text[/.\z/mu] || ''
    if RUBY_VERSION >= '1.9'
      lead_length = lead.bytesize
      last_length = last.bytesize
    else
      lead_length = lead.length
      last_length = last.length
    end
    tokens.pop
    tokens << Regexp::Token.new(:literal, :literal, lead, token.ts,
              (token.te - last_length), nesting, set_nesting, conditional_nesting)
    tokens << Regexp::Token.new(:literal, :literal, last,
              (token.ts + lead_length),
              token.te, nesting, set_nesting, conditional_nesting)
  end
end

def descend(type, token)
  case type
  when :group, :assertion
    self.nesting = nesting + 1 if OPENING_TOKENS.include?(token)
  when :set
    self.set_nesting = set_nesting + 1 if token == :open
  when :conditional
    self.conditional_nesting = conditional_nesting + 1 if token == :open
  end
end

def lex(input, syntax = "ruby/#{RUBY_VERSION}", &block)
  syntax = Regexp::Syntax.new(syntax)
  self.tokens = []
  self.nesting = 0
  self.set_nesting = 0
  self.conditional_nesting = 0
  last = nil
  Regexp::Scanner.scan(input) do |type, token, text, ts, te|
    type, token = *syntax.normalize(type, token)
    syntax.check! type, token
    ascend(type, token)
    break_literal(last) if type == :quantifier and
      last and last.type == :literal
    current = Regexp::Token.new(type, token, text, ts, te,
              nesting, set_nesting, conditional_nesting)
    current = merge_literal(current) if type == :literal and
      set_nesting == 0 and
      last and last.type == :literal
    current = merge_condition(current) if type == :conditional and
      [:condition, :condition_close].include?(token)
    last.next = current if last
    current.previous = last if last
    tokens << current
    last = current
    descend(type, token)
  end
  if block_given?
    tokens.map { |t| block.call(t) }
  else
    tokens
  end
end

def merge_condition(current)
  last = tokens.pop
  Regexp::Token.new(:conditional, :condition, last.text + current.text,
    last.ts, current.te, nesting, set_nesting, conditional_nesting)
end

def merge_literal(current)
  last = tokens.pop
  Regexp::Token.new(
    :literal,
    :literal,
    last.text + current.text,
    last.ts,
    current.te,
    nesting,
    set_nesting,
    conditional_nesting,
  )
end