class ActionDispatch::Journey::Parser

def self.parse(string)
  new.parse string
end

def advance_token
  @next_token = @scanner.next_token
end

def do_parse
  parse_expressions
end

def initialize
  @scanner = Scanner.new
  @next_token = nil
end

def parse(string)
  @scanner.scan_setup(string)
  advance_token
  do_parse
end

def parse_expression
  if @next_token == :STAR
    parse_star
  elsif @next_token == :LPAREN
    parse_group
  else
    parse_terminal
  end
end

def parse_expressions
  node = parse_expression
  while @next_token
    case @next_token
    when :RPAREN
      break
    when :OR
      node = parse_or(node)
    else
      node = Cat.new(node, parse_expressions)
    end
  end
  node
end

def parse_group
  advance_token
  node = parse_expressions
  if @next_token == :RPAREN
    node = Group.new(node)
    advance_token
    node
  else
    raise ArgumentError, "missing right parenthesis."
  end
end

def parse_or(lhs)
  advance_token
  node = parse_expression
  Or.new([lhs, node])
end

def parse_star
  node = Star.new(Symbol.new(@scanner.last_string, Symbol::GREEDY_EXP))
  advance_token
  node
end

def parse_terminal
  node = case @next_token
  when :SYMBOL
    Symbol.new(@scanner.last_string)
  when :LITERAL
    Literal.new(@scanner.last_literal)
  when :SLASH
    Slash.new("/")
  when :DOT
    Dot.new(".")
  end
  advance_token
  node
end