RegexEngine.rb

require_relative 'NFA'

module Pattern
  def bracket(outer_precedence)
    if precedence < outer_precedence
      '(' + to_s + ')'
    else
      to_s
    end
  end

  def inspect
    "/#{self}/"
  end

  def match?(str)
    to_nfa_design.accepts?(str)
  end
end

class Empty
  include Pattern
  def to_s
    ''
  end

  def precedence
    3
  end

  def to_nfa_design
    start_state = Object.new
    accept_states = [start_state]
    rulebook = NFARulebook.new([])

    NFADesign.new(start_state, accept_states, rulebook)
  end
end

class Literal < Struct.new(:character)
  include Pattern

  def to_s
    character
  end

  def precedence
    3
  end

  def to_nfa_design
    start_state = Object.new
    accept_state = Object.new
    rule = FARule.new(start_state, character, accept_state)
    rulebook = NFARulebook.new([rule])

    NFADesign.new(start_state, [accept_state], rulebook)
  end
end

class Concatenate < Struct.new(:first, :second)
  include Pattern

  def to_s
    [first, second].map { |pattern| pattern.bracket(precedence) }.join
  end

  def precedence
    1
  end
end

class Choose < Struct.new(:first, :second)
  include Pattern

  def to_s
    [first, second].map { |pattern| pattern.bracket(precedence) }.join('|')
  end

  def precedence
    0
  end
end

class Repeat < Struct.new(:pattern)
  include Pattern

  def to_s
    pattern.bracket(precedence) + '*'
  end

  def precedence
    2
  end
end

nfa_design = Empty.new.to_nfa_design
puts nfa_design.accepting?('')