Dictionary Replacer

replacer.rb

require 'minitest/autorun'

require 'stringio'

class DictionaryReplacer
  MARK   = '$'
  ESCAPE = '\\'

  def initialize(to_replace, dictionary)
    @to_replace = StringIO.new(to_replace)
    @dictionary = dictionary
  end

  def translate
    result = ''
    key    = ''

    in_match = false
    escaping = false

    while c = @to_replace.getc
      if c == ESCAPE
        escaping = true
        result += c

      elsif escaping
        result += c
        escaping = false

      elsif c == MARK
        in_match = !in_match

        if in_match
          key = ''
        else
          if translation = @dictionary[key.to_sym]
            result += translation
          else
            raise SyntaxError
          end
        end

      else
        if in_match
          key += c
        else
          result += c
        end
      end
    end

    raise SyntaxError if in_match
    result
  end
end

describe "dictionary replacement" do
  describe "converting strings" do
    it "converts matching strings" do
      DictionaryReplacer.new('foo $bar$', {bar: 'baz'}).translate.
        must_equal 'foo baz'
    end

    it "doesn't consider escaped MARKs as tokens" do
      DictionaryReplacer.new('foo\$ $bar$', {bar: 'baz'}).translate.
        must_equal 'foo\$ baz'
    end

    it "converts strings with two substitutions" do
      DictionaryReplacer.new('foo $bar$ what $fizz$',
                             {bar: 'baz', fizz: 'faz'}).translate.
        must_equal 'foo baz what faz'
    end

    it "raises a SyntaxError when the token is not found in the dictionary" do
      -> { DictionaryReplacer.new('$foo$',{ }).translate }.
        must_raise SyntaxError
    end

    it "raises an error if the input string is invalid" do
      -> { DictionaryReplacer.new('$foo$ $', {foo: 'bar'}).translate }.
        must_raise(SyntaxError)
    end

  end
end