balancing symbols

require 'minitest/autorun'

# detect if a string symbols are balanced
# '[()]' => true
# '({)' => false
# '(([[]]))'

class Algo
  TABLE = {
    '(' => ')',
    '[' => ']',
    '{' => '}'
  }

  OPENINGS = TABLE.keys

  def is_balanced str
    stack = []
    str.each_char do |char|
      if OPENINGS.include?(char)
        stack << char
      else
        open_char = stack.pop
        if !open_char or TABLE[open_char] != char
          return false
        end
      end
    end

    stack.empty? ? true : false
  end
end

describe Algo do
  before do
    @algo = Algo.new
  end

  describe "balanced symbols" do
    it "return true if symbols are balanced" do
      assert_equal true, @algo.is_balanced( '()' )
      assert_equal true, @algo.is_balanced( '[()]' )
      assert_equal true, @algo.is_balanced( '[[[({})]]]' )
      assert_equal true, @algo.is_balanced( '' )
    end

    it "return false if symbols are not balanced" do
      assert_equal false, @algo.is_balanced( '{()' )
      assert_equal false, @algo.is_balanced( '{' )
    end
  end
end