balanced.rb

require 'minitest/autorun'
require 'pry'

PAIRS = { '{' => '}', '[' => ']', '(' => ')' }.freeze

def balanced?(string)
  return false if string.length.odd?
  chars = string.chars

  PAIRS.keys
       .map do |opening|
         [opening, chars.count { |char| char == opening }]
       end
       .reject { |_char, count| count.zero? }
       .all? do |opening, expected_count|
         chars.count { |char| char == PAIRS[opening] } == expected_count
       end
end

class BalancedTest < MiniTest::Test # :nodoc:
  def test_balanced
    assert balanced?('(())')
  end

  def test_unbalanced
    refute balanced?('((()')
  end
end