Well formed strings solution

string_validator.rb

@@ -0,0 +1,44 @@
+class StringValidator
+  attr_reader :token_stack
+
+  def initialize
+    @token_stack = []
+  end
+
+  def pairings
+    {
+      "(" => ")",
+      "[" => "]",
+      "{" => "}",
+    }
+  end
+
+  def matched_pair?(t1, t2)
+    pairings[t1] == t2
+  end
+
+  def left?(token)
+    pairings.keys.include?(token)
+  end
+
+  def right?(token)
+    pairings.values.include?(token)
+  end
+
+  def validate(target)
+    answer = pairs_match?(target) && token_stack.empty?
+    token_stack.clear
+    return answer
+  end
+
+  def pairs_match?(target)
+    target.chars.all? do |token|
+      if left?(token)
+        token_stack.push(token)
+      elsif right?(token)
+        left = token_stack.pop
+        matched_pair?(left, token)
+      end
+    end
+  end
+end

string_validator_test.rb

@@ -0,0 +1,64 @@
+require 'minitest/autorun'
+require './string_validator'
+
+class StringValidatorTest < Minitest::Test
+  def test_it_exists
+    assert StringValidator
+  end
+
+  def test_an_empty_string_is_valid
+    sv = StringValidator.new
+    assert sv.validate("")
+  end
+
+  def test_single_parens_are_valid
+    sv = StringValidator.new
+    assert sv.validate("()")
+  end
+
+  def test_single_inverted_parens_are_not_valid
+    sv = StringValidator.new
+    refute sv.validate(")(")
+  end
+
+  def test_single_square_brackets_are_valid
+    sv = StringValidator.new
+    assert sv.validate("[]")
+  end
+
+  def test_single_curly_brackets_are_valid
+    sv = StringValidator.new
+    assert sv.validate("{}")
+  end
+
+  def test_two_pairs_of_matched_brackets_are_valid
+    sv = StringValidator.new
+    assert sv.validate("{}()")
+  end
+
+  def test_mismatched_second_brackets_are_invalid
+    sv = StringValidator.new
+    refute sv.validate("{})(")
+  end
+
+  def test_complex_nesting_is_valid
+    sv = StringValidator.new
+    assert sv.validate("([{}[]])")
+  end
+
+  def test_complex_nesting_with_some_trash_is_invalid
+    sv = StringValidator.new
+    refute sv.validate("([{}[]])]")
+  end
+
+  def test_complex_nesting_with_some_other_trash_is_invalid
+    sv = StringValidator.new
+    refute sv.validate("([{}[]])[")
+  end
+
+  def test_validating_a_second_input_works_correctly
+    sv = StringValidator.new
+    refute sv.validate("([{}[]])[")
+    assert sv.validate("()")
+  end
+end