shmibs's solution to Bracket Push on the Crystal track

Instructions

Test suite

Solution

Given a string containing brackets [], braces {}, parentheses (), or any combination thereof, verify that any and all pairs are matched and nested correctly.

Setup

Follow the setup instructions for Crystal here:

http://exercism.io/languages/crystal

More help installing can be found here:

http://crystal-lang.org/docs/installation/index.html

Making the Test Suit Pass

Execute the tests with:

$ crystal spec

In each test suite all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by changing pending to it.

Source

Ginna Baker

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

bracket_push_spec.cr

require "spec"
require "../src/*"

describe "Brackets" do
  describe "#are_valid?" do
    it "paired square brackets" do
      Brackets.are_valid?("[]").should be_true
    end
    it "empty string" do
      Brackets.are_valid?("").should be_true
    end
    it "unpaired brackets" do
      Brackets.are_valid?("[[").should be_false
    end
    it "wrong ordered brackets" do
      Brackets.are_valid?("}{").should be_false
    end
    it "paired with whitespace" do
      Brackets.are_valid?("{ }").should be_true
    end
    it "simple nested brackets" do
      Brackets.are_valid?("{[]}").should be_true
    end
    it "several paired brackets" do
      Brackets.are_valid?("{}[]").should be_true
    end
    it "paired and nested brackets" do
      Brackets.are_valid?("([{}({}[])])").should be_true
    end
    it "unopened closing brackets" do
      Brackets.are_valid?("{[)][]}").should be_false
    end
    it "unpaired and nested brackets" do
      Brackets.are_valid?("([{])").should be_false
    end
    it "paired and wrong nested brackets" do
      Brackets.are_valid?("[({]})").should be_false
    end
    it "math expression" do
      Brackets.are_valid?("(((185 + 223.85) * 15) - 543)/2").should be_true
    end
    it "complex latex expression" do
      Brackets.are_valid?("\left(\begin{array}{cc} \frac{1}{3} & x\\ \mathrm{e}^{x} &... x^2 \end{array}\right)").should be_true
    end
  end
end

module Brackets

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

  def self.are_valid?(s : String) : Bool
    a = [] of Char

    s.each_char { |c|
      pc = PAIRS[c]?

      if PAIRS.values.includes?(c)
        a.push(c)
      elsif pc
        if a.last? == nil || a.last != pc
          return false
        end
        a.pop
      end
    }

    a.empty?
  end

end

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shmibs's solution

to Bracket Push in the Crystal Track