Ruby Weekly Challenge - Pattern Matcher

pattern_matcher.rb

# Description:

# word_pattern(pattern, string)
# that given a pattern and a string str, find if str follows the same sequence as pattern.

# For example:

# word_pattern('abab', 'truck car truck car') == true
# word_pattern('aaaa', 'dog dog dog dog') == true
# word_pattern('abab', 'apple banana banana apple') == false
# word_pattern('aaaa', 'cat cat dog cat') == false

def word_pattern(pattern, string)
 WordPattern.new(pattern: pattern, string: string).match?
end

class WordPattern
  attr_reader :under_test, :pattern, :string, :expectations

  def initialize(pattern:, string:)
    fail ArgumentError, 'pattern and string must not be nil.' if pattern.nil? || string.nil?

    @under_test = string.split(" ")
    fail ArgumentError, 'pattern and string must be of equal length' unless pattern.length == under_test.length

    @pattern = pattern
    @string = string
    @expectations = {}
  end

  def match?
    (0..pattern.size-1).each do |i|
      return false if new_pattern_but_expectation_exists(pattern[i], under_test[i]) ||
        pattern_exists_but_new_expectation(pattern[i], under_test[i])

      expectations[pattern[i]] = under_test[i] if new_pattern?(i)

      return false unless expectations[pattern[i]] == under_test[i]
    end
    return true
  end

  def pattern_exists_but_new_expectation(p, s)
    !new_pattern?(p) && new_string?(s)
  end

  def new_pattern_but_expectation_exists(p, s)
    new_pattern?(p) && !new_string?(s)
  end

  def new_pattern?(p)
    !expectations.keys.include?(p)
  end

  def new_string?(s)
    !expectations.values.include?(s)
  end
end

pattern_matcher_spec.rb

require_relative '../pattern_matcher'

describe "word_pattern" do
  context "when the pattern matches" do
    it "returns true" do
      expect(word_pattern('abab', 'apple banana apple banana')).to be(true)
    end
    it "returns true" do
      expect(word_pattern('abba', 'car truck truck car')).to be(true)
    end
  end

  context "when the pattern does not match" do
    it "returns false" do
      expect(word_pattern('abab', 'apple banana banana apple')).to be(false)
    end

    it "returns false" do
      expect(word_pattern('abba', 'dog dog dog dog')).to be(false)
    end

    it "returns false" do
      expect(word_pattern('abcd', 'dog cat dog dog')).to be(false)
    end
  end
end