angelikatyborska's solution to Palindrome Products on the Ruby track

Instructions

Test suite

Solution

Detect palindrome products in a given range.

A palindromic number is a number that remains the same when its digits are reversed. For example, 121 is a palindromic number but 112 is not.

Given a range of numbers, find the largest and smallest palindromes which are products of numbers within that range.

Your solution should return the largest and smallest palindromes, along with the factors of each within the range. If the largest or smallest palindrome has more than one pair of factors within the range, then return all the pairs.

Example 1

Given the range [1, 9] (both inclusive)...

And given the list of all possible products within this range: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 15, 21, 24, 27, 20, 28, 32, 36, 25, 30, 35, 40, 45, 42, 48, 54, 49, 56, 63, 64, 72, 81]

The palindrome products are all single digit numbers (in this case): [1, 2, 3, 4, 5, 6, 7, 8, 9]

The smallest palindrome product is 1. Its factors are (1, 1). The largest palindrome product is 9. Its factors are (1, 9) and (3, 3).

Example 2

Given the range [10, 99] (both inclusive)...

The smallest palindrome product is 121. Its factors are (11, 11). The largest palindrome product is 9009. Its factors are (91, 99).

For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby palindrome_products_test.rb

To include color from the command line:

ruby -r minitest/pride palindrome_products_test.rb

Source

Problem 4 at Project Euler http://projecteuler.net/problem=4

Submitting Incomplete Solutions

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

palindrome_products_test.rb

require 'minitest/autorun'
require_relative 'palindrome_products'

class PalindromesTest < Minitest::Test
  def test_largest_palindrome_from_single_digit_factors
    palindromes = Palindromes.new(max_factor: 9)
    palindromes.generate
    largest = palindromes.largest
    assert_equal 9, largest.value
    assert_includes [[[3, 3], [1, 9]], [[1, 9], [3, 3]]], largest.factors
  end

  def test_largest_palindrome_from_double_digit_factors
    skip
    palindromes = Palindromes.new(max_factor: 99, min_factor: 10)
    palindromes.generate
    largest = palindromes.largest
    assert_equal 9009, largest.value
    assert_equal [[91, 99]], largest.factors
  end

  def test_smallest_palindrome_from_double_digit_factors
    skip
    palindromes = Palindromes.new(max_factor: 99, min_factor: 10)
    palindromes.generate
    smallest = palindromes.smallest
    assert_equal 121, smallest.value
    assert_equal [[11, 11]], smallest.factors
  end

  def test_largest_palindrome_from_triple_digit_factors
    skip
    palindromes = Palindromes.new(max_factor: 999, min_factor: 100)
    palindromes.generate
    largest = palindromes.largest
    assert_equal 906_609, largest.value
    assert_equal [[913, 993]], largest.factors
  end

  def test_smallest_palindrome_from_triple_digit_factors
    skip
    palindromes = Palindromes.new(max_factor: 999, min_factor: 100)
    palindromes.generate
    smallest = palindromes.smallest
    assert_equal 10_201, smallest.value
    assert_equal [[101, 101]], smallest.factors
  end
end

require 'set'

class Palindromes
  def initialize(max_factor:, min_factor: 1)
    @min_factor = min_factor
    @max_factor = max_factor
  end

  def generate
    @palindromes = all_factors.each_with_object({}) do |factor1, palindromes|
      all_factors.each do |factor2|
        factors = [factor1, factor2]
        product = factors.reduce(:*)

        next unless palindrome?(product)

        palindromes[product] ||= CompositeNumber.new(product)
        palindromes[product].add_factors(factors)
      end
    end
  end

  def largest
    palindromes[palindromes.keys.max]
  end

  def smallest
    palindromes[palindromes.keys.min]
  end

  private

  def palindromes
    @palindromes || generate
  end

  def palindrome?(value)
    value.to_s.reverse == value.to_s
  end

  def all_factors
    @all_factors ||= @min_factor..@max_factor
  end
end

class CompositeNumber
  attr_reader :value

  def initialize(value)
    @value = value
    @factors = Set.new
  end

  def add_factors(factors)
    @factors << factors.sort
  end

  def factors
    @factors.to_a
  end
end

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angelikatyborska's solution

to Palindrome Products in the Ruby Track