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w1zeman1p's solution

to Prime Factors in the Ruby Track

Published at Feb 26 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

Example

What are the prime factors of 60?

  • Our first divisor is 2. 2 goes into 60, leaving 30.
  • 2 goes into 30, leaving 15.
    • 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
  • 3 goes cleanly into 15, leaving 5.
    • 3 does not go cleanly into 5. The next possible factor is 4.
    • 4 does not go cleanly into 5. The next possible factor is 5.
  • 5 does go cleanly into 5.
  • We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

  • 2 * 2 * 3 * 5
  • = 4 * 15
  • = 60
  • Success!

For installation and learning resources, refer to the Ruby resources 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 prime_factors_test.rb

To include color from the command line:

ruby -r minitest/pride prime_factors_test.rb

Source

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

Submitting Incomplete Solutions

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

prime_factors_test.rb

require 'minitest/autorun'
require_relative 'prime_factors'

class PrimeFactorsTest < Minitest::Test
  def test_1
    assert_equal [], PrimeFactors.for(1)
  end

  def test_2
    skip
    assert_equal [2], PrimeFactors.for(2)
  end

  def test_3
    skip
    assert_equal [3], PrimeFactors.for(3)
  end

  def test_4
    skip
    assert_equal [2, 2], PrimeFactors.for(4)
  end

  def test_6
    skip
    assert_equal [2, 3], PrimeFactors.for(6)
  end

  def test_8
    skip
    assert_equal [2, 2, 2], PrimeFactors.for(8)
  end

  def test_9
    skip
    assert_equal [3, 3], PrimeFactors.for(9)
  end

  def test_27
    skip
    assert_equal [3, 3, 3], PrimeFactors.for(27)
  end

  def test_625
    skip
    assert_equal [5, 5, 5, 5], PrimeFactors.for(625)
  end

  def test_901255
    skip
    assert_equal [5, 17, 23, 461], PrimeFactors.for(901_255)
  end

  def test_93819012551
    skip
    assert_equal [11, 9539, 894_119], PrimeFactors.for(93_819_012_551)
  end
end
require 'byebug'

class PrimeGenerator

  def initialize
    @primes = []
    @i = 2
  end

  def build(n)
    while @primes.length < n
      if is_prime?(@i)
        @primes << @i
      end
      @i += 1
    end
  end

  def next
    build(@primes.length + 1)
  end

  def last
    @primes.last
  end

  def is_prime?(x)
    @primes.each do |prime|
      # Return early if the number in question is evenly divisible by a prime.
      return false if x != prime && x % prime == 0
    end
    true
  end
end

class PrimeFactors
  def self.for(n)
    return [] if n == 1
    return [2] if n == 2
    factors = []

    prime_generator = PrimeGenerator.new
    prime_generator.next
    current = n
    while current > 1
      if current % prime_generator.last == 0
        current /= prime_generator.last
        factors << prime_generator.last
      else
        prime_generator.next
      end
    end
    factors
  end
end

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