prime_factors

prime_factors.rb

require 'sieve'

def prime_factors_parent(number)
  primes = prime_sift(number)
  prime_factors(number, primes)
end

def prime_factors(number, primes, factors=[])
  if primes.include?(number)
    factors << number
  else
    first_prime = primes.find { |prime| number % prime == 0 }
    factors << first_prime 
    prime_factors(number / first_prime, primes, factors)
  end
end

prime_factors_spec.rb

require 'rspec'
require 'prime_factors'

describe 'prime_factors' do
  it 'returns an array for the prime factors of any number greater than one' do
    expect { prime_factors_parent(1) }.to raise_exception
  end

  it 'returns an array of prime factors up through 16' do
    prime_factors_parent(9).should eq [3, 3]
    prime_factors_parent(8).should eq [2, 2, 2]
    prime_factors_parent(16).should eq [2, 2, 2, 2]
    prime_factors_parent(625).should eq [5, 5, 5, 5]
  end
end