Euler Problem 3

3.rb

num = 600851475143

# Brute force
def is_prime?(n)
  limit = Math.sqrt(n).round
  (2..limit).each do |i|
    return false if n % i == 0
  end
  return true
end

def is_factor?(num, n)
  num % n == 0
end

largest_prime = nil
Math.sqrt(num).round.downto 1 do |n|
  if is_factor?(num, n) && is_prime?(n)
    largest_prime = n
    break
  end
end

puts "Brute force: #{largest_prime}"