projecteuler-trianglenumber.rb

# See: http://projecteuler.net/index.php?section=problems&id=12
#
# We don't need to actually store the individual factors so long as we know how many were identified
def divisors(limit)
  triangle, triangle_sum, found_divisors = 0, 0, 0
  while found_divisors < limit
    triangle_sum += (triangle += 1)
    next unless triangle_sum % 2 == 0 # Only caring about even numbers

    found_divisors = 2 # default = [1, triangle_sum]
    position, last_max_factor = 2, triangle_sum
    while position < last_max_factor
      if triangle_sum % position == 0
        last_max_factor = triangle_sum / position
        found_divisors += 2
      end
      position += 1
    end
  end
  triangle_sum
end

puts divisors(ARGV[0].to_i)

results.txt

gus@jaknowlden:trinumbers> time ruby1.9 projecteuler-trianglenumber.rb 500
76576500

real	0m2.930s
user	0m2.899s
sys	0m0.015s