Ruby function to detect if a Fixednum is a prime number

isPrime.rb

# @author David Wortham <djwortham+programming@gmail.com>
# @date 2011-04-30
# @license MIT License: http://www.opensource.org/licenses/mit-license.php

# isPrime
# Function to detect if the input (asserted Fixednum) is a prime number
# uses an optional cache (asserted Hash) to speed up subsequent calls
def isPrime(number, cache)
  return false if number < 1
  return true if number == 1
  # if our cache contains the result for this number, then just use the cached result
  return cache[number] unless cache.nil? || cache[number].nil?

  # there is no need to test numbers larger than (number + 1) / 2 for prime factors
  maxTest = (number + 1) / 2
  i = 2
  # no reason to retest numbers we already have cached; need to check to make sure this method is optimal
  unless cache.nil? || cache.empty?
    i = cache.length
  end

  while i < maxTest
    if number % i == 0
      cache[number] = false unless cache.nil?
      return false
    else
      cache[number] = true unless cache.nil?
    end
    i = i + 1
  end
  return true
end

# usage (without cache):
# puts "is 19 prime? #{isPrime(19)}"

# usage (with cache):
# primeCache = {}
# puts "is 34 prime? #{isPrime(34, primeCache)}"
# puts "is 19 prime? #{isPrime(19, primeCache)}" # this result has already been cached by the previous call