primality_checker.rb

class Integer
  def is_prime?
    return false if self < 2
    (2..Math::sqrt(self)).each do |i|
      return false if self % i == 0
    end
    return true
  end
end

primality_checker_benchmark.rb

require 'benchmark'

class Integer

  def is_prime_v0?
    return false if self < 2
    (2...self).each { |i| return false if self % i == 0 }
    return true
  end
  
  def is_prime_v1?
    return false if self < 2
    (2..Math::sqrt(self)).each { |i| return false if self % i == 0 }
    return true
  end
  
  def is_prime_v2?
    return false if self < 2
    i = 2
    while i*i <= self do
      return false if self % i == 0  
      i += 1
    end
    return true
  end
  
  def is_prime_v3?
    return true if self == 2
    return false if (self < 2) || (self % 2 == 0)
    i = 3
    while i*i <= self do
      return false if self % i == 0  
      i += 2
    end
    return true
  end
  
end

trial_num = 1
[10, 25, 50, 75, 100, 500, 1000, 20000].each do |n|
  puts "\n\nTrial #{trial_num} (n = #{n}) :\n"
  Benchmark.bmbm do |x|
    x.report("v0: all to n:")          { for i in 1..n; i.is_prime_v0?; end }
    x.report("v1: all to sqrt-n:")     { for i in 1..n; i.is_prime_v1?; end }
    x.report("v2: all to n by n^2:")   { for i in 1..n; i.is_prime_v2?; end }
    x.report("v3: by n^2, skipping:")  { for i in 1..n; i.is_prime_v3?; end }
  end
  trial_num += 1
end