Reimplemented the RSA::Math.primes method to generate pseudo-primes d…

…irectly.

This enables compatibility with Ruby 1.8, where the Prime standard library isn't yet available.

This also has the effect of significantly speeding up other RSA::Math methods relying on prime generation. It appears that Ruby 1.9's pseudo-prime generator implementation is rather inefficient compared to a more straightforward implementation of the same algorithm.

lib/rsa/math.rb

@@ -7,19 +7,29 @@ module Math
     class ArithmeticError < ArgumentError; end
 
     ##
-    # Yields the infinite sequence of prime numbers.
+    # Yields an infinite pseudo-prime number sequence.
+    #
+    # This is a pseudo-prime generator that simply yields the initial values
+    # 2 and 3, followed by all positive integers that are not divisible by 2
+    # or 3.
+    #
+    # It works identically to `Prime::Generator23`, the Ruby 1.9 standard
+    # library's default pseudo-prime generator implementation.
     #
     # @example
     #   RSA::Math.primes.take(5)                       #=> [2, 3, 5, 7, 11]
     #
-    # @yield  [prime] each prime number
-    # @yieldparam [Integer] prime a prime number
-    # @return [Enumerator]
+    # @yield  [p] each pseudo-prime number
+    # @yieldparam [Integer] p a pseudo-prime number
+    # @return [Enumerator] yielding pseudo-primes
     # @see    http://ruby-doc.org/core-1.9/classes/Prime.html
     def self.primes(&block)
-      require 'prime' unless defined?(Prime) # Ruby 1.9+ only
-      primes = Prime::Generator23.new
-      block_given? ? primes.each(&block) : primes.to_enum
+      if block_given?
+        yield 2; yield 3; yield 5
+        prime, step = 5, 4
+        loop { yield prime += (step = 6 - step) }
+      end
+      enum_for(:primes)
     end
 
     ##

spec/math_spec.rb

@@ -6,8 +6,9 @@
       RSA::Math.primes.should be_an(Enumerator)
     end
 
-    it "returns the prime number sequence" do
-      RSA::Math.primes.take(5).should == [2, 3, 5, 7, 11]
+    it "returns a pseudo-prime number sequence" do
+      RSA::Math.primes.take(5).should    == [2, 3, 5, 7, 11]
+      RSA::Math.primes.take(1000).should == Prime::Generator23.new.take(1000) if RUBY_VERSION > '1.9' # Ruby 1.9+ only
     end
   end
 

spec/spec_helper.rb

@@ -1,2 +1,4 @@
 require 'rsa'
 include RSA
+
+require 'prime' if RUBY_VERSION > '1.9' # Ruby 1.9+ only