* lib/prime.rb: Optimize prime?

  Adapted from patch by Jabari Zakiya [#12665]

* test/test_prime.rb: Improve test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55856 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

3 files changed, 20 insertions, 15 deletions

diff --git a/ChangeLog b/ChangeLog
index 8f923c2264..db63f49a6e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Aug 11 03:16:59 2016  Marc-Andre Lafortune  <ruby-core@marc-andre.ca>
+
+	* lib/prime.rb: Optimize prime?
+	  Adapted from patch by Jabari Zakiya [#12665]
+
+	* test/test_prime.rb: Improve test
+
 Wed Aug 10 22:37:01 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (command_rhs, arg_rhs): introduce new rules to reduce
diff --git a/lib/prime.rb b/lib/prime.rb
index c64c0c2cf1..a6700e3e8b 100644
--- a/lib/prime.rb
+++ b/lib/prime.rb
@@ -33,11 +33,12 @@ class Integer
   # Returns true if +self+ is a prime number, else returns false.
   def prime?
     return self >= 2 if self <= 3
-    return false if self % 2 == 0 or self % 3 == 0
-    (5..(self**0.5).floor).step(6).each do |i|
-      if self % i == 0 || self % (i + 2) == 0
-        return false
-      end
+    return true if self == 5
+    return false unless 30.gcd(self) == 1
+    (7..Math.sqrt(self).to_i).step(30) do |p|
+      return false if
+        self%(p)    == 0 || self%(p+4)  == 0 || self%(p+6)  == 0 || self%(p+10) == 0 ||
+        self%(p+12) == 0 || self%(p+16) == 0 || self%(p+22) == 0 || self%(p+24) == 0
     end
     true
   end
diff --git a/test/test_prime.rb b/test/test_prime.rb
index b48ccae319..7f15894abf 100644
--- a/test/test_prime.rb
+++ b/test/test_prime.rb
@@ -140,17 +140,14 @@ class TestPrime < Test::Unit::TestCase
     end
 
     def test_prime?
-      # zero and unit
-      assert_not_predicate(0, :prime?)
-      assert_not_predicate(1, :prime?)
-
-      # small primes
-      assert_predicate(2, :prime?)
-      assert_predicate(3, :prime?)
+      PRIMES.each do |p|
+        assert_predicate(p, :prime?)
+      end
 
-      # squared prime
-      assert_not_predicate(4, :prime?)
-      assert_not_predicate(9, :prime?)
+      composites = (0..PRIMES.last).to_a - PRIMES
+      composites.each do |c|
+        assert_not_predicate(c, :prime?)
+      end
 
       # mersenne numbers
       assert_predicate((2**31-1), :prime?)