gistfile1.diff

diff --git a/lib/rollout.rb b/lib/rollout.rb
index 4727b97..fe469f8 100644
--- a/lib/rollout.rb
+++ b/lib/rollout.rb
@@ -1,7 +1,10 @@
 class Rollout
-  def initialize(redis)
-    @redis  = redis
-    @groups = {"all" => lambda { |user| true }}
+  attr_accessor :population
+
+  def initialize(redis, population = 1.0)
+    @redis      = redis
+    @groups     = {"all" => lambda { |user| true }}
+    @population = population
   end
 
   def activate_group(feature, group)
@@ -72,6 +75,6 @@ class Rollout
     def user_within_active_percentage?(feature, user)
       percentage = @redis.get(percentage_key(feature))
       return false if percentage.nil?
-      user.id % 100 < percentage.to_i
+      user.id * @population % 100 < percentage.to_i
     end
 end
diff --git a/spec/rollout_spec.rb b/spec/rollout_spec.rb
index 0941965..a5a3ccd 100644
--- a/spec/rollout_spec.rb
+++ b/spec/rollout_spec.rb
@@ -116,7 +116,7 @@ describe "Rollout" do
     end
   end
 
-  describe "activating a feature for a percentage of users" do
+  describe "activating a feature for a bigger percentage of users" do
     before do
       @rollout.activate_percentage(:chat, 20)
     end
@@ -126,6 +126,24 @@ describe "Rollout" do
     end
   end
 
+  describe "activating a feature for a percentage of users with holes in the id space" do
+    before do
+      @standard_poulation, @rollout.population = @rollout.population, 0.1
+      @rollout.activate_percentage(:chat, 3)
+    end
+
+    after do
+      @rollout.population = @standard_population
+    end
+
+    it "activates the feature for that percentage of the users" do
+      id = 3
+      step = 10
+
+      100.times.select { @rollout.active?(:chat, stub(:id => id += step)) }.length.should == 3
+    end
+  end
+
   describe "activating a feature for a percentage of users" do
     before do
       @rollout.activate_percentage(:chat, 5)