add rank command

lib/rrrdtool.rb

@@ -78,6 +78,11 @@ def last(set, num, options = {})
     options.key?(:with_scores) ? Hash[*e] : e
   end
 
+  def rank(set, key)
+    union_epochs(set)
+    @db.zrevrank("#{set}:union", key)
+  end
+
   def stats(set)
     stats = {}
 

spec/rrrdtool_spec.rb

@@ -122,6 +122,21 @@
     end
   end
 
+  it "should return rank of key across all epochs" do
+    rr.set("test", "key1", 2)
+    rr.set("test", "key2", 1)
+
+    time_travel_to(Time.now + 10) do
+      rr.epoch("test").should match(/test:1/)
+
+      rr.set("test", "key1", 1)
+      rr.set("test", "key2", 3)
+
+      rr.rank("test", "key2").should == 0
+      rr.rank("test", "key1").should == 1
+    end
+  end
+
   it "should footprint stats" do
     rr.incr("test", "key")
     rr.incr("test", "key2")