hll-err.rb added to test error rate of Redis HyperLogLog.

utils/hyperloglog/hll-err.rb

+# hll-err.rb - Copyright (C) 2014 Salvatore Sanfilippo
+# BSD license, See the COPYING file for more information.
+#
+# Check error of HyperLogLog Redis implementation for different set sizes.
+
+require 'rubygems'
+require 'redis'
+require 'digest/sha1'
+
+r = Redis.new
+r.del('hll')
+(1..1000000000).each{|i|
+    ele = Digest::SHA1.hexdigest(i.to_s)
+    r.hlladd('hll',ele)
+    if i != 0 && (i%10000) == 0
+        approx = r.hllcount('hll')
+        abs_err = (approx-i).abs
+        rel_err = 100.to_f*abs_err/i
+        puts "#{i} vs #{approx}: #{rel_err}%"
+    end
+}