make hyperLogLogMap public with configurable serializer

src/main/scala/com/twitter/scalding/ReduceOperations.scala

@@ -94,20 +94,23 @@ trait ReduceOperations[+Self <: ReduceOperations[Self]] extends java.io.Serializ
    * }}}
    */
   def approxUniques(f : (Fields, Fields), errPercent : Double = 1.0) = {
-    derivedHll(f, errPercent) { (monoid,hll) => monoid.estimateSize(hll) }
+    hyperLogLogMap(f, errPercent) { (monoid,hll) => monoid.estimateSize(hll) }
+      { _.toString.getBytes("UTF-8") }
   }
 
   def hyperLogLog(f : (Fields, Fields), errPercent : Double = 1.0) = {
-    derivedHll(f, errPercent) { (_,hll) => hll }
+    hyperLogLogMap(f, errPercent) { (_,hll) => hll }
+      { _.toString.getBytes("UTF-8") }
   }
 
-  private[this] def derivedHll[T](f : (Fields, Fields), errPercent : Double)
-                                          (fn : (HyperLogLogMonoid,HLL) => T) = {
+  def hyperLogLogMap[T,U](f : (Fields, Fields), errPercent : Double = 1.0)
+                         (fn : (HyperLogLogMonoid,HLL) => U)
+                         (toBytes : (T) => Array[Byte]) = {
     //bits = log(m) == 2 *log(104/errPercent) = 2log(104) - 2*log(errPercent)
     def log2(x : Double) = scala.math.log(x)/scala.math.log(2.0)
     val bits = 2 * scala.math.ceil(log2(104) - log2(errPercent)).toInt
     implicit val hmm = new HyperLogLogMonoid(bits)
-    mapPlusMap(f) { (in : CTuple) => hmm.create(in.toString.getBytes("UTF-8")) }
+    mapPlusMap(f) { (t : T) => hmm.create(toBytes(t)) }
       { hll => fn(hmm,hll) }
   }