Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/2667#discussion_r19129863
--- Diff:
mllib/src/main/scala/org/apache/spark/mllib/evaluation/RankingMetrics.scala ---
@@ -0,0 +1,157 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.mllib.evaluation
+
+import scala.reflect.ClassTag
+
+import org.apache.spark.Logging
+import org.apache.spark.SparkContext._
+import org.apache.spark.annotation.Experimental
+import org.apache.spark.rdd.RDD
+
+/**
+ * ::Experimental::
+ * Evaluator for ranking algorithms.
+ *
+ * @param predictionAndLabels an RDD of (predicted ranking, ground truth
set) pairs.
+ */
+@Experimental
+class RankingMetrics[T: ClassTag](predictionAndLabels: RDD[(Array[T],
Array[T])])
+ extends Logging with Serializable {
+
+ /**
+ * Compute the average precision of all the queries, truncated at
ranking position k.
+ *
+ * If for a query, the ranking algorithm returns n (n < k) results, the
precision value will be
+ * computed as #(relevant items retrived) / k. This formula also applies
when the size of the
+ * ground truth set is less than k.
+ *
+ * If a query has an empty ground truth set, zero will be returned
together with a log warning.
+ *
+ * See the following paper for detail:
+ *
+ * IR evaluation methods for retrieving highly relevant documents.
+ * K. Jarvelin and J. Kekalainen
+ *
+ * @param k the position to compute the truncated precision, must be
positive
+ * @return the average precision at the first k ranking positions
+ */
+ def precisionAt(k: Int): Double = {
+ require (k > 0,"ranking position k should be positive")
+ predictionAndLabels.map { case (pred, lab) =>
+ val labSet = lab.toSet
+ val n = math.min(pred.length, k)
+ var i = 0
+ var cnt = 0
+
+ while (i < n) {
+ if (labSet.contains(pred(i))) {
+ cnt += 1
+ }
+ i += 1
+ }
+ if (labSet.size == 0) {
+ logWarning("Empty ground truth set, check input data")
+ 0.0
+ } else {
+ cnt.toDouble / k
+ }
+ }.mean
+ }
+
+ /**
+ * Returns the mean average precision (MAP) of all the queries.
+ * If a query has an empty ground truth set, the average precision will
be zero and a log
+ * warining is generated.
+ */
+ lazy val meanAveragePrecision: Double = {
+ predictionAndLabels.map { case (pred, lab) =>
+ val labSet = lab.toSet
+ val labSetSize = labSet.size
+ var i = 0
+ var cnt = 0
+ var precSum = 0.0
+ val n = pred.length
+
+ while (i < n) {
+ if (labSet.contains(pred(i))) {
+ cnt += 1
+ precSum += cnt.toDouble / (i + 1)
+ }
+ i += 1
+ }
+ if (labSetSize == 0) {
+ logWarning("Empty ground truth set, check input data")
+ 0.0
+ } else {
+ precSum / labSet.size
+ }
+ }.mean
+ }
+
+ /**
+ * Compute the average NDCG value of all the queries, truncated at
ranking position k.
+ * The discounted cumulative gain at position k is computed as:
+ * \sum_{i=1}^k (2^{relevance of ith item} - 1) / log(i + 1),
+ * and the NDCG is obtained by dividing the DCG value on the ground
truth set. In the current
+ * implementation, the relevance value is binary.
+ *
+ * If for a query, the ranking algorithm returns n (n < k) results, the
NDCG value at position n
+ * will be used. If the ground truth set contains n (n < k) results, the
first n items will be
+ * used to compute the DCG value on the ground truth set.
--- End diff --
This paragraph is not necessary because those cases are compatible with the
definition of NDCG.
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