Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/15148#discussion_r79338430
--- Diff: mllib/src/main/scala/org/apache/spark/ml/lsh/LSH.scala ---
@@ -0,0 +1,270 @@
+/*
+ * 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.ml.lsh
+
+import scala.util.Random
+
+import org.apache.spark.ml.{Estimator, Model}
+import org.apache.spark.ml.linalg.{Vector, VectorUDT}
+import org.apache.spark.ml.param.{IntParam, ParamMap, ParamValidators}
+import org.apache.spark.ml.param.shared.{HasInputCol, HasOutputCol}
+import org.apache.spark.sql._
+import org.apache.spark.sql.expressions.UserDefinedFunction
+import org.apache.spark.sql.functions._
+import org.apache.spark.sql.types._
+
+/**
+ * Params for [[LSH]].
+ */
+private[ml] trait LSHParams extends HasInputCol with HasOutputCol {
+ /**
+ * Param for output dimension.
+ *
+ * @group param
+ */
+ final val outputDim: IntParam = new IntParam(this, "outputDim", "output
dimension",
+ ParamValidators.gt(0))
+
+ /** @group getParam */
+ final def getOutputDim: Int = $(outputDim)
+
+ setDefault(outputDim -> 1)
+
+ setDefault(outputCol -> "lsh_output")
+
+ /**
+ * Transform the Schema for LSH
+ * @param schema The schema of the input dataset without outputCol
+ * @return A derived schema with outputCol added
+ */
+ final def transformLSHSchema(schema: StructType): StructType = {
+ val outputFields = schema.fields :+
+ StructField($(outputCol), new VectorUDT, nullable = false)
+ StructType(outputFields)
+ }
+}
+
+/**
+ * Model produced by [[LSH]].
+ */
+abstract class LSHModel[KeyType, T <: LSHModel[KeyType, T]] private[ml]
+ extends Model[T] with LSHParams {
+ override def copy(extra: ParamMap): T = defaultCopy(extra)
+ /**
+ * :: DeveloperApi ::
+ *
+ * The hash function of LSH, mapping a predefined KeyType to a Vector
+ * @return The mapping of LSH function.
+ */
+ protected[this] val hashFunction: KeyType => Vector
+
+ /**
+ * :: DeveloperApi ::
+ *
+ * Calculate the distance between two different keys using the distance
metric corresponding
+ * to the hashFunction
+ * @param x One of the point in the metric space
+ * @param y Another the point in the metric space
+ * @return The distance between x and y in double
+ */
+ protected[ml] def keyDistance(x: KeyType, y: KeyType): Double
+
+ /**
+ * :: DeveloperApi ::
+ *
+ * Calculate the distance between two different hash Vectors. By
default, the distance is the
+ * minimum distance of two hash values in any dimension.
+ *
+ * @param x One of the hash vector
+ * @param y Another hash vector
+ * @return The distance between hash vectors x and y in double
+ */
+ protected[ml] def hashDistance(x: Vector, y: Vector): Double = {
+ (x.asBreeze - y.asBreeze).toArray.map(math.abs).min
+ }
+
+ /**
+ * Transforms the input dataset.
+ */
+ override def transform(dataset: Dataset[_]): DataFrame = {
+ transformSchema(dataset.schema, logging = true)
+ val transformUDF = udf(hashFunction, new VectorUDT)
+ dataset.withColumn($(outputCol), transformUDF(dataset($(inputCol))))
+ }
+
+ /**
+ * :: DeveloperApi ::
+ *
+ * Check transform validity and derive the output schema from the input
schema.
+ *
+ * Typical implementation should first conduct verification on schema
change and parameter
+ * validity, including complex parameter interaction checks.
+ */
+ override def transformSchema(schema: StructType): StructType = {
+ transformLSHSchema(schema)
+ }
+
+ /**
+ * Given a large dataset and an item, approximately find at most k items
which have the closest
+ * distance to the item.
+ * @param dataset the dataset to look for the key
+ * @param key The key to hash for the item
+ * @param k The maximum number of items closest to the key
+ * @param distCol The column to store the distance between pairs
+ * @return A dataset containing at most k items closest to the key. A
distCol is added to show
+ * the distance between each record and the key.
+ */
+ def approxNearestNeighbors(dataset: Dataset[_], key: KeyType, k: Int = 1,
+ distCol: String = "distance"): Dataset[_] = {
+ if (k < 1) {
--- End diff --
Usually we use `assert` for this. And more informative error message might
be `The number of nearest neighbors cannot be less than 1`.
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