Github user sethah commented on a diff in the pull request:
https://github.com/apache/spark/pull/15148#discussion_r86719955
--- Diff: mllib/src/main/scala/org/apache/spark/ml/feature/MinHash.scala ---
@@ -0,0 +1,194 @@
+/*
+ * 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.feature
+
+import scala.util.Random
+
+import org.apache.hadoop.fs.Path
+
+import org.apache.spark.annotation.{Experimental, Since}
+import org.apache.spark.ml.linalg.{Vector, Vectors, VectorUDT}
+import org.apache.spark.ml.param.ParamMap
+import org.apache.spark.ml.param.shared.HasSeed
+import org.apache.spark.ml.util._
+import org.apache.spark.sql.types.StructType
+
+/**
+ * :: Experimental ::
+ *
+ * Model produced by [[MinHash]], where multiple hash functions are
stored. Each hash function is
+ * a perfect hash function:
+ * `h_i(x) = (x * k_i mod prime) mod numEntries`
+ * where `k_i` is the i-th coefficient, and both `x` and `k_i` are from
`Z_prime^*`
+ *
+ * Reference:
+ * [[https://en.wikipedia.org/wiki/Perfect_hash_function Wikipedia on
Perfect Hash Function]]
+ *
+ * @param numEntries The number of entries of the hash functions.
+ * @param randCoefficients An array of random coefficients, each used by
one hash function.
+ */
+@Experimental
+@Since("2.1.0")
+class MinHashModel private[ml] (
+ override val uid: String,
+ @Since("2.1.0") val numEntries: Int,
+ @Since("2.1.0") val randCoefficients: Array[Int])
+ extends LSHModel[MinHashModel] {
+
+ @Since("2.1.0")
+ override protected[ml] val hashFunction: Vector => Vector = {
+ elems: Vector =>
+ require(elems.numNonzeros > 0, "Must have at least 1 non zero
entry.")
+ val elemsList = elems.toSparse.indices.toList
+ val hashValues = randCoefficients.map({ randCoefficient: Int =>
+ elemsList.map({elem: Int =>
+ (1 + elem) * randCoefficient.toLong % MinHash.prime %
numEntries
+ }).min.toDouble
+ })
+ Vectors.dense(hashValues)
+ }
+
+ @Since("2.1.0")
+ override protected[ml] def keyDistance(x: Vector, y: Vector): Double = {
+ val xSet = x.toSparse.indices.toSet
+ val ySet = y.toSparse.indices.toSet
+ val intersectionSize = xSet.intersect(ySet).size.toDouble
+ val unionSize = xSet.size + ySet.size - intersectionSize
+ assert(unionSize > 0, "The union of two input sets must have at least
1 elements")
+ 1 - intersectionSize / unionSize
+ }
+
+ @Since("2.1.0")
+ override protected[ml] def hashDistance(x: Vector, y: Vector): Double = {
+ // Since it's generated by hashing, it will be a pair of dense vectors.
+ x.toDense.values.zip(y.toDense.values).map(pair => math.abs(pair._1 -
pair._2)).min
--- End diff --
Does this even make sense for `MinHash`? For the `RandomProjection` class I
understand that the absolute difference between their hash values is a measure
of their similarity, but for `MinHash` I don't think it is. It is true that
dissimilar items have a lower likelihood of hash collisions, but it should not
be true that they have a low likelihood to hash to buckets near each other. We
use this `hashDistance` to ensure that we get enough near-neighbor candidates,
but I don't see how this `hashDistance` corresponds to similarity in the case
where there are no zero distance elements.
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