[SPARK-7140] [MLLIB] only scan the first 16 entries in Vector.hashCode

The Python SerDe calls `Object.hashCode`, which is very expensive for Vectors. It is not necessary to scan the whole vector, especially for large ones. In this PR, we only scan the first 16 nonzeros. srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #5697 from mengxr/SPARK-7140 and squashes the following commits:

2abc86d [Xiangrui Meng] typo
8fb7d74 [Xiangrui Meng] update impl
1ebad60 [Xiangrui Meng] only scan the first 16 nonzeros in Vector.hashCode

mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala

@@ -52,7 +52,7 @@ sealed trait Vector extends Serializable {
 
   override def equals(other: Any): Boolean = {
     other match {
-      case v2: Vector => {
+      case v2: Vector =>
         if (this.size != v2.size) return false
         (this, v2) match {
           case (s1: SparseVector, s2: SparseVector) =>
@@ -63,21 +63,29 @@ sealed trait Vector extends Serializable {
             Vectors.equals(0 until d1.size, d1.values, s1.indices, s1.values)
           case (_, _) => util.Arrays.equals(this.toArray, v2.toArray)
         }
-      }
       case _ => false
     }
   }
 
+  /**
+   * Returns a hash code value for the vector. The hash code is based on its size and its nonzeros
+   * in the first 16 entries, using a hash algorithm similar to [[java.util.Arrays.hashCode]].
+   */
   override def hashCode(): Int = {
-    var result: Int = size + 31
-    this.foreachActive { case (index, value) =>
-      // ignore explict 0 for comparison between sparse and dense
+    // This is a reference implementation. It calls return in foreachActive, which is slow.
+    // Subclasses should override it with optimized implementation.
+    var result: Int = 31 + size
+    this.foreachActive { (index, value) =>
+      if (index < 16) {
+        // ignore explicit 0 for comparison between sparse and dense
         if (value != 0) {
           result = 31 * result + index
-        // refer to {@link java.util.Arrays.equals} for hash algorithm
           val bits = java.lang.Double.doubleToLongBits(value)
           result = 31 * result + (bits ^ (bits >>> 32)).toInt
         }
+      } else {
+        return result
+      }
     }
     result
   }
@@ -317,7 +325,7 @@ object Vectors {
       case SparseVector(n, ids, vs) => vs
       case v => throw new IllegalArgumentException("Do not support vector type " + v.getClass)
     }
-    val size = values.size
+    val size = values.length
 
     if (p == 1) {
       var sum = 0.0
@@ -371,8 +379,8 @@ object Vectors {
         val v1Indices = v1.indices
         val v2Values = v2.values
         val v2Indices = v2.indices
-        val nnzv1 = v1Indices.size
-        val nnzv2 = v2Indices.size
+        val nnzv1 = v1Indices.length
+        val nnzv2 = v2Indices.length
 
         var kv1 = 0
         var kv2 = 0
@@ -401,7 +409,7 @@ object Vectors {
 
       case (DenseVector(vv1), DenseVector(vv2)) =>
         var kv = 0
-        val sz = vv1.size
+        val sz = vv1.length
         while (kv < sz) {
           val score = vv1(kv) - vv2(kv)
           squaredDistance += score * score
@@ -422,7 +430,7 @@ object Vectors {
     var kv2 = 0
     val indices = v1.indices
     var squaredDistance = 0.0
-    val nnzv1 = indices.size
+    val nnzv1 = indices.length
     val nnzv2 = v2.size
     var iv1 = if (nnzv1 > 0) indices(kv1) else -1
 
@@ -451,8 +459,8 @@ object Vectors {
       v1Values: Array[Double],
       v2Indices: IndexedSeq[Int],
       v2Values: Array[Double]): Boolean = {
-    val v1Size = v1Values.size
-    val v2Size = v2Values.size
+    val v1Size = v1Values.length
+    val v2Size = v2Values.length
     var k1 = 0
     var k2 = 0
     var allEqual = true
@@ -493,7 +501,7 @@ class DenseVector(val values: Array[Double]) extends Vector {
 
   private[spark] override def foreachActive(f: (Int, Double) => Unit) = {
     var i = 0
-    val localValuesSize = values.size
+    val localValuesSize = values.length
     val localValues = values
 
     while (i < localValuesSize) {
@@ -501,6 +509,22 @@ class DenseVector(val values: Array[Double]) extends Vector {
       i += 1
     }
   }
+
+  override def hashCode(): Int = {
+    var result: Int = 31 + size
+    var i = 0
+    val end = math.min(values.length, 16)
+    while (i < end) {
+      val v = values(i)
+      if (v != 0.0) {
+        result = 31 * result + i
+        val bits = java.lang.Double.doubleToLongBits(values(i))
+        result = 31 * result + (bits ^ (bits >>> 32)).toInt
+      }
+      i += 1
+    }
+    result
+  }
 }
 
 object DenseVector {
@@ -522,8 +546,8 @@ class SparseVector(
     val values: Array[Double]) extends Vector {
 
   require(indices.length == values.length, "Sparse vectors require that the dimension of the" +
-    s" indices match the dimension of the values. You provided ${indices.size} indices and " +
-    s" ${values.size} values.")
+    s" indices match the dimension of the values. You provided ${indices.length} indices and " +
+    s" ${values.length} values.")
 
   override def toString: String =
     s"($size,${indices.mkString("[", ",", "]")},${values.mkString("[", ",", "]")})"
@@ -547,7 +571,7 @@ class SparseVector(
 
   private[spark] override def foreachActive(f: (Int, Double) => Unit) = {
     var i = 0
-    val localValuesSize = values.size
+    val localValuesSize = values.length
     val localIndices = indices
     val localValues = values
 
@@ -556,6 +580,28 @@ class SparseVector(
       i += 1
     }
   }
+
+  override def hashCode(): Int = {
+    var result: Int = 31 + size
+    val end = values.length
+    var continue = true
+    var k = 0
+    while ((k < end) & continue) {
+      val i = indices(k)
+      if (i < 16) {
+        val v = values(k)
+        if (v != 0.0) {
+          result = 31 * result + i
+          val bits = java.lang.Double.doubleToLongBits(v)
+          result = 31 * result + (bits ^ (bits >>> 32)).toInt
+        }
+      } else {
+        continue = false
+      }
+      k += 1
+    }
+    result
+  }
 }
 
 object SparseVector {