[SPARK-9954] [MLLIB] use first 128 nonzeros to compute Vector.hashCode

This could help reduce hash collisions, e.g., in `RDD[Vector].repartition`. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #8182 from mengxr/SPARK-9954.

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

@@ -71,20 +71,22 @@ sealed trait Vector extends Serializable {
   }
 
   /**
-   * 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]].
+   * Returns a hash code value for the vector. The hash code is based on its size and its first 128
+   * nonzero entries, using a hash algorithm similar to [[java.util.Arrays.hashCode]].
    */
   override def hashCode(): Int = {
     // 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
+    var nnz = 0
     this.foreachActive { (index, value) =>
-      if (index < 16) {
+      if (nnz < Vectors.MAX_HASH_NNZ) {
         // ignore explicit 0 for comparison between sparse and dense
         if (value != 0) {
           result = 31 * result + index
           val bits = java.lang.Double.doubleToLongBits(value)
           result = 31 * result + (bits ^ (bits >>> 32)).toInt
+          nnz += 1
         }
       } else {
         return result
@@ -536,6 +538,9 @@ object Vectors {
     }
     allEqual
   }
+
+  /** Max number of nonzero entries used in computing hash code. */
+  private[linalg] val MAX_HASH_NNZ = 128
 }
 
 /**
@@ -578,13 +583,15 @@ class DenseVector @Since("1.0.0") (
   override def hashCode(): Int = {
     var result: Int = 31 + size
     var i = 0
-    val end = math.min(values.length, 16)
-    while (i < end) {
+    val end = values.length
+    var nnz = 0
+    while (i < end && nnz < Vectors.MAX_HASH_NNZ) {
       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
+        nnz += 1
       }
       i += 1
     }
@@ -707,19 +714,16 @@ class SparseVector @Since("1.0.0") (
   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
+    var nnz = 0
+    while (k < end && nnz < Vectors.MAX_HASH_NNZ) {
+      val v = values(k)
+      if (v != 0.0) {
+        val i = indices(k)
+        result = 31 * result + i
+        val bits = java.lang.Double.doubleToLongBits(v)
+        result = 31 * result + (bits ^ (bits >>> 32)).toInt
+        nnz += 1
       }
       k += 1
     }