general fixes to Distribution, plus some tests

core/src/main/scala/spark/util/Distribution.scala

@@ -6,8 +6,11 @@ import java.io.PrintStream
  * util for getting some stats from a small sample of numeric values, with some handy summary functions
  *
  * Entirely in memory, not intended as a good way to compute stats over large data sets.
+ *
+ * assumes you are giving it a non-empty set of data
  */
 class Distribution(val data: Array[Double], val startIdx: Int, val endIdx: Int) {
+  require(startIdx < endIdx)
   def this(data: Traversable[Double]) = this(data.toArray, 0, data.size)
   java.util.Arrays.sort(data, startIdx, endIdx)
   val length = endIdx - startIdx
@@ -19,34 +22,43 @@ class Distribution(val data: Array[Double], val startIdx: Int, val endIdx: Int)
    * given from 0 to 1
    * @param probabilities
    */
-  def getQuantiles(probabilities: Traversable[Double]) = {
-    probabilities.map{q =>data((q * length).toInt + startIdx)}
+  def getQuantiles(probabilities: Traversable[Double] = defaultProbabilities) = {
+    probabilities.toIndexedSeq.map{p:Double => data(closestIndex(p))}
   }
 
-  def showQuantiles(out: PrintStream = System.out, probabilities: Traversable[Double] = defaultProbabilities) = {
-    out.println("min\t25%\t50%\t75%max")
-    probabilities.foreach{q => out.print(q + "\t")}
+  private def closestIndex(p: Double) = {
+    math.min((p * length).toInt + startIdx, endIdx - 1)
+  }
+
+  def showQuantiles(out: PrintStream = System.out) = {
+    out.println("min\t25%\t50%\t75%\tmax")
+    getQuantiles(defaultProbabilities).foreach{q => out.print(q + "\t")}
     out.println
   }
 
-  def summary : (StatCounter, Traversable[Double]) = {
-    (StatCounter(data), getQuantiles(defaultProbabilities))
-  }
+  def statCounter = StatCounter(data.slice(startIdx, endIdx))
 
   /**
    * print a summary of this distribution to the given PrintStream.
    * @param out
    */
   def summary(out: PrintStream = System.out) {
-    val (statCounter, quantiles) = summary
     out.println(statCounter)
-    Distribution.showQuantiles(out, quantiles)
+    showQuantiles(out)
   }
 }
 
 object Distribution {
+
+  def apply(data: Traversable[Double]): Option[Distribution] = {
+    if (data.size > 0)
+      Some(new Distribution(data))
+    else
+      None
+  }
+
   def showQuantiles(out: PrintStream = System.out, quantiles: Traversable[Double]) {
-    out.println("min\t25%\t50%\t75%max")
+    out.println("min\t25%\t50%\t75%\tmax")
     quantiles.foreach{q => out.print(q + "\t")}
     out.println
   }

core/src/test/scala/spark/util/DistributionSuite.scala

@@ -0,0 +1,25 @@
+package spark.util
+
+import org.scalatest.FunSuite
+import org.scalatest.matchers.ShouldMatchers
+
+/**
+ *
+ */
+
+class DistributionSuite extends FunSuite with ShouldMatchers {
+  test("summary") {
+    val d = new Distribution((1 to 100).toArray.map{_.toDouble})
+    val stats = d.statCounter
+    stats.count should be (100)
+    stats.mean should be (50.5)
+    stats.sum should be (50 * 101)
+
+    val quantiles = d.getQuantiles()
+    quantiles(0) should be (1)
+    quantiles(1) should be (26)
+    quantiles(2) should be (51)
+    quantiles(3) should be (76)
+    quantiles(4) should be (100)
+  }
+}