Use reduceLeft for union on Interval

src/main/scala/ai/lum/common/Interval.scala

@@ -396,7 +396,7 @@ object Interval {
   def union(col: Seq[Interval]): Interval = {
     val sorted = col.sorted
     try {
-      sorted.reduceRight(_ union _)
+      sorted.reduceLeft(_ union _)
     } catch {
       case _: IllegalArgumentException =>
         throw new IllegalArgumentException("gap in intervals: " + sorted)

src/test/scala/ai/lum/common/TestInterval.scala

@@ -0,0 +1,17 @@
+package ai.lum.common
+
+import org.scalatest._
+
+class TestInterval extends FlatSpec with Matchers {
+
+  behavior of "Interval"
+
+  it should "perform unions in large quantities" in {
+    val count = 1000000
+    val intervals = 1.to(count).map { length => Interval.ofLength(length - 1, length) }.toList
+    val expectedInterval = Interval.open(0, count + count - 1)
+    val actualInterval = Interval.union(intervals)
+
+    actualInterval should be (expectedInterval)
+  }
+}