Added ArrayBuilder methods (hail-is#2168)

* Added ArrayBuilder methods

* addressed comments

src/main/scala/is/hail/utils/ArrayBuilder.scala

@@ -17,6 +17,11 @@ class ArrayBuilder[@specialized T](initialCapacity: Int)(implicit tct: ClassTag[
     b(i)
   }
 
+  def update(i: Int, x: T) {
+    require(i >= 0 && i < size)
+    b(i) = x
+  }
+
   def ensureCapacity(n: Int) {
     if (b.length < n) {
       val newCapacity = (b.length * 2).max(n)
@@ -36,6 +41,15 @@ class ArrayBuilder[@specialized T](initialCapacity: Int)(implicit tct: ClassTag[
     size_ += 1
   }
 
+  def ++=(a: Array[T]): Unit = ++=(a, a.length)
+
+  def ++=(a: Array[T], length: Int) {
+    require(length >= 0 && length <= a.length)
+    ensureCapacity(size_ + length)
+    System.arraycopy(a, 0, b, size_, length)
+    size_ += length
+  }
+
   def result(): Array[T] = {
     val r = new Array[T](size_)
     Array.copy(b, 0, r, 0, size_)

src/test/scala/is/hail/utils/ArrayBuilderSuite.scala

@@ -8,7 +8,37 @@ class ArrayBuilderSuite extends SparkSuite {
     val ab = new ArrayBuilder[Int](0)
     ab += 3
     val a = ab.result()
-    assert(a.size == 1)
+    assert(a.length == 1)
     assert(a(0) == 3)
   }
+
+  @Test def addArray() {
+    val ab = new ArrayBuilder[Int](0)
+    ab ++= Array.fill[Int](5)(2)
+    val a = ab.result()
+    assert(a.length == 5)
+    assert(a.forall(_ == 2))
+
+    val ab2 = new ArrayBuilder[Int](0)
+    ab2 ++= (Array.fill[Int](4)(3), 2)
+    val a2 = ab2.result()
+    assert(a2.length == 2)
+    assert(a2.forall(_ == 3))
+
+    ab2(0) = 5
+    val ab2Update = ab2.result()
+    assert(ab2Update sameElements Array(5, 3))
+
+    val ab3 = new ArrayBuilder[Int]
+    ab3 += 1
+    ab3 += 5
+    ab3 ++= Array.fill[Int](2)(3)
+    ab3 += 4
+    ab3 += 6
+
+    assert(ab3.result() sameElements Array(1, 5, 3, 3, 4, 6))
+
+    ab3(2) = 20
+    assert(ab3.result() sameElements Array(1, 5, 20, 3, 4, 6))
+  }
 }