SI-7443 Use typeclass instance for {Range,NumericRange}.sum

Previously both Range and NumeriRange used formula for sum of elements
of arithmetic series and thus always assumed that provided Numeric is
regular one.
Bug is now fixed by conservatively checking if Numeric is one of
default ones and the formula still holds.

2 files changed, 48 insertions, 6 deletions

diff --git a/src/library/scala/collection/immutable/NumericRange.scala b/src/library/scala/collection/immutable/NumericRange.scala
index 486c2b6c8f..249d76584d 100644
--- a/src/library/scala/collection/immutable/NumericRange.scala
+++ b/src/library/scala/collection/immutable/NumericRange.scala
@@ -175,9 +175,36 @@ extends AbstractSeq[T] with IndexedSeq[T] with Serializable {
     catch { case _: ClassCastException => false }
 
   final override def sum[B >: T](implicit num: Numeric[B]): B = {
-    if (isEmpty) this.num fromInt 0
-    else if (numRangeElements == 1) head
-    else ((this.num fromInt numRangeElements) * (head + last) / (this.num fromInt 2))
+    // arithmetic series formula  can be used for regular addition
+    if ((num eq scala.math.Numeric.IntIsIntegral)||
+        (num eq scala.math.Numeric.BigIntIsIntegral)||
+        (num eq scala.math.Numeric.ShortIsIntegral)||
+        (num eq scala.math.Numeric.ByteIsIntegral)||
+        (num eq scala.math.Numeric.CharIsIntegral)||
+        (num eq scala.math.Numeric.LongIsIntegral)||
+        (num eq scala.math.Numeric.FloatAsIfIntegral)||
+        (num eq scala.math.Numeric.BigDecimalIsFractional)||
+        (num eq scala.math.Numeric.DoubleAsIfIntegral)) {
+      val numAsIntegral = num.asInstanceOf[Integral[B]]
+      import numAsIntegral._
+      if (isEmpty) num fromInt 0
+      else if (numRangeElements == 1) head
+      else ((num fromInt numRangeElements) * (head + last) / (num fromInt 2))
+    } else {
+      // user provided custom Numeric, we cannot rely on arithmetic series formula
+      if (isEmpty) num.zero
+      else {
+        var acc = num.zero
+        var i = head
+        var idx = 0
+        while(idx < length) {
+          acc = num.plus(acc, i)
+          i = i + step
+          idx = idx + 1
+        }
+        acc
+      }
+    }
   }
 
   override lazy val hashCode = super.hashCode()
diff --git a/src/library/scala/collection/immutable/Range.scala b/src/library/scala/collection/immutable/Range.scala
index 00f398a4b0..786b18cd21 100644
--- a/src/library/scala/collection/immutable/Range.scala
+++ b/src/library/scala/collection/immutable/Range.scala
@@ -259,9 +259,24 @@ extends scala.collection.AbstractSeq[Int]
   final def contains(x: Int) = isWithinBoundaries(x) && ((x - start) % step == 0)
 
   final override def sum[B >: Int](implicit num: Numeric[B]): Int = {
-    if (isEmpty) 0
-    else if (numRangeElements == 1) head
-    else (numRangeElements.toLong * (head + last) / 2).toInt
+    if (num eq scala.math.Numeric.IntIsIntegral) {
+      // this is normal integer range with usual addition. arithmetic series formula can be used
+      if (isEmpty) 0
+      else if (numRangeElements == 1) head
+      else (numRangeElements.toLong * (head + last) / 2).toInt
+    } else {
+      // user provided custom Numeric, we cannot rely on arithmetic series formula
+      if (isEmpty) num.toInt(num.zero)
+      else {
+        var acc = num.zero
+        var i = head
+        while(i != terminalElement) {
+          acc = num.plus(acc, i)
+          i = i + step
+        }
+        num.toInt(acc)
+      }
+    }
   }
 
   override def toIterable = this