introduce computeValidSum for spark sql reuse

velox/functions/lib/aggregates/SumAggregateBase.h

@@ -185,19 +185,11 @@ class DecimalSumAggregate
 
   virtual int128_t computeFinalValue(
       functions::aggregate::LongDecimalWithOverflowState* accumulator) final {
-    // Value is valid if the conditions below are true.
-    int128_t sum = accumulator->sum;
-    if ((accumulator->overflow == 1 && accumulator->sum < 0) ||
-        (accumulator->overflow == -1 && accumulator->sum > 0)) {
-      sum = static_cast<int128_t>(
-          DecimalUtil::kOverflowMultiplier * accumulator->overflow +
-          accumulator->sum);
-    } else {
-      VELOX_CHECK(accumulator->overflow == 0, "Decimal overflow");
-    }
-
-    DecimalUtil::valueInRange(sum);
-    return sum;
+    auto sum =
+        DecimalUtil::computeValidSum(accumulator->sum, accumulator->overflow);
+    VELOX_CHECK(sum.has_value(), "Decimal overflow");
+    DecimalUtil::valueInRange(sum.value());
+    return sum.value();
   }
 };
 

velox/type/DecimalUtil.h

@@ -304,7 +304,23 @@ class DecimalUtil {
     }
   }
 
-  /// Origins from java side BigInteger#bitLength.
+  inline static std::optional<int128_t> computeValidSum(
+      int128_t sum,
+      int64_t overflow) {
+    // Value is valid if the conditions below are true.
+    int128_t validSum = sum;
+    if ((overflow == 1 && sum < 0) || (overflow == -1 && sum > 0)) {
+      validSum = static_cast<int128_t>(
+          DecimalUtil::kOverflowMultiplier * overflow + sum);
+    } else {
+      if (overflow != 0) {
+        return std::nullopt;
+      }
+    }
+    return validSum;
+  }
+
+  / // Origins from java side BigInteger#bitLength.
   ///
   /// Returns the number of bits in the minimal two's-complement
   /// representation of this BigInteger, <em>excluding</em> a sign bit.