refactor: RescaleWouldCauseDataLoss

Author: zhjwpku

src/iceberg/test/decimal_test.cc

@@ -717,14 +717,24 @@ TEST(DecimalTest, Rescale) {
 
 TEST(DecimalTest, Compare) {
   // max positive unscaled value
-  ASSERT_EQ(Decimal::Compare(Decimal("170141183460469231731687303715884105727"),
-                             Decimal("170141183460469231731687303715884105727"), 2, 3),
+  // 10^38 - 1 scale cause overflow
+  ASSERT_EQ(Decimal::Compare(Decimal("99999999999999999999999999999999999999"),
+                             Decimal("99999999999999999999999999999999999999"), 2, 3),
             std::partial_ordering::greater);
+  // 10^37 - 1 scale no overflow
+  ASSERT_EQ(Decimal::Compare(Decimal("9999999999999999999999999999999999999"),
+                             Decimal("99999999999999999999999999999999999999"), 2, 3),
+            std::partial_ordering::less);
 
   // min negative unscaled value
-  ASSERT_EQ(Decimal::Compare(Decimal("-170141183460469231731687303715884105728"),
-                             Decimal("-170141183460469231731687303715884105728"), 2, 3),
+  // -10^38 + 1 scale cause overflow
+  ASSERT_EQ(Decimal::Compare(Decimal("-99999999999999999999999999999999999999"),
+                             Decimal("-99999999999999999999999999999999999999"), 2, 3),
             std::partial_ordering::less);
+  // -10^37 + 1 scale no overflow
+  ASSERT_EQ(Decimal::Compare(Decimal("-9999999999999999999999999999999999999"),
+                             Decimal("-99999999999999999999999999999999999999"), 2, 3),
+            std::partial_ordering::greater);
 
   // equal values with different scales
   ASSERT_EQ(Decimal::Compare(Decimal("123456789"), Decimal("1234567890"), 2, 3),

src/iceberg/util/decimal.cc

@@ -47,6 +47,15 @@ namespace {
 static constexpr int32_t kMinDecimalBytes = 1;
 static constexpr int32_t kMaxDecimalBytes = 16;
 
+// The maximum decimal value that can be represented with kMaxPrecision digits.
+// 10^38 - 1
+static constexpr Decimal kMaxDecimalValue(5421010862427522170LL, 687399551400673279ULL);
+// The mininum (most negative) decimal value that can be represented with kMaxPrecision
+// digits.
+// - (10^38 - 1)
+static constexpr Decimal kMinDecimalValue(-5421010862427522171LL,
+                                          17759344522308878337ULL);
+
 struct DecimalComponents {
   std::string_view while_digits;
   std::string_view fractional_digits;
@@ -278,15 +287,15 @@ bool RescaleWouldCauseDataLoss(const Decimal& value, int32_t delta_scale,
     return res->second != 0;
   }
 
-  int128_t max_safe_value = std::numeric_limits<int128_t>::max() / multiplier.value();
-  int128_t min_safe_value = std::numeric_limits<int128_t>::min() / multiplier.value();
-  if (value.value() > max_safe_value || value.value() < min_safe_value) {
+  auto max_safe_value = kMaxDecimalValue / multiplier;
+  auto min_safe_value = kMinDecimalValue / multiplier;
+  if (value > max_safe_value || value < min_safe_value) {
     // Overflow would happen — treat as data loss
     return true;
   }
 
   *result = value * multiplier;
-  return (value < 0) ? *result > value : *result < value;
+  return false;
 }
 
 }  // namespace