[GitHub] [arrow] bkietz commented on a change in pull request #10823: ARROW-9948: [C++] Fix scale handling in Decimal{128, 256}::FromString

[GitBox <gi...@apache.org>]

bkietz commented on a change in pull request #10823:
URL: https://github.com/apache/arrow/pull/10823#discussion_r679930351



##########
File path: cpp/src/arrow/util/basic_decimal.h
##########
@@ -54,6 +61,20 @@ class ARROW_EXPORT BasicDecimal128 {
       : high_bits_(high), low_bits_(low) {}
 #endif
 
+  /// \brief Create a BasicDecimal256 from the two's complement representation.
+  /// Input array is assumed to be in native endianness.
+#if ARROW_LITTLE_ENDIAN
+  constexpr BasicDecimal128(const std::array<uint64_t, 2>& array) noexcept
+      : low_bits_(array[0]), high_bits_(static_cast<int64_t>(array[1])) {}
+#else
+  constexpr BasicDecimal128(const std::array<uint64_t, 2>& array) noexcept
+      : high_bits_(static_cast<int64_t>(array[0])), low_bits_(array[1]) {}
+#endif
+
+  // XXX
+  BasicDecimal128(LittleEndianArrayTag, const std::array<uint64_t, 2>& array) noexcept
+      : BasicDecimal128(BitUtil::LittleEndianArray::ToNative(array)) {}

Review comment:
       I'd say it's not idiomatic arrow; I'd expect something like:
   ```suggestion
     static BasicDecimal128 FromLittleEndianArray(const std::array<uint64_t, 2>& array) noexcept {
       return {BitUtil::LittleEndianArray::ToNative(array)};
     }
   ```
   ... but then we'll have to repeat the declaration for non Basic decimal types.




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