Cleanup iteration a bit

src/parquet/arrow/writer.cc

@@ -797,6 +797,7 @@ Status FileWriter::Impl::TypedWriteBatch<FLBAType, ::arrow::DecimalType>(
   const auto& data = static_cast<const Decimal128Array&>(*array);
   const int64_t length = data.length();
 
+  // TODO(phillipc): This is potentially very wasteful if we have a lot of nulls
   std::vector<uint64_t> big_endian_values(static_cast<size_t>(length) * 2);
 
   RETURN_NOT_OK(data_buffer_.Resize(length * sizeof(FLBA), false));
@@ -808,7 +809,13 @@ Status FileWriter::Impl::TypedWriteBatch<FLBAType, ::arrow::DecimalType>(
   const int32_t offset =
       decimal_type.byte_width() - DecimalSize(decimal_type.precision());
 
-  if (writer->descr()->schema_node()->is_required() || data.null_count() == 0) {
+  const bool does_not_have_nulls =
+      writer->descr()->schema_node()->is_required() || data.null_count() == 0;
+
+  // TODO(phillipc): Look into whether our compilers will perform loop unswitching so we
+  // don't have to keep writing two loops to handle the case where we know there are no
+  // nulls
+  if (does_not_have_nulls) {
     // no nulls, just dump the data
     // todo(advancedxy): use a writeBatch to avoid this step
     for (int64_t i = 0, j = 0; i < length; ++i, j += 2) {
@@ -819,17 +826,13 @@ Status FileWriter::Impl::TypedWriteBatch<FLBAType, ::arrow::DecimalType>(
           reinterpret_cast<const uint8_t*>(&big_endian_values[j]) + offset);
     }
   } else {
-    int32_t buffer_idx = 0;
-    int32_t j = 0;
-
-    for (int64_t i = 0; i < length; ++i) {
+    for (int64_t i = 0, buffer_idx = 0, j = 0; i < length; ++i) {
       if (!data.IsNull(i)) {
         auto unsigned_64_bit = reinterpret_cast<const uint64_t*>(data.GetValue(i));
         big_endian_values[j] = ::arrow::BitUtil::ToBigEndian(unsigned_64_bit[1]);
         big_endian_values[j + 1] = ::arrow::BitUtil::ToBigEndian(unsigned_64_bit[0]);
-        buffer_ptr[buffer_idx] = FixedLenByteArray(
+        buffer_ptr[buffer_idx++] = FixedLenByteArray(
             reinterpret_cast<const uint8_t*>(&big_endian_values[j]) + offset);
-        ++buffer_idx;
         j += 2;
       }
     }