etseidl commented on code in PR #37940:
URL: https://github.com/apache/arrow/pull/37940#discussion_r1341515246
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cpp/src/parquet/encoding.cc:
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@@ -2250,17 +2269,17 @@ void DeltaBitPackEncoder<DType>::FlushBlock() {
std::min(values_per_mini_block_, values_current_block_);
const uint32_t start = i * values_per_mini_block_;
- const UT max_delta = *std::max_element(
+ const T max_delta = *std::max_element(
deltas_.begin() + start, deltas_.begin() + start +
values_current_mini_block);
// The minimum number of bits required to write any of values in deltas_
vector.
// See overflow comment above.
- const auto bit_width = bit_width_data[i] =
- bit_util::NumRequiredBits(max_delta - min_delta);
+ const auto bit_width = bit_width_data[i] = bit_util::NumRequiredBits(
+ static_cast<UT>(SafeSignedSubtractSigned(max_delta, min_delta)));
Review Comment:
Yes, you are correct. Leaving everything unsigned, we have (unsigned)(0 - 1)
= 0xffffffffU. `min_delta == 1`, `max_delta == 0xffffffffU`, so `max_delta -
min_delta == 0xfffffffeU`, which requires the full 32 bits to encode.
The end result is correct, but uses more space than it needs to.
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