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GH-50601: [C++] Refactor TranslateTo for clearer semantics #50602

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Merged
pitrou merged 1 commit into from
Jul 22, 2026
Merged

GH-50601: [C++] Refactor TranslateTo for clearer semantics #50602

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6 changes: 3 additions & 3 deletions cpp/src/arrow/compute/kernels/vector_sort.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -115,7 +115,7 @@ class ChunkedArraySorter : public TypeVisitor {

std::vector<ChunkedNullLikePartition> chunk_sorted(num_chunks);
for (int i = 0; i < num_chunks; ++i) {
chunk_sorted[i] = sorted[i].TranslateTo(indices_.data(), chunked_indices.data());
chunk_sorted[i] = sorted[i].TranslateTo(indices_, chunked_indices);
}

// merge function for merging ranges where the first sort key is equal
Expand Down Expand Up @@ -155,7 +155,7 @@ class ChunkedArraySorter : public TypeVisitor {

// Reverse everything
sorted.resize(1);
sorted[0] = chunk_sorted[0].TranslateTo(chunked_indices.data(), indices_.data());
sorted[0] = chunk_sorted[0].TranslateTo(chunked_indices, indices_);

RETURN_NOT_OK(chunked_mapper.PhysicalToLogical());
}
Expand Down Expand Up @@ -681,7 +681,7 @@ class TableSorter {

std::vector<ChunkedNullLikePartition> chunk_sorted(num_batches);
for (int64_t i = 0; i < num_batches; ++i) {
chunk_sorted[i] = sorted[i].TranslateTo(indices_.data(), chunked_indices.data());
chunk_sorted[i] = sorted[i].TranslateTo(indices_, chunked_indices);
}

struct Visitor {
Expand Down
33 changes: 22 additions & 11 deletions cpp/src/arrow/compute/kernels/vector_sort_internal.h
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Expand Up @@ -135,19 +135,30 @@ struct GenericNullLikePartition {
return std::max(non_null_like_end(), null_end());
}

// Note that "_begin" is not actually the begin of the stored ranges, but can be much
// smaller. I.e. this function can be and is used when the Partition object is pointing
// into a larger buffer and translate it into another larger buffer at the same offset
// Re-express this partition's ranges as offsets into `target`, at the same
// positions they occupy within `source`. `source` and `target` are the (possibly
// larger) buffers the partition points into: this partition must lie within `source`,
// and the translated partition lies within `target` at the same offsets.
template <typename TargetIndexType>
GenericNullLikePartition<TargetIndexType> TranslateTo(
IndexType* indices_begin, TargetIndexType* target_indices_begin) const {
size_t non_null_offset = non_null_like_range.data() - indices_begin;
size_t nan_offset = nan_range.data() - indices_begin;
size_t null_offset = null_range.data() - indices_begin;
return {.non_null_like_range = {target_indices_begin + non_null_offset,
non_null_like_range.size()},
.nan_range = {target_indices_begin + nan_offset, nan_range.size()},
.null_range = {target_indices_begin + null_offset, null_range.size()}};
std::span<IndexType> source, std::span<TargetIndexType> target) const {
ARROW_DCHECK_EQ(source.size(), target.size());
// This partition must lie within `source`.
ARROW_DCHECK_GE(overall_begin(), source.data());
ARROW_DCHECK_LE(overall_end(), source.data() + source.size());

size_t non_null_offset = non_null_like_range.data() - source.data();
size_t nan_offset = nan_range.data() - source.data();
size_t null_offset = null_range.data() - source.data();
GenericNullLikePartition<TargetIndexType> result{
.non_null_like_range = {target.data() + non_null_offset,
non_null_like_range.size()},
.nan_range = {target.data() + nan_offset, nan_range.size()},
.null_range = {target.data() + null_offset, null_range.size()}};
// The translated partition must lie within `target`.
ARROW_DCHECK_GE(result.overall_begin(), target.data());
ARROW_DCHECK_LE(result.overall_end(), target.data() + target.size());
return result;
}

static GenericNullLikePartition FromCounts(std::span<IndexType> indices,
Expand Down
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