Copilot commented on code in PR #45001:
URL: https://github.com/apache/arrow/pull/45001#discussion_r3644971978
##########
cpp/src/arrow/compute/api_scalar.h:
##########
@@ -1807,5 +1807,42 @@ ARROW_EXPORT Result<Datum> NanosecondsBetween(const
Datum& left, const Datum& ri
/// \note API not yet finalized
ARROW_EXPORT Result<Datum> MapLookup(const Datum& map, MapLookupOptions
options,
ExecContext* ctx = NULLPTR);
+
+/// \brief Construct a hash value for each row of the input.
+///
+/// The result is an Array of length equal to the length of the input;
however, the output
+/// shall be a UInt32Array, with each element being a hash constructed from
each row of
+/// the input. If the input Array is a NestedArray, this means that each
"attribute" or
+/// "field" of the input NestedArray corresponding to the same "row" will
collectively
+/// produce a single uint32_t hash. At the moment, this function does not take
options,
+/// though these may be added in the future.
Review Comment:
The Hash32/Hash64 API docstrings state the result is always an Array and
refer to a "NestedArray" type; however these functions take a Datum and (like
other compute APIs) may return a Scalar/Array/ChunkedArray depending on the
input kind, and for nested *types* (struct/list/map) the semantics are “combine
child hashes per top-level element”. It would also help to mention the NULL
sentinel behavior (OUTPUT_NOT_NULL) and that hash values are not stable across
Arrow versions. Please update the Hash32 docs accordingly (and mirror the same
edits for Hash64).
##########
cpp/src/arrow/compute/kernels/scalar_hash.cc:
##########
@@ -0,0 +1,387 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <algorithm>
+#include <utility>
+
+#include "arrow/array/array_base.h"
+#include "arrow/array/builder_primitive.h"
+#include "arrow/compute/kernels/common_internal.h"
+#include "arrow/compute/key_hash_internal.h"
+#include "arrow/compute/light_array_internal.h"
+#include "arrow/compute/registry_internal.h"
+#include "arrow/compute/util.h"
+#include "arrow/result.h"
+#include "arrow/util/bitmap_ops.h"
+
+namespace arrow {
+namespace compute {
+namespace internal {
+
+// Define symbols visible within `arrow::compute::internal` in this file;
+// these symbols are not visible outside of this file.
+namespace {
+
+// ------------------------------
+// Kernel implementations
+// It is expected that HashArrowType is either UInt32Type or UInt64Type
(default)
+
+// Not dependent on the ArrowType/Hasher template arguments below, so defined
+// as a free function to avoid unnecessary code generation per instantiation.
+// Never called with a nested (list-like or struct) array: HashArray handles
those
+// itself, either via HashChild (which reduces them to a plain UInt32/UInt64
array
+// before it ever reaches here) or the is_list_like branch directly.
+Result<KeyColumnArray> ToColumnArray(const ArraySpan& array) {
+ KeyColumnMetadata metadata;
+ const uint8_t* validity_buffer = nullptr;
+ const uint8_t* fixed_length_buffer = nullptr;
+ const uint8_t* var_length_buffer = nullptr;
+
+ if (array.GetBuffer(0) != nullptr) {
+ validity_buffer = array.GetBuffer(0)->data();
+ }
+ if (array.GetBuffer(1) != nullptr) {
+ fixed_length_buffer = array.GetBuffer(1)->data();
+ }
+
+ auto type = array.type;
+ auto type_id = type->id();
+ if (type_id == Type::NA) {
+ metadata = KeyColumnMetadata(true, 0, true);
+ } else if (type_id == Type::BOOL) {
+ metadata = KeyColumnMetadata(true, 0);
+ } else if (is_fixed_width(type_id)) {
+ metadata = KeyColumnMetadata(true, type->bit_width() / 8);
+ } else if (is_binary_like(type_id)) {
+ metadata = KeyColumnMetadata(false, sizeof(uint32_t));
+ if (array.GetBuffer(2) != nullptr) {
+ var_length_buffer = array.GetBuffer(2)->data();
+ }
+ } else if (is_large_binary_like(type_id)) {
+ metadata = KeyColumnMetadata(false, sizeof(uint64_t));
+ if (array.GetBuffer(2) != nullptr) {
+ var_length_buffer = array.GetBuffer(2)->data();
+ }
+ } else {
+ return Status::TypeError("Unsupported column data type ", type->name(),
+ " used with hash32/hash64 compute kernel");
+ }
+
+ return KeyColumnArray(metadata, array.length, validity_buffer,
fixed_length_buffer,
+ var_length_buffer);
+}
+
+// Zeroes out[i] wherever array is null.
+template <typename c_type>
+void ZeroNulls(const ArraySpan& array, c_type* out) {
+ if (array.GetBuffer(0) == nullptr) {
+ return;
+ }
+ for (int64_t i = 0; i < array.length; i++) {
+ if (array.IsNull(i)) {
+ out[i] = 0;
+ }
+ }
+}
+
+// Folds one row's child hashes into a single hash. Seeded with
CombineHashes(0, 0),
+// not 0, so an empty list doesn't collide with a null list (zeroed separately
below).
+// Free function since it only depends on c_type/Hasher, not ArrowType.
+template <typename c_type, typename Hasher>
+c_type CombineRange(const c_type* value_hashes, int64_t start, int64_t end) {
+ c_type combined = Hasher::CombineHashes(0, 0);
+ for (int64_t j = start; j < end; j++) {
+ combined = Hasher::CombineHashes(combined, value_hashes[j]);
+ }
+ return combined;
+}
+
+// Combines rows for LIST/LARGE_LIST/MAP, whose offsets buffers differ only in
width.
+// `bias` is values.offset + rel_start, so offsets[i] - bias locates row i.
+template <typename c_type, typename Hasher, typename OffsetT>
+void CombineOffsetRows(int64_t length, const OffsetT* offsets, int64_t bias,
+ const c_type* value_hash_data, c_type* out) {
+ for (int64_t i = 0; i < length; i++) {
+ out[i] = CombineRange<c_type, Hasher>(value_hash_data, offsets[i] - bias,
+ offsets[i + 1] - bias);
+ }
+}
+
+template <typename ArrowType, typename Hasher>
+struct FastHashScalar {
+ using c_type = typename ArrowType::c_type;
+
+ // Hashes the [offset, offset + length) slice of `child`.
+ static Result<std::shared_ptr<ArrayData>> HashChild(const ArraySpan& child,
+ int64_t offset, int64_t
length,
+ LightContext* hash_ctx,
+ MemoryPool* memory_pool)
{
+ auto sliced = child;
+ sliced.SetSlice(offset, length);
+ auto arrow_type = TypeTraits<ArrowType>::type_singleton();
+ auto buffer_size = sliced.length * sizeof(c_type);
+ ARROW_ASSIGN_OR_RAISE(auto buffer, AllocateBuffer(buffer_size,
memory_pool));
+ ARROW_RETURN_NOT_OK(
+ HashArray(sliced, hash_ctx, memory_pool,
buffer->mutable_data_as<c_type>()));
+
+ std::shared_ptr<Buffer> validity;
+ if (sliced.GetBuffer(0) != nullptr) {
+ // sliced.GetBuffer(0) is unshifted: reading logical row i requires bit
+ // `sliced.offset + i`. But the returned ArrayData has offset 0, and
callers
+ // build a KeyColumnArray directly from its buffers without applying that
+ // offset themselves (see ToColumnArray, which always treats bit/byte 0
of a
+ // buffer as row 0). Repack into a fresh, 0-based bitmap so it's
self-consistent
+ // with the hash values buffer, which is already 0-based.
+ ARROW_ASSIGN_OR_RAISE(validity, ::arrow::internal::CopyBitmap(
+ memory_pool,
sliced.GetBuffer(0)->data(),
+ sliced.offset, sliced.length));
+ }
+ return ArrayData::Make(arrow_type, sliced.length,
+ {std::move(validity), std::move(buffer)},
sliced.null_count);
+ }
+
+ static Status HashStructArray(const ArraySpan& array, LightContext* hash_ctx,
+ MemoryPool* memory_pool, c_type* out) {
+ if (array.child_data.empty()) {
+ // No fields to combine (e.g. struct<>): HashMultiColumn requires at
least one
+ // column (it reads cols[0] unconditionally), so give every row the same
defined
+ // hash here instead, then let ZeroNulls below zero out the
actually-null rows.
+ c_type empty_struct_hash = Hasher::CombineHashes(0, 0);
+ for (int64_t i = 0; i < array.length; i++) {
+ out[i] = empty_struct_hash;
+ }
+ ZeroNulls(array, out);
+ return Status::OK();
+ }
+ std::vector<std::shared_ptr<ArrayData>>
child_hashes(array.child_data.size());
+ std::vector<KeyColumnArray> columns(array.child_data.size());
+ KeyColumnArray column;
+ for (size_t i = 0; i < array.child_data.size(); i++) {
+ auto child = array.child_data[i];
+ int64_t slice_offset = array.offset;
+ if (is_nested(child.type->id())) {
+ ARROW_ASSIGN_OR_RAISE(
+ child_hashes[i],
+ HashChild(child, child.offset, child.length, hash_ctx,
memory_pool));
+ ARROW_ASSIGN_OR_RAISE(column, ToColumnArray(*child_hashes[i]));
+ // child_hashes[i] starts at offset 0; only the struct's own offset
applies.
+ } else {
+ ARROW_ASSIGN_OR_RAISE(column, ToColumnArray(child));
+ // `child` may have its own offset independent of the struct's (a
struct field
+ // can itself be a slice; see StructArray::GetFlattenedField), so add
both.
+ slice_offset += child.offset;
+ }
+ columns[i] = column.Slice(slice_offset, array.length);
Review Comment:
In HashStructArray, nested child fields are hashed for the entire child
array (`HashChild(child, child.offset, child.length, ...)`) and only then
sliced. Since StructArray::Slice() doesn’t reslice child_data, hashing
`child.length` makes hashing a small struct slice O(size of the original
struct) whenever a field is nested (e.g. struct<list<...>>). Consider hashing
only the struct slice’s referenced range (and then slicing from 0) to keep
sliced structs proportional to the slice size, similar to the list/map paths.
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