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Fix segmented adjacent_difference across partition boundaries - #7882
Segmented adjacent_difference must calculate the first element of each partition from the input element and the preceding element in the overall range. Processing each partition separately instead copies the first value of a partition leading to incorrect results.
Changes
Apply the independent input and output partition traversal approach from equal/mismatch PR #7815 to segmented adjacent_difference.
Splits traversal into bounded chunks
The last input value from a partition is carried onto the next chunk for calculation
Handles empty ranges without dereferencing the end interator object
New tests created for the issue mentioned above
Tests
partitioned_vector_adjacent_difference1_test passed on one locality.
partitioned_vector_adjacent_difference1_test passed across two localities.
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📝 Summary
Summary by CodeRabbit
Bug Fixes
Improved adjacent_difference results for partitioned vectors, including across partition boundaries and when input and output use different partition layouts.
Corrected output-end reporting for segmented processing.
New Features
Added support for in-place processing and empty ranges with sequential, parallel, and task-based execution policies.
Added coverage for custom operations and differing input and output partition layouts.
The sequential and parallel segmented adjacent_difference implementations now process bounded chunks and carry the preceding input value across chunk boundaries. The change adds sequential support for an initial previous value and tests in-place, empty, and differently partitioned ranges.
Changes: Segmented adjacent difference
Layer / File(s)
Summary
Previous-value sequential support libs/core/algorithms/include/hpx/parallel/algorithms/detail/adjacent_difference.hpp, libs/core/algorithms/include/hpx/parallel/algorithms/adjacent_difference.hpp
A sequential overload accepts a previous value, initializes the accumulator from it, and processes the input range.
The sequential implementation processes bounded input and output chunks. It carries the preceding input value between chunks and uses captured dispatch when source and destination partition IDs differ.
Parallel segmented chunk processing and tests libs/full/segmented_algorithms/include/hpx/parallel/segmented_algorithms/adjacent_difference.hpp, libs/full/segmented_algorithms/tests/unit/partitioned_vector_adjacent_difference1.cpp
The parallel implementation dispatches chunks asynchronously, waits for segment futures, and writes saved boundary results. Tests cover in-place, empty, and differently partitioned ranges under sequential and parallel policies.
TheHPXProject/hpx#7815: This PR adds capture_async and get_capture_result, which this change uses to fetch remote input chunks.
Suggested labels:category: algorithms
Merge Risk: Minimal · up to e19b3
The inspected partition-boundary paths preserve the preceding input value, and no merge-blocking issue remains after normal checks.
Pre-merge checks: 5
Check name
Status
Explanation
Title check
The title clearly and concisely describes the primary change: fixing segmented adjacent_difference across partition boundaries.
Description check
The description directly explains the partition-boundary bug, the implementation changes, and the added test coverage.
Linked Issues check
The PR addresses [#7881]. The segmented implementation uses bounded input and output chunks and carries the preceding input value across chunk and partition boundaries. The sequential and parallel pat…
Out of Scope Changes check
The changes remain within [#7881]. The added core overload supports the carried previous value. The segmented algorithm changes implement cross-partition traversal. The tests cover the affected behavi…
Docstring Coverage
Docstring coverage is inconclusive: files from earlier reviews still need to be rechecked within the check's analysis limits. Not blocking.
Full details: Linked Issues check
Explanation
The PR addresses [#7881]. The segmented implementation uses bounded input and output chunks and carries the preceding input value across chunk and partition boundaries. The sequential and parallel paths therefore calculate each later partition's first difference from the preceding overall-range value. The empty-range path avoids dereferencing the end iterator. Tests cover sequential, parallel, task, in-place, empty-range, and differing-layout cases.
Full details: Out of Scope Changes check
Explanation
The changes remain within [#7881]. The added core overload supports the carried previous value. The segmented algorithm changes implement cross-partition traversal. The tests cover the affected behavior and related policy, aliasing, empty-range, and layout cases. No unrelated change is established.
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Actionable comments posted: 2
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at
@libs/full/segmented_algorithms/include/hpx/parallel/segmented_algorithms/adjacent_difference.hpp:
- Around line 172-186: In the sequential and parallel segmented
adjacent-difference loops, skip exhausted input or output partitions before
computing chunk_size: advance the corresponding segment iterator, reset its
local iterator to that partition’s beginning, and continue. Preserve the
existing chunk processing for non-empty partitions.
- Around line 216-236: Update the remote-chunk else branch in the
adjacent-difference flow to execute the sequential computation on destination_id
using its local output iterator, rather than writing each result through the
segmented output_position. Preserve the has_previous behavior and pass the
previous value when required.
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Reviewing files that changed from the base of the PR and between 55ef914 and 04458ad.
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Fixes #7881
Description
Segmented
adjacent_differencemust calculate the first element of each partition from the input element and the preceding element in the overall range. Processing each partition separately instead copies the first value of a partition leading to incorrect results.Changes
Tests