[beojan]

Where the first (result) argument to a fold callable is a tuple, this represents a multi-output fold. This matches the behaviour of transforms, though we might want to discuss how ergonomic it is here.

Resolves #947.

• Code

  • Added multi-output support for fold nodes.
  • Derived fold output counts from number_types<result_type> instead of assuming one output.
  • Added tuple support to send, including per-element sendability handling.
  • Updated accumulator release logic to produce and store all configured outputs.
  • Updated fold_api to derive the result type from the algorithm’s first parameter.

• Tests

  • Added a concurrent multi-output fold test.
  • Verified separate sum and count products with expected values of 10 and 5.
  • Verified execution counts for the fold and both downstream observers.

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📝 Walkthrough

Fold result types now determine output counts, and tuple results can be transformed for sending. The accumulator releases every specified product into the product store. A test checks separate sum and count products from a multi-output fold.

Changes: Multi-output fold

Layer / File(s)	Summary
Tuple results and output counts phlex/core/fold/send.hpp, phlex/core/registration_api.hpp, phlex/core/fold_node.hpp	Tuple elements are transformed for sending. Fold registration and nodes derive output counts from result types.
Product release and validation phlex/core/detail/accumulator_node.hpp, test/fold_test.cpp	The accumulator adds all released products to the product store. The test checks sum and count products for two runs and verifies fold and observer execution counts.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant FoldInputs
  participant add_and_count
  participant fold_node
  participant accumulator_message
  participant product_store
  participant observers
  FoldInputs->>add_and_count: provide event numbers
  add_and_count->>fold_node: return accumulated sum and count
  fold_node->>accumulator_message: provide the final result and output specifications
  accumulator_message->>product_store: store all released products
  product_store->>observers: provide sum and count products

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Merge Risk: 🔵 Low · up to 363d9

Folds with move-only tuple outputs may not compile. Other multi-output cases are covered, but this supported case should be fixed or explicitly deferred before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 12.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding support for fold callables with multiple outputs.
Linked Issues check		The PR meets the coding requirements in #947. fold_api and fold_node derive the output count from the fold result type. accumulator::release_as_products() releases all result objects and passes …
Out of Scope Changes check		The changes stay within #947. The tuple support in send.hpp and the accumulator changes are required to represent and release multiple fold outputs. The changes in fold_node.hpp and `registration_…

Full details: Linked Issues check

Explanation

The PR meets the coding requirements in #947. fold_api and fold_node derive the output count from the fold result type. accumulator::release_as_products() releases all result objects and passes them to product_store. Tuple results support sendable elements. upstream_predicates::output_product_suffixes() validates the suffix count with a compile-time assertion. The new Multi-output fold test consumes run_sum and run_count through separate observers and checks their values and execution counts.

Full details: Out of Scope Changes check

Explanation

The changes stay within #947. The tuple support in send.hpp and the accumulator changes are required to represent and release multiple fold outputs. The changes in fold_node.hpp and registration_api.hpp implement output-count deduction. The added test verifies the requested downstream consumption. No unrelated change is demonstrated.

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Actionable comments posted: 1

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Inline comments:
Review comments at @phlex/core/fold/send.hpp:
- Around line 66-72: Extend the tuple send API with a mutable overload of
send(std::tuple<Ts...>&) that forwards elements, using remove_cvref_t and
forward to preserve move-only non-has_send values; retain the existing const
tuple overload for copyable callers and add the required type_traits and utility
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📒 Files selected for processing: 5

• phlex/core/detail/accumulator_node.hpp
• phlex/core/fold/send.hpp
• phlex/core/fold_node.hpp
• phlex/core/registration_api.hpp
• test/fold_test.cpp

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Files:

• phlex/core/fold_node.hpp
• test/fold_test.cpp
• phlex/core/registration_api.hpp
• phlex/core/detail/accumulator_node.hpp
• phlex/core/fold/send.hpp

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phlex/core/fold_node.hpp: 1

71-71: LGTM!

test/fold_test.cpp: 1

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phlex/core/detail/accumulator_node.hpp: 1

40-54: 🗄️ Data Integrity & Integration

No change needed: fold suffix counts are checked during registration.

fold_api passes number_types<result_type> to upstream_predicates. Its output_product_suffixes overload uses a static_assert to require the supplied suffix count to match that value. Empty suffixes are expanded to the required count before to_product_specifications asserts the final sizes. A mismatch cannot reach add_all.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #994      +/-   ##
==========================================
+ Coverage   88.59%   88.61%   +0.02%     
==========================================
  Files         187      187              
  Lines        7769     7776       +7     
  Branches     1012     1012              
==========================================
+ Hits         6883     6891       +8     
  Misses        641      641              
+ Partials      245      244       -1     

Flag	Coverage Δ
scripts	86.35% <ø> (ø)
unittests	89.25% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/core/detail/accumulator_node.hpp	98.09% <100.00%> (+0.03%)	⬆️
phlex/core/fold/send.hpp	100.00% <100.00%> (ø)
phlex/core/fold_node.hpp	100.00% <ø> (ø)
phlex/core/registration_api.hpp	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[knoepfel]

@beojan, this looks quite good. Some comments below for your consideration.

One thing we should consider: I could imagine us making the API a little more ergonomic:

- void add_and_count(std::tuple<std::atomic<unsigned int>, std::atomic<unsigned int>>& result,
+ void add_and_count(std::atomic<unsigned int>& sum, std::atomic<unsigned int>& count,
                     unsigned int number)

Thoughts?

[beojan]

That's what I was thinking. It does mean it's harder to distinguish state / result arguments from input arguments.

[knoepfel]

That's what I was thinking. It does mean it's harder to distinguish state / result arguments from input arguments.

It certainly makes it harder to distinguish for the reader (at a glance). The pattern would be unambiguous, though—accumulators come first in the function signature, followed by data product access, followed by resource access (e.g.):

void fold_op(Result1&, Result2&, DataProduct1 const& d1, phlex::handle<DataProduct2> d2, Resource)

This can be the focus of a different PR, though.

[knoepfel]

@CodeRabbit, make an issue (with labels "enhancement" and "discussion" and type "Feature") that covers the changes describes at #994 (review) and at #994 (comment).

[beojan]

That's what I was thinking. It does mean it's harder to distinguish state / result arguments from input arguments.

It certainly makes it harder to distinguish for the reader (at a glance). The pattern would be unambiguous, though—accumulators come first in the function signature, followed by data product access, followed by resource access (e.g.):

void fold_op(Result1&, Result2&, DataProduct1 const& d1, phlex::handle<DataProduct2> d2, Resource)

This can be the focus of a different PR, though.

Specifically, the first non-(non-const lvalue reference) argument is the first input argument.

I agree, this should be a separate PR, and an option rather than the only way to specify a multi-output fold.