zihanx opened a new pull request, #16572:
URL: https://github.com/apache/lucene/pull/16572
### Description
Adds a reusable utility for fetching "return fields" — one value per hit for
a set of
global doc IDs — concurrently across leaves, returning results 1:1 in the
caller's input
order. This is the follow-up to the `ReaderUtil.partitionByLeaf`
scatter/gather API
#16496; it builds the retriever the ordinal tracking was designed for.
### Motivation
Fetching field values for a page of hits (in ranking order) is a common
need, and getting
the leaf partitioning + concurrency + input-order reassembly right is tricky
and easy to
get subtly wrong. Today callers hand-roll this. This packages it once.
### What's included
Two classes:
- **`ReturnFieldsRetriever`** — the generic engine. Owns only the reusable
machinery:
scatter doc IDs to leaves via `ReaderUtil.partitionByLeaf`, run each leaf
as an
independent task on a supplied `Executor` (via `TaskExecutor`), and gather
per-leaf
results back into input order. It is agnostic about *what* is produced per
hit: the
caller supplies a `LeafVisitorFactory<T>` that turns a document into a
value of type `T`.
```java
public static <T> T[] retrieve(
IndexReader reader, int[] globalDocIds,
LeafVisitorFactory<T> factory, IntFunction<T[]> arrayFactory,
Executor executor)
```
- **`DoubleValuesRetriever`** — a thin `DoubleValuesSource`-specific
specialization built
on the engine. Adapts each source to a `LeafVisitor<double[]>`:
```java
public static double[][] retrieve(
IndexReader reader, int[] globalDocIds, DoubleValuesSource[]
sources, Executor executor)
```
### Design notes
- **Concurrency contract:** leaves run concurrently, so `newLeafVisitor` is
called once per
leaf and must return a fresh, single-threaded visitor. Each leaf writes
disjoint slots of the result array,
so no synchronization is needed.
- **Input order preserved:** `result[i]` is the value for `globalDocIds[i]`.
The input
array is not mutated.
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