[frankmcsherry]

Replaces #1019 and #1020: the proxy half-join tactic now lives in dogs3, next to the operator it drives, and is built only from differential's public int_proxy surface, as any outside implementor would build a tactic over proxies.

The half join had no seam between its time logic and its data: dogs3's HalfJoinTactic reads the arrangement's batches itself, so nothing can wrap it, for example to read an arrangement as of a frontier. ProxyHalfJoinTactic implements HalfJoinTactic over a ProxyHalfJoinBackend, in the shape of differential's proxy join and reduce.

• The tactic owns the comparison of times, strictness, cutting work into blocks of keys, the diff product, and the rule for output times: the update's own time, carrying its data time joined with the arranged time.
• The backend owns staging the released updates, presenting each block of both sides by key, comparing real keys where the key proxy is a hash, and building output containers.

Notes:

• The comparison is Ord, which Timestamp promises is a linear extension of the partial order. Nothing needs TotalOrder, so product times work.
• Arranged times are presented as stored. Advancing them by a frontier can move them in the total order relative to stream times they are incomparable with. For example, with lower = {(1, 0)}, (0, 5) sorts before (1, 0), but advances to (1, 5), which sorts after it. So the backend is given no lower bound to advance by.
• The arranged side has the join's contract, sorted and consolidated by ((key, val), time). The stream side is in key order only.
• Matches are pairs of positions into the block's two presented sides, which cross is handed back, so a backend can evaluate the time rule in its own layout.
• BlobList is public. Every HalfJoinTactic needs a batcher that agrees with it on when an update is released, and BlobList is the one for Vec containers.

What it needed from differential

One change: int_proxy::debug_assert_sorted_bridge is now pub. It checks the bridge contract every proxy backend owes its tactic, so an outside tactic should run the same check rather than a copy that can drift. Everything else was already public:

• ProxyBridge and KeyPosition;
• Semigroup, Multiply, Lattice, consolidate_updates and Hashable;
• ChunkBatch and VecChunk.

Two private helpers weren't worth exposing, so dogs3 has its own: a batch type alias over public types, and an eight-line search.

Tests

tests/proxy_half_join.rs runs the proxy half join over VecHalfJoinBackend in two ways:

• against the cursor half join in one dataflow, on random inputs, strict and not, at one and two workers;
• against the definition over Product<u64, u64> times, with several block sizes and colliding hashes.

The dogs3 tests and cargo test -p differential-dataflow pass, and clippy reports nothing in the files touched.

DDIR's port to this tactic is #1021, which builds on this PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_019XsakwFGwtESwZ1gSYwG4o