Background

#21694 wires GLOAS head selection (getHeadGloas) onto the incremental indexedWeightStore: votes are maintained in a directVotes index per attestation (IndexVote/RemoveVote), and weights are computed lazily by a subtree DFS per GetWeight. This is correct in steady state (differential test vs the full-scan store) and far faster than the previous O(V) full-scan — but it diverges from the canonical fork-choice design and keeps a per-attestation maintenance cost on the hot path.

#21698 removed that per-attestation index maintenance pre-GLOAS (where its result was never read). Under GLOAS it returns, because the index is maintained on every GLOAS attestation and getHeadGloas now reads it. #21694 trims it (allocation-free RemoveVote, no per-vote getCheckpointState), but RemoveVote is still an O(E) scan per attestation under the fork-choice lock.

What Prysm and Lighthouse do (verified on glamsterdam-devnet-5)

Both use the proto-array / doubly-linked-tree model:

• Per attestation: O(1), no weight work — just record the validator's vote (current/next root + payload-present). Prysm ProcessAttestation updates f.votes[index]; Lighthouse process_attestation sets vote.next_root / next_payload_present.
• Per head: delta propagation over a maintained per-node weight tree. Lighthouse apply_score_changes computes per-validator deltas from vote changes and applies them to node.weight, propagated up — O(nodes + changed votes). Prysm applyWeightChangesConsensusNode / ...PayloadNode recompute weight = balance + Σ children bottom-up.
• (root, payload_status) handled per node. Lighthouse: each ProtoNode carries empty_delta / full_delta and attestation_score(payload_status). Prysm: a two-tier tree of consensus Node → EMPTY/FULL PayloadNode.

Net: votes stored per validator (cheap), per-node cumulative weights materialized incrementally; head = follow best-descendant; the debug dump is free.

Proposal

Adopt the delta model for Caplin GLOAS fork choice:

1. Stop maintaining the directVotes index per attestation; keep votes in latestMessages (already O(1)/attestation).
2. Maintain a per-node cumulative weight (with Empty/Full payload-status variants), updated by deltas computed from latestMessages changes at head time and propagated over the filtered tree — O(nodes + changed votes).
3. getHeadGloas follows best-descendant over the maintained weights.

Benefits

• Eliminates the per-attestation maintenance cost entirely (no IndexVote/RemoveVote on the hot path) under GLOAS as well as pre-GLOAS — finishing what cl/phase1/forkchoice: don't maintain GLOAS indexed weight store pre-GLOAS #21698 started.
• Replaces the per-GetWeight subtree DFS with O(nodes) delta propagation.
• Materializes per-node weights → fixes the GLOAS f.weights/ForkNodes gap (/eth/v1/debug/fork_choice is currently empty under GLOAS because getHeadGloas doesn't populate f.weights).
• Matches the canonical design both major clients ship.
• Removes the interim transition seed (seedFromLatestMessages + the seeded flag) added in cl/phase1/forkchoice: use the incremental indexed weight store for the GLOAS head #21694: deriving weights from the continuously-maintained latestMessages leaves no cold index to seed.

Cost / notes

• Larger rework: Caplin currently uses latestMessages + on-demand scoring, not a proto-array-style node tree; this adds a maintained weight tree.
• Not urgent: GLOAS isn't live and the cl/phase1/forkchoice: use the incremental indexed weight store for the GLOAS head #21694 index is a correct interim. Validate on a GLOAS devnet (head agreement vs a reference client + per-attestation cost under high-validator-count load).

References

• Follows up cl/phase1/forkchoice: use the incremental indexed weight store for the GLOAS head #21694; addresses the perf concern raised in cl/phase1/forkchoice: don't maintain GLOAS indexed weight store pre-GLOAS #21698.
• Current Caplin code: cl/phase1/forkchoice/weight_store_indexed.go, get_head.go (getHeadGloas), on_attestation.go (setLatestMessage).
• Prysm glamsterdam-devnet-5: beacon-chain/forkchoice/doubly-linked-tree/{gloas.go,forkchoice.go,node.go}.
• Lighthouse glamsterdam-devnet-5: consensus/proto_array/src/{proto_array.rs,proto_array_fork_choice.rs}.