f4cfd6c81f
Milestone 8 (phases 1-4): - Shard-aware WAL segment naming, BatchHeader v2, ShardRouter - Transport trait, InProcessTransport, WalShipper, FollowerDb - HLC, PNCounter, LWWRegister, CrdtSignalState, ReconciliationEngine - Session replication bridge with SeqNo/HWM, idempotency store Forage application: - Multi-source discovery engine with MAB exploration - Embedding-based label system, server handlers, UI refresh Other: - QUICKSTART.md, README.md, milestone-8 planning docs - Hard negative union semantics, RLHF export enhancements - Recovery benchmark and visibility test expansions - Split 8 oversized source files per CODING_GUIDELINES §9 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
91 lines
3.2 KiB
Markdown
91 lines
3.2 KiB
Markdown
# Task 02: PNCounter
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## Delivers
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`PNCounter` in `tidal/src/replication/crdt/pn_counter.rs`. Per-node P and N vectors (backed by `HashMap<ShardId, u64>`). Supports `increment`, `decrement`, `merge`, `value`. Property tests verify commutativity, monotonicity, and associativity (CMA) across 100K random operations over 5 nodes.
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## Complexity: M
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## Dependencies
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- Phase 8.1 (ShardId)
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## Technical Design
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```rust
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// tidal/src/replication/crdt/pn_counter.rs
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/// Positive-Negative Counter CRDT.
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///
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/// Each node (ShardId) maintains its own P (increment) and N (decrement)
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/// totals. The global value = sum(P) - sum(N). Merge takes the per-node
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/// max of each component -- safe because values only ever increase within
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/// a node.
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///
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/// Properties:
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/// - Commutative: merge(A, B) == merge(B, A)
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/// - Associative: merge(A, merge(B, C)) == merge(merge(A, B), C)
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/// - Idempotent: merge(A, A) == A
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#[derive(Debug, Clone, Default, PartialEq, serde::Serialize, serde::Deserialize)]
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pub struct PNCounter {
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positive: HashMap<ShardId, u64>,
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negative: HashMap<ShardId, u64>,
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}
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impl PNCounter {
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pub fn new() -> Self {
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Self::default()
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}
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/// Increment by `amount` for this node.
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pub fn increment(&mut self, node: ShardId, amount: u64) {
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*self.positive.entry(node).or_default() += amount;
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}
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/// Decrement by `amount` for this node.
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pub fn decrement(&mut self, node: ShardId, amount: u64) {
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*self.negative.entry(node).or_default() += amount;
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}
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/// Merge another counter into this one.
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///
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/// Takes the per-node maximum of both P and N components.
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/// Safe because each node's contribution only grows.
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pub fn merge(&mut self, other: &PNCounter) {
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for (&node, &val) in &other.positive {
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let entry = self.positive.entry(node).or_default();
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*entry = (*entry).max(val);
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}
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for (&node, &val) in &other.negative {
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let entry = self.negative.entry(node).or_default();
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*entry = (*entry).max(val);
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}
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}
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/// Returns the current value: sum(P) - sum(N).
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///
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/// Saturates at 0 (never negative).
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pub fn value(&self) -> u64 {
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let p: u64 = self.positive.values().sum();
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let n: u64 = self.negative.values().sum();
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p.saturating_sub(n)
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}
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/// Total positive contributions across all nodes.
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pub fn total_positive(&self) -> u64 {
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self.positive.values().sum()
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}
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}
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```
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## Acceptance Criteria
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- [ ] `PNCounter::increment(node, amount)` increases the P component for `node`
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- [ ] `PNCounter::decrement(node, amount)` increases the N component for `node`
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- [ ] `PNCounter::value()` returns `sum(P) - sum(N)`, saturating at 0
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- [ ] `PNCounter::merge` is commutative: `merge(A, B) == merge(B, A)` (property test: 100K random sequences, 5 nodes)
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- [ ] `PNCounter::merge` is associative: `merge(A, merge(B, C)) == merge(merge(A, B), C)` (property test)
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- [ ] `PNCounter::merge` is idempotent: `merge(A, A) == A` (property test)
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- [ ] No double-counting: after merging two counters that each received N independent increments (no overlap), `value() == N * 2` (property test)
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- [ ] `cargo clippy -D warnings` and `cargo fmt` pass
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