# Task 03: LWWRegister

## Delivers

`LWWRegister<T>` in `tidal/src/replication/crdt/lww_register.rs`. HLC-timestamped value with `merge` taking the higher timestamp. Tie-breaking by `node_id`. Used for hard negatives (hide/mute/block) which require last-writer-wins semantics across regions.

## Complexity: S

## Dependencies

- Task 01 (HlcTimestamp)

## Technical Design

```rust
// tidal/src/replication/crdt/lww_register.rs

/// Last-Writer-Wins register with HLC timestamp.
///
/// Resolves concurrent writes by `HlcTimestamp` ordering:
/// - Higher `wall_ns` wins
/// - Same wall, higher `logical` wins
/// - Same wall + logical, higher `node_id` wins (deterministic tie-break)
///
/// The value `None` represents "not yet written."
#[derive(Debug, Clone, PartialEq, serde::Serialize, serde::Deserialize)]
pub struct LWWRegister<T: Clone + PartialEq> {
    value: Option<T>,
    timestamp: Option<HlcTimestamp>,
}

impl<T: Clone + PartialEq> LWWRegister<T> {
    pub fn empty() -> Self {
        Self { value: None, timestamp: None }
    }

    /// Write a new value with the given HLC timestamp.
    ///
    /// Only advances the register if `ts > self.timestamp`.
    pub fn write(&mut self, value: T, ts: HlcTimestamp) {
        if self.timestamp.map_or(true, |cur| ts > cur) {
            self.value = Some(value);
            self.timestamp = Some(ts);
        }
    }

    /// Merge another register into this one.
    ///
    /// The register with the higher timestamp wins.
    pub fn merge(&mut self, other: &LWWRegister<T>) {
        if let Some(other_ts) = other.timestamp {
            if self.timestamp.map_or(true, |cur| other_ts > cur) {
                self.value = other.value.clone();
                self.timestamp = other.timestamp;
            }
        }
    }

    /// Current value of the register.
    pub fn get(&self) -> Option<&T> {
        self.value.as_ref()
    }

    /// The HLC timestamp of the last write.
    pub fn timestamp(&self) -> Option<HlcTimestamp> {
        self.timestamp
    }
}

impl<T: Clone + PartialEq> Default for LWWRegister<T> {
    fn default() -> Self {
        Self::empty()
    }
}
```

## Acceptance Criteria

- [ ] `LWWRegister::write(value, ts)` accepts writes with higher timestamps only
- [ ] `LWWRegister::merge` takes the value with the higher HLC timestamp
- [ ] Concurrent writes at the same wall time resolve by `logical` then `node_id`
- [ ] `LWWRegister::merge` is commutative: `merge(A, B) == merge(B, A)` (property test)
- [ ] `LWWRegister::merge` is associative and idempotent (property tests)
- [ ] `T: Clone + PartialEq` bound is sufficient; no `Ord` required
- [ ] Used for `HardNegAction` in Phase 8.4; `T` will be `HardNegAction` enum
- [ ] `cargo clippy -D warnings` and `cargo fmt` pass
