# Task 06: ReplicationState

## Delivers

`ReplicationState` in `tidal/src/replication/state.rs` tracking per-shard high-water-mark seqno with `AtomicU64` for lock-free reads. Serialize/deserialize for checkpoint persistence. Used by followers to track which segments have been applied.

## Complexity: S

## Dependencies

- Task 05 (NodeConfig -- establishes the set of known shards)

## Technical Design

```rust
// tidal/src/replication/state.rs

use crate::replication::shard::ShardId;
use std::collections::HashMap;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::Arc;

/// Tracks the per-shard replication high-water-mark.
///
/// Each shard tracks the last WAL segment seqno that has been fully
/// applied to the local state machine. Segments with seqno <=
/// high_water_mark are idempotent no-ops on replay.
///
/// Thread-safe: all fields are atomic. Clone is O(n_shards) -- clones
/// the snapshot, not the atomics.
#[derive(Debug)]
pub struct ReplicationState {
    /// Per-shard high-water-mark seqno.
    /// `AtomicU64::MAX` means "no segments applied yet" (initial state).
    applied: HashMap<ShardId, Arc<AtomicU64>>,
}

impl ReplicationState {
    /// Create a new `ReplicationState` tracking the given shards.
    ///
    /// All high-water-marks start at 0 (no segments applied).
    pub fn new(shards: &[ShardId]) -> Self {
        let applied = shards
            .iter()
            .map(|&s| (s, Arc::new(AtomicU64::new(0))))
            .collect();
        Self { applied }
    }

    /// Create a single-node `ReplicationState` (tracks only `ShardId(0)`).
    pub fn single() -> Self {
        Self::new(&[ShardId::SINGLE])
    }

    /// Get the high-water-mark seqno for a shard.
    ///
    /// Returns `None` if the shard is unknown to this state.
    pub fn applied_seqno(&self, shard_id: ShardId) -> Option<u64> {
        self.applied.get(&shard_id).map(|a| a.load(Ordering::Acquire))
    }

    /// Update the high-water-mark for a shard.
    ///
    /// Only advances forward -- a seqno smaller than the current
    /// high-water-mark is silently ignored.
    pub fn advance(&self, shard_id: ShardId, seqno: u64) {
        if let Some(atomic) = self.applied.get(&shard_id) {
            let mut current = atomic.load(Ordering::Acquire);
            loop {
                if seqno <= current {
                    break; // already at or past this seqno
                }
                match atomic.compare_exchange_weak(
                    current,
                    seqno,
                    Ordering::AcqRel,
                    Ordering::Acquire,
                ) {
                    Ok(_) => break,
                    Err(actual) => current = actual,
                }
            }
        }
    }

    /// Returns all tracked shards and their current seqnos.
    pub fn snapshot(&self) -> HashMap<ShardId, u64> {
        self.applied
            .iter()
            .map(|(&s, a)| (s, a.load(Ordering::Acquire)))
            .collect()
    }

    /// Serialize for checkpoint persistence.
    pub fn to_checkpoint_bytes(&self) -> Vec<u8> {
        let snap = self.snapshot();
        serde_json::to_vec(&snap).expect("ReplicationState serialization is infallible")
    }

    /// Restore from checkpoint bytes.
    pub fn from_checkpoint_bytes(bytes: &[u8], shards: &[ShardId]) -> Self {
        let snap: HashMap<ShardId, u64> = serde_json::from_slice(bytes)
            .unwrap_or_default();
        let applied = shards
            .iter()
            .map(|&s| {
                let seqno = snap.get(&s).copied().unwrap_or(0);
                (s, Arc::new(AtomicU64::new(seqno)))
            })
            .collect();
        Self { applied }
    }
}
```

## Acceptance Criteria

- [ ] `ReplicationState::single()` tracks only `ShardId(0)`; initial seqno = 0
- [ ] `ReplicationState::advance(shard, seqno)` atomically advances the high-water-mark; never decreases
- [ ] `ReplicationState::applied_seqno(shard)` returns `None` for unknown shards
- [ ] `advance` is safe to call from multiple threads concurrently (CAS loop)
- [ ] `to_checkpoint_bytes` + `from_checkpoint_bytes` roundtrip preserves all shard seqnos
- [ ] `ReplicationState` is `Send + Sync`
- [ ] Unit tests: advance monotonicity, concurrent advance from 4 threads, checkpoint roundtrip
- [ ] `cargo clippy -D warnings` and `cargo fmt` pass