//! M8 UAT: End-to-end acceptance tests for the Distributed Fabric milestone.
//!
//! Validates all five UAT steps:
//! 1. Cross-region signal replication (convergence within timeout)
//! 2. Leader crash and follower promotion (no data loss)
//! 3. Degraded query during partition (query succeeds with partial data)
//! 4. Partition heal and reconciliation (CRDT merge, hard negatives)
//! 5. Tenant migration state machine (zero-downtime transitions)
//!
//! Plus three performance assertions:
//! - Replication latency p99 < 2000ms (in-process)
//! - Failover (promotion) < 10s
//! - Reconciliation of 1000 events < 100ms
#![allow(
    clippy::unwrap_used,
    clippy::items_after_statements,
    clippy::doc_markdown,
    clippy::significant_drop_tightening,
    clippy::suboptimal_flops,
    clippy::cast_precision_loss,
    clippy::needless_pass_by_value,
    clippy::cast_possible_truncation
)]

use std::sync::{Arc, RwLock};
use std::time::{Duration, Instant};

use tidaldb::replication::crdt::{CrdtSignalState, HlcTimestamp, LWWRegister};
use tidaldb::replication::lag::ReplicationLagGauge;
use tidaldb::replication::reconcile::{HardNegAction, ReconciliationEngine, StateSnapshot};
use tidaldb::replication::state::ReplicationState;
use tidaldb::replication::{
    ClusterTopology, ControlPlane, MigrationState, RegionId, ShardId, TenantConfig, TenantId,
    TenantMigration, TenantRouter,
};
use tidaldb::schema::{DecaySpec, EntityId, EntityKind, SchemaBuilder, Timestamp, Window};
use tidaldb::signals::SignalTypeId;
use tidaldb::testing::cluster::{ClusterConfig, SimulatedCluster};
use tidaldb::testing::faults::NetworkPartition;

// ── Shared helpers ───────────────────────────────────────────────────────

fn m8_schema() -> tidaldb::schema::Schema {
    let mut builder = SchemaBuilder::new();
    let _ = builder
        .signal(
            "view",
            EntityKind::Item,
            DecaySpec::Exponential {
                half_life: Duration::from_secs(7 * 24 * 3600),
            },
        )
        .windows(&[Window::OneHour, Window::TwentyFourHours])
        .velocity(false)
        .add();
    let _ = builder
        .signal(
            "like",
            EntityKind::Item,
            DecaySpec::Exponential {
                half_life: Duration::from_secs(24 * 3600),
            },
        )
        .windows(&[Window::OneHour])
        .velocity(false)
        .add();
    builder.build().unwrap()
}

fn three_region_config() -> ClusterConfig {
    ClusterConfig {
        regions: vec![RegionId(0), RegionId(1), RegionId(2)],
        leader_region: RegionId(0),
        schema: m8_schema(),
    }
}

fn two_region_config() -> ClusterConfig {
    ClusterConfig {
        regions: vec![RegionId(0), RegionId(1)],
        leader_region: RegionId(0),
        schema: m8_schema(),
    }
}

// ── UAT Step 1: Cross-region signal replication ──────────────────────────

/// Write 25 signals to the leader (region 0). After convergence,
/// verify that all three regions report the same decay score within
/// floating-point epsilon.
#[test]
fn uat_step1_cross_region_replication() {
    let cluster = SimulatedCluster::build(three_region_config());

    let item = EntityId::new(1);

    // Write 25 signals in region 0 (leader).
    for _ in 0..25 {
        cluster.write_signal("view", item, 1.0).unwrap();
    }

    // Wait for convergence (< 2 seconds on in-process relay).
    cluster.await_convergence(Duration::from_secs(2));

    // Read decay scores from all regions.
    let score_east = cluster
        .read_decay_score(RegionId(0), item, "view")
        .expect("leader must have score");
    let score_west = cluster
        .read_decay_score(RegionId(1), item, "view")
        .expect("follower region 1 must have score");
    let score_south = cluster
        .read_decay_score(RegionId(2), item, "view")
        .expect("follower region 2 must have score");

    // All regions should report the same score within epsilon.
    // The small difference comes from the fact that leader writes and
    // follower replays happen at slightly different wall-clock times,
    // causing slightly different decay factors. With a 7-day half-life
    // and sub-second elapsed time, the difference is negligible.
    let epsilon = 0.5; // generous epsilon for wall-clock timing differences
    assert!(
        (score_east - score_west).abs() < epsilon,
        "region 1 score {score_west} diverges from leader score {score_east} by > {epsilon}"
    );
    assert!(
        (score_east - score_south).abs() < epsilon,
        "region 2 score {score_south} diverges from leader score {score_east} by > {epsilon}"
    );

    // All scores must be significantly greater than zero (25 events accumulated).
    assert!(score_east > 10.0, "leader score {score_east} is too low");
    assert!(
        score_west > 10.0,
        "follower 1 score {score_west} is too low"
    );
    assert!(
        score_south > 10.0,
        "follower 2 score {score_south} is too low"
    );

    // Verify relay log has all 25 events.
    assert_eq!(cluster.relay_log_len(), 25);

    // Verify all followers applied all events.
    assert_eq!(cluster.applied_count(RegionId(1)), 25);
    assert_eq!(cluster.applied_count(RegionId(2)), 25);
}

// ── UAT Step 2: Leader crash and follower promotion ──────────────────────

/// Write 100 signals to the leader. Converge. "Crash" the leader by
/// promoting a follower. Verify the new leader has all signals.
/// Write post-crash signals to the new leader.
#[test]
fn uat_step2_leader_crash_and_failover() {
    let cluster = SimulatedCluster::build(three_region_config());

    let item = EntityId::new(2);

    // Write 100 signals on the leader (region 0).
    for _ in 0..100 {
        cluster.write_signal("view", item, 1.0).unwrap();
    }

    // Wait for all followers to receive the events.
    cluster.await_convergence(Duration::from_secs(2));

    // Record pre-crash score on region 1 (will become new leader).
    let pre_crash_score = cluster
        .read_decay_score(RegionId(1), item, "view")
        .expect("follower must have score before crash");
    assert!(
        pre_crash_score > 50.0,
        "pre-crash score {pre_crash_score} too low; 100 events should yield > 50"
    );

    // "Crash" the leader: promote region 1.
    let start = Instant::now();
    cluster.promote_leader(RegionId(1));
    let failover_time = start.elapsed();

    // Failover must complete instantly (in-process, no election protocol).
    assert!(
        failover_time < Duration::from_secs(10),
        "failover took {:?}, must be < 10s",
        failover_time
    );

    // Verify the new leader is region 1.
    assert_eq!(cluster.leader_region(), RegionId(1));

    // New leader must have all 100 signals.
    let new_leader_score = cluster
        .read_decay_score(RegionId(1), item, "view")
        .expect("new leader must have score");
    assert!(
        new_leader_score > 50.0,
        "new leader score {new_leader_score} too low after promotion"
    );

    // Write 10 more signals to the new leader.
    for _ in 0..10 {
        cluster.write_signal("view", item, 1.0).unwrap();
    }

    // Converge: region 2 should receive the 10 new signals.
    cluster.await_convergence(Duration::from_secs(2));

    let score_r2 = cluster
        .read_decay_score(RegionId(2), item, "view")
        .expect("region 2 must have score");
    // Region 2 should have 100 (original) + 10 (post-crash) events.
    assert!(
        score_r2 > 50.0,
        "region 2 score {score_r2} too low after post-crash writes"
    );
}

// ── UAT Step 3: Degraded query during partition ──────────────────────────

/// Inject a partition isolating region 2. Write more signals.
/// Query the leader -- query must succeed. Region 2 does not receive
/// the new signals.
#[test]
fn uat_step3_degraded_query_during_partition() {
    let cluster = SimulatedCluster::build(three_region_config());

    let item = EntityId::new(3);

    // Seed some data before partition.
    for _ in 0..10 {
        cluster.write_signal("view", item, 1.0).unwrap();
    }
    cluster.await_convergence(Duration::from_secs(1));

    // Inject partition: region 2 is isolated.
    let _partition = NetworkPartition::isolate(RegionId(2), cluster.partitioned_regions());

    // Write more signals during the partition.
    for _ in 0..5 {
        cluster.write_signal("view", item, 1.0).unwrap();
    }

    // Converge: only region 1 should get the new signals.
    cluster.await_convergence(Duration::from_secs(1));

    // Leader query must succeed.
    let leader_score = cluster
        .read_decay_score(RegionId(0), item, "view")
        .expect("leader must have score");
    assert!(
        leader_score > 5.0,
        "leader score {leader_score} must reflect all 15 events"
    );

    // Region 1 (non-partitioned follower) should also have the new events.
    let r1_score = cluster
        .read_decay_score(RegionId(1), item, "view")
        .expect("region 1 must have score");
    assert!(
        r1_score > 5.0,
        "region 1 score {r1_score} must reflect 15 events"
    );

    // Region 2 (partitioned) should still have only the 10 pre-partition events.
    let r2_score = cluster
        .read_decay_score(RegionId(2), item, "view")
        .expect("region 2 must have pre-partition score");
    let r2_applied = cluster.applied_count(RegionId(2));
    assert_eq!(
        r2_applied, 10,
        "region 2 should only have 10 events applied"
    );

    // Region 2 score < leader score (it's missing 5 events).
    assert!(
        r2_score < leader_score,
        "partitioned region 2 ({r2_score}) should lag behind leader ({leader_score})"
    );
}

// ── UAT Step 4: Partition heal and reconciliation ────────────────────────

/// Heal the partition from Step 3. Verify region 2 catches up.
/// Also test CRDT reconciliation of diverged signal states and
/// hard negative propagation.
#[test]
fn uat_step4_partition_heal_and_reconciliation() {
    let cluster = SimulatedCluster::build(three_region_config());

    let item = EntityId::new(4);

    // Phase 1: Write some events, then partition region 2.
    for _ in 0..20 {
        cluster.write_signal("view", item, 1.0).unwrap();
    }
    cluster.await_convergence(Duration::from_secs(1));

    // Partition region 2.
    {
        let _partition = NetworkPartition::isolate(RegionId(2), cluster.partitioned_regions());

        // Write 30 more events to the leader during partition.
        for _ in 0..30 {
            cluster.write_signal("view", item, 1.0).unwrap();
        }

        // Converge region 1 only (region 2 is partitioned).
        cluster.await_convergence(Duration::from_secs(1));

        // Verify region 2 is behind.
        assert_eq!(
            cluster.applied_count(RegionId(2)),
            20,
            "region 2 should only have 20 events"
        );
    }
    // Partition dropped here -- healed automatically.

    // Phase 2: Converge after partition heal.
    cluster.await_convergence(Duration::from_secs(2));

    // Region 2 should now have all 50 events.
    assert_eq!(
        cluster.applied_count(RegionId(2)),
        50,
        "region 2 should have all 50 events after partition heal"
    );

    let score_east = cluster.read_decay_score(RegionId(0), item, "view").unwrap();
    let score_south = cluster.read_decay_score(RegionId(2), item, "view").unwrap();

    // Scores should be close (within wall-clock timing epsilon).
    let epsilon = 1.0;
    assert!(
        (score_east - score_south).abs() < epsilon,
        "post-heal scores diverge: leader={score_east}, region2={score_south}"
    );

    // ── CRDT reconciliation sub-test ─────────────────────────────────

    // Test the ReconciliationEngine with diverged CRDT states.
    let lambda = std::f64::consts::LN_2 / (7.0 * 24.0 * 3600.0);
    let now_ns = Timestamp::now().as_nanos();

    // Node A (shard 0) has 50 events.
    let mut state_a = CrdtSignalState::new(lambda);
    for _ in 0..50 {
        state_a.on_signal(ShardId(0), 1.0, now_ns);
    }

    // Node B (shard 1) has 30 events (accumulated during partition).
    let mut state_b = CrdtSignalState::new(lambda);
    for _ in 0..30 {
        state_b.on_signal(ShardId(1), 1.0, now_ns);
    }

    let mut snap_a = StateSnapshot::new();
    snap_a.add_signal_state(item, SignalTypeId::new(0), state_a.clone());

    let mut snap_b = StateSnapshot::new();
    snap_b.add_signal_state(item, SignalTypeId::new(0), state_b.clone());

    // Use a standalone ledger and hard-neg index for reconciliation.
    let recon_schema = m8_schema();
    let ledger = Arc::new(tidaldb::signals::SignalLedger::new(
        recon_schema,
        Box::new(tidaldb::signals::NoopWalWriter),
    ));
    let hard_negs = Arc::new(tidaldb::entities::HardNegIndex::new());

    let engine = ReconciliationEngine::new(Arc::clone(&ledger), Arc::clone(&hard_negs));
    let plan = engine.plan(&snap_a, &snap_b);

    // Plan should have 1 signal merge (entity 4, signal type 0).
    assert_eq!(
        plan.signal_merges.len(),
        1,
        "plan should have exactly 1 signal merge"
    );

    // Verify merged score = sum of both sides.
    let merged = &plan.signal_merges[0];
    let merged_score = merged.merged_state.decay_score(now_ns);
    let expected = state_a.decay_score(now_ns) + state_b.decay_score(now_ns);
    assert!(
        (merged_score - expected).abs() < 1e-6,
        "merged CRDT score {merged_score} != expected {expected}"
    );

    // Apply the plan.
    engine.apply(&plan).unwrap();

    // ── Hard negative sub-test ───────────────────────────────────────

    let user = EntityId::new(100);

    // Simulate: region 2 hides an item during partition (newer timestamp).
    let ts_hide = HlcTimestamp {
        wall_ns: 200,
        logical: 0,
        node_id: 2,
    };
    let ts_older = HlcTimestamp {
        wall_ns: 100,
        logical: 0,
        node_id: 0,
    };

    let mut snap_local = StateSnapshot::new();
    // Local side: an older unhide.
    let mut reg_local: LWWRegister<HardNegAction> = LWWRegister::empty();
    reg_local.write(HardNegAction::Unhide, ts_older);
    snap_local.add_hardneg_register(user, item, reg_local);

    let mut snap_remote = StateSnapshot::new();
    // Remote side: hide at a later timestamp.
    let mut reg_remote: LWWRegister<HardNegAction> = LWWRegister::empty();
    reg_remote.write(HardNegAction::Hide, ts_hide);
    snap_remote.add_hardneg_register(user, item, reg_remote);

    let plan = engine.plan(&snap_local, &snap_remote);
    assert_eq!(
        plan.hardneg_resolutions.len(),
        1,
        "should have 1 hard-neg resolution"
    );
    assert_eq!(
        plan.hardneg_resolutions[0].action,
        Some(HardNegAction::Hide),
        "LWW should resolve to Hide (newer timestamp)"
    );

    engine.apply(&plan).unwrap();

    // Verify hard negative is applied.
    assert!(
        hard_negs.is_negative(user.as_u64(), item.as_u64() as u32),
        "hard negative must be applied after reconciliation"
    );
}

// ── UAT Step 5: Tenant migration state machine ──────────────────────────

/// Drive the `TenantMigration` state machine through all transitions
/// while simultaneously writing signals. Verify all transitions succeed.
#[test]
fn uat_step5_tenant_migration() {
    let cluster = SimulatedCluster::build(three_region_config());

    let tenant = TenantId(42);
    let item = EntityId::new(5);

    // Register tenant on the leader's router.
    cluster
        .leader()
        .db
        .tenant_router()
        .register_tenant(TenantConfig {
            tenant_id: tenant,
            max_signals_per_sec: None,
            max_entities: None,
            max_storage_bytes: None,
            required_regions: vec![RegionId(0)],
            label: "migrating-tenant".into(),
        });

    // Write 100 signals before migration.
    for _ in 0..100 {
        cluster
            .leader()
            .db
            .signal_for_tenant(tenant, "view", item, 1.0, Timestamp::now())
            .unwrap();
    }

    cluster.await_convergence(Duration::from_secs(1));

    // Build the TenantMigration state machine.
    let topo = Arc::new(RwLock::new(ClusterTopology::single()));
    let router = Arc::new(TenantRouter::new(Arc::clone(&topo)));
    let rep_state = Arc::new(ReplicationState::single());
    let lag = Arc::new(ReplicationLagGauge::new(ShardId::SINGLE, rep_state));
    let cp = Arc::new(ControlPlane::new(topo, Arc::clone(&router), lag));

    let migration = TenantMigration::new(tenant, ShardId(0), ShardId(1), cp, router);

    // Step 1: Idle -> PreparingTarget
    assert_eq!(migration.current_state(), MigrationState::Idle);
    let seqno = migration.prepare_target(42).unwrap();
    assert_eq!(seqno, 42);
    assert!(matches!(
        migration.current_state(),
        MigrationState::PreparingTarget { .. }
    ));

    // Step 2: PreparingTarget -> DualWrite
    let cutover = migration.enter_dual_write(100).unwrap();
    assert_eq!(cutover, 100);
    assert!(matches!(
        migration.current_state(),
        MigrationState::DualWrite { .. }
    ));

    // Write 50 more signals during dual-write.
    for _ in 0..50 {
        cluster
            .leader()
            .db
            .signal_for_tenant(tenant, "view", item, 1.0, Timestamp::now())
            .unwrap();
    }

    // Step 3: DualWrite -> Finalizing
    migration.finalize(150).unwrap();
    assert!(matches!(
        migration.current_state(),
        MigrationState::Finalizing { .. }
    ));

    // Step 4: Finalizing -> Complete (gc_window_ns = 0 for instant GC).
    migration.gc_source(0).unwrap();
    assert_eq!(migration.current_state(), MigrationState::Complete);

    // Verify all 150 signals are present on the leader.
    let score = cluster
        .read_decay_score(RegionId(0), item, "view")
        .expect("leader must have score");
    assert!(
        score > 50.0,
        "leader score {score} must reflect 150 signals"
    );
}

// ── Performance: Replication latency p99 ─────────────────────────────────

/// Measure time from `write_signal()` on leader to `read_decay_score()`
/// on follower returning `Some`. 1000 samples, p99 < 2000ms.
#[test]
fn perf_replication_latency_p99() {
    let cluster = SimulatedCluster::build(two_region_config());

    let mut latencies_us: Vec<u128> = Vec::with_capacity(1000);

    for i in 0u64..1000 {
        let item = EntityId::new(1_000_000 + i);
        let before = Instant::now();

        cluster.write_signal("view", item, 1.0).unwrap();
        cluster.await_convergence(Duration::from_secs(3));

        let score = cluster.read_decay_score(RegionId(1), item, "view");
        assert!(
            score.is_some(),
            "follower must have score for entity {i} after convergence"
        );

        let elapsed = before.elapsed().as_micros();
        latencies_us.push(elapsed);
    }

    latencies_us.sort_unstable();
    let p99_idx = (latencies_us.len() as f64 * 0.99) as usize;
    let p99_us = latencies_us[p99_idx.min(latencies_us.len() - 1)];
    let p99_ms = p99_us / 1_000;

    println!(
        "Replication latency: p50={}us p99={}us ({}ms)",
        latencies_us[latencies_us.len() / 2],
        p99_us,
        p99_ms,
    );

    assert!(
        p99_ms < 2000,
        "replication latency p99 = {p99_ms}ms, must be < 2000ms"
    );
}

// ── Performance: Failover under 10s ──────────────────────────────────────

/// Measure time to promote a follower after leader crash.
#[test]
fn perf_failover_under_10s() {
    let cluster = SimulatedCluster::build(three_region_config());

    // Write some data.
    for _ in 0..50 {
        cluster
            .write_signal("view", EntityId::new(10), 1.0)
            .unwrap();
    }
    cluster.await_convergence(Duration::from_secs(1));

    let start = Instant::now();
    cluster.promote_leader(RegionId(1));
    let elapsed = start.elapsed();

    println!("Failover completed in {:?}", elapsed);
    assert!(
        elapsed < Duration::from_secs(10),
        "failover must complete within 10 seconds, took {:?}",
        elapsed
    );

    // Verify the new leader can accept writes.
    cluster
        .write_signal("view", EntityId::new(10), 1.0)
        .unwrap();
    let score = cluster
        .read_decay_score(RegionId(1), EntityId::new(10), "view")
        .expect("new leader must have score");
    assert!(score > 0.0, "new leader must have positive score");
}

// ── Performance: Reconciliation overhead ─────────────────────────────────

/// Reconcile 1000 events (500 per side). Measure time. Assert < 100ms.
#[test]
fn perf_reconciliation_overhead() {
    let lambda = std::f64::consts::LN_2 / (7.0 * 24.0 * 3600.0);
    let now_ns = Timestamp::now().as_nanos();

    // Build two snapshots: 500 distinct entities per side.
    let mut snap_a = StateSnapshot::new();
    let mut snap_b = StateSnapshot::new();

    for i in 0u64..500 {
        let mut state = CrdtSignalState::new(lambda);
        state.on_signal(ShardId(0), 1.0, now_ns);
        snap_a.add_signal_state(EntityId::new(i), SignalTypeId::new(0), state);
    }
    for i in 500u64..1000 {
        let mut state = CrdtSignalState::new(lambda);
        state.on_signal(ShardId(1), 1.0, now_ns);
        snap_b.add_signal_state(EntityId::new(i), SignalTypeId::new(0), state);
    }

    let schema = m8_schema();
    let ledger = Arc::new(tidaldb::signals::SignalLedger::new(
        schema,
        Box::new(tidaldb::signals::NoopWalWriter),
    ));
    let hard_negs = Arc::new(tidaldb::entities::HardNegIndex::new());
    let engine = ReconciliationEngine::new(Arc::clone(&ledger), Arc::clone(&hard_negs));

    let start = Instant::now();
    let plan = engine.plan(&snap_a, &snap_b);
    engine.apply(&plan).unwrap();
    let elapsed = start.elapsed();

    println!(
        "Reconciliation of {} signal merges took {:?}",
        plan.signal_merges.len(),
        elapsed
    );

    assert_eq!(
        plan.signal_merges.len(),
        1000,
        "plan should have 1000 signal merges (500 + 500 disjoint entities)"
    );
    assert!(
        elapsed < Duration::from_millis(100),
        "reconciliation took {:?}, must be < 100ms",
        elapsed
    );
}