reddb_server/storage/unified/

segment.rs

//! Unified Segment System
//!
//! Implements the Growing → Sealed segment lifecycle pattern. Segments are the
//! fundamental unit of storage that handle entities of all types.
//!
//! # Write Path
//!
//! 1. WAL write (durability)
//! 2. Growing-segment entity storage in RAM (HashMap mode or flat-vector mode
//!    with epoch-published lock-free reads)
//! 3. `seal()` freezes the growing segment into a Sealed segment and builds
//!    pruning summaries over the immutable data
//!
//! # Read Path
//!
//! 1. Growing segment entity storage (most recent writes)
//! 2. Sealed segments (older, immutable, zone-map guarded)
//!
//! # Lifecycle
//!
//! ```text
//! Growing (in-memory, accepts writes)
//!    ↓ seal() when full or manually triggered
//! Sealed (immutable, zone maps built)
//!    ↓ flush() for persistence
//! Flushed (on disk, can be mmap'd)
//!    ↓ archive() for cold storage
//! Archived (compressed, infrequently accessed)
//! ```

use std::collections::{BTreeMap, HashMap, HashSet};
use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

use super::entity::{CrossRef, EntityData, EntityId, EntityKind, RefType, UnifiedEntity};
use super::metadata::{Metadata, MetadataStorage};
use super::segment_graph_index::SegmentGraphIndex;
use super::segment_key_lookup::SegmentKeyLookup;
use super::segment_unique_key::SegmentUniqueKey;
use crate::storage::query::value_compare::partial_compare_values;
use reddb_types::{value_to_canonical_key, CanonicalKey, Value};

/// Unique identifier for a segment
pub type SegmentId = u64;

/// Segment state in its lifecycle
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SegmentState {
    /// Accepts writes, partial/no index
    Growing,
    /// Transitioning, building indices
    Sealing,
    /// Immutable, fully indexed
    Sealed,
    /// Persisted to disk
    Flushed,
    /// Compressed, cold storage
    Archived,
}

impl SegmentState {
    /// Check if segment accepts writes
    pub fn is_writable(&self) -> bool {
        matches!(self, Self::Growing)
    }

    /// Check if segment is queryable
    pub fn is_queryable(&self) -> bool {
        !matches!(self, Self::Sealing)
    }

    /// Check if segment is immutable
    pub fn is_immutable(&self) -> bool {
        matches!(self, Self::Sealed | Self::Flushed | Self::Archived)
    }
}

/// Configuration for segments
#[derive(Debug, Clone)]
pub struct SegmentConfig {
    /// Maximum entities before auto-sealing
    pub max_entities: usize,
    /// Maximum memory bytes before auto-sealing
    pub max_bytes: usize,
    /// Maximum age in seconds before auto-sealing
    pub max_age_secs: u64,
    /// Enable vector indexing when sealed
    pub build_vector_index: bool,
    /// Enable graph indexing when sealed
    pub build_graph_index: bool,
    /// Compression level for archived segments (0-9)
    pub compression_level: u8,
}

impl Default for SegmentConfig {
    fn default() -> Self {
        Self {
            max_entities: 100_000,
            max_bytes: 256 * 1024 * 1024, // 256 MB
            max_age_secs: 3600,           // 1 hour
            build_vector_index: true,
            build_graph_index: true,
            compression_level: 6,
        }
    }
}

/// Segment statistics
#[derive(Debug, Clone, Default)]
pub struct SegmentStats {
    /// Number of entities
    pub entity_count: usize,
    /// Number of deleted entities
    pub deleted_count: usize,
    /// Approximate memory usage in bytes
    pub memory_bytes: usize,
    /// Number of vectors
    pub vector_count: usize,
    /// Number of graph nodes
    pub node_count: usize,
    /// Number of graph edges
    pub edge_count: usize,
    /// Number of table rows
    pub row_count: usize,
    /// Number of cross-references
    pub cross_ref_count: usize,
}

/// Segment error types
#[derive(Debug, Clone)]
pub enum SegmentError {
    /// Segment is not writable
    NotWritable,
    /// Entity not found
    NotFound(EntityId),
    /// Entity already exists
    AlreadyExists(EntityId),
    /// Segment is full
    Full,
    /// Invalid operation for current state
    InvalidState(SegmentState),
    /// Internal error
    Internal(String),
}

impl std::fmt::Display for SegmentError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::NotWritable => write!(f, "segment is not writable"),
            Self::NotFound(id) => write!(f, "entity not found: {}", id),
            Self::AlreadyExists(id) => write!(f, "entity already exists: {}", id),
            Self::Full => write!(f, "segment is full"),
            Self::InvalidState(state) => write!(f, "invalid operation for state: {:?}", state),
            Self::Internal(msg) => write!(f, "internal error: {}", msg),
        }
    }
}

impl std::error::Error for SegmentError {}

fn current_unix_secs() -> u64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_default()
        .as_secs()
}

fn entity_id_from_probe_key(key: &[u8]) -> Option<EntityId> {
    if let Ok(text) = std::str::from_utf8(key) {
        if let Ok(raw) = text.parse::<u64>() {
            return Some(EntityId::new(raw));
        }
    }

    let bytes: [u8; 8] = key.try_into().ok()?;
    Some(EntityId::new(u64::from_le_bytes(bytes)))
}

const SEALED_MULTI_ZONE_MAX_INTERVALS: usize = 4;

/// Approximate cost of one entry in the `deleted` tombstone map: the entity
/// id and deletion ordinal plus hashbrown's control byte and load-factor slack.
///
/// Tombstones outlive the entity they bury — a sealed segment holds its
/// `deleted` map for as long as the segment lives. That map is the memory
/// consolidation reclaims even in HashMap mode, where the entity body itself
/// was already freed by `force_delete`.
const TOMBSTONE_ENTRY_BYTES: u64 = 24;

#[derive(Debug, Clone)]
struct UpdateIndexSnapshot {
    pk_column_name: Option<String>,
    pk_value: Option<Value>,
    pk_index_key: Option<(String, String)>,
    cross_refs: Vec<CrossRef>,
}

impl UpdateIndexSnapshot {
    fn from_entity(entity: &UnifiedEntity) -> Self {
        let (pk_column_name, pk_value) = match &entity.data {
            EntityData::Row(row) => (
                row.schema
                    .as_deref()
                    .and_then(|schema| schema.first().cloned()),
                row.columns.first().cloned(),
            ),
            _ => (None, None),
        };
        let pk_index_key = pk_value
            .as_ref()
            .map(|value| (entity.kind.collection().to_string(), format!("{:?}", value)));
        Self {
            pk_column_name,
            pk_value,
            pk_index_key,
            cross_refs: entity.cross_refs().to_vec(),
        }
    }
}

/// A unified segment that stores all entity types
pub trait UnifiedSegment: Send + Sync {
    /// Get segment ID
    fn id(&self) -> SegmentId;

    /// Get current state
    fn state(&self) -> SegmentState;

    /// Get collection/namespace name
    fn collection(&self) -> &str;

    /// Get statistics
    fn stats(&self) -> SegmentStats;

    /// O(1) live entity count (entities minus tombstones)
    fn entity_count(&self) -> usize;

    /// Check if entity exists
    fn contains(&self, id: EntityId) -> bool;

    /// Get an entity by ID
    fn get(&self, id: EntityId) -> Option<&UnifiedEntity>;

    /// Get mutable reference to entity
    fn get_mut(&mut self, id: EntityId) -> Option<&mut UnifiedEntity>;

    /// Insert a new entity
    fn insert(&mut self, entity: UnifiedEntity) -> Result<EntityId, SegmentError>;

    /// Update an existing entity
    fn update(&mut self, entity: UnifiedEntity) -> Result<(), SegmentError>;

    /// HOT-update: like update but receives the set of field names that actually
    /// changed. Allows skipping index work when indexed columns are unaffected.
    /// Default: falls back to full update.
    fn update_hot(
        &mut self,
        entity: UnifiedEntity,
        modified_columns: &[String],
    ) -> Result<(), SegmentError> {
        let _ = modified_columns;
        self.update(entity)
    }

    /// Delete an entity
    fn delete(&mut self, id: EntityId) -> Result<bool, SegmentError>;

    /// Get metadata for an entity
    fn get_metadata(&self, id: EntityId) -> Option<Metadata>;

    /// Set metadata for an entity
    fn set_metadata(&mut self, id: EntityId, metadata: Metadata) -> Result<(), SegmentError>;

    /// Seal the segment (make immutable)
    fn seal(&mut self) -> Result<(), SegmentError>;

    /// Check if should auto-seal based on config
    fn should_seal(&self, config: &SegmentConfig) -> bool;

    /// Iterate over all entities
    fn iter(&self) -> Box<dyn Iterator<Item = &UnifiedEntity> + '_>;

    /// Iterate over entities of a specific kind
    fn iter_kind(&self, kind_filter: &str) -> Box<dyn Iterator<Item = &UnifiedEntity> + '_>;

    /// Search entities by metadata filter
    fn filter_metadata(
        &self,
        filters: &[(String, super::metadata::MetadataFilter)],
    ) -> Vec<EntityId>;
}

// ─────────────────────────────────────────────────────────────────────────────
// Zone map: per-column min/max for segment pruning
// ─────────────────────────────────────────────────────────────────────────────

/// Tracks the min and max observed `Value` for one column in a segment.
/// Used to skip segments that cannot satisfy a range or equality predicate.
#[derive(Debug, Clone)]
pub struct ColZone {
    pub min: Value,
    pub max: Value,
    min_key: Option<CanonicalKey>,
    max_key: Option<CanonicalKey>,
}

impl ColZone {
    fn payload_bytes(&self) -> usize {
        super::memory_size::value_heap_bytes(&self.min)
            .saturating_add(super::memory_size::value_heap_bytes(&self.max))
            .saturating_add(
                self.min_key
                    .as_ref()
                    .map_or(0, super::memory_size::canonical_heap_bytes),
            )
            .saturating_add(
                self.max_key
                    .as_ref()
                    .map_or(0, super::memory_size::canonical_heap_bytes),
            )
    }

    fn new(v: Value) -> Self {
        Self {
            min_key: value_to_canonical_key(&v),
            max_key: value_to_canonical_key(&v),
            min: v.clone(),
            max: v,
        }
    }

    fn with_bounds(min: Value, max: Value) -> Self {
        Self {
            min_key: value_to_canonical_key(&min),
            max_key: value_to_canonical_key(&max),
            min,
            max,
        }
    }

    fn update(&mut self, v: &Value) {
        if compare_zone_values(v, None, &self.min, self.min_key.as_ref())
            .map(|o| o == std::cmp::Ordering::Less)
            .unwrap_or(false)
        {
            self.min = v.clone();
            self.min_key = value_to_canonical_key(v);
        }
        if compare_zone_values(v, None, &self.max, self.max_key.as_ref())
            .map(|o| o == std::cmp::Ordering::Greater)
            .unwrap_or(false)
        {
            self.max = v.clone();
            self.max_key = value_to_canonical_key(v);
        }
    }
}

#[derive(Debug, Clone, Default)]
pub struct MultiColZone {
    pub intervals: Vec<ColZone>,
}

impl MultiColZone {
    fn can_skip(&self, pred: &ZoneColPred<'_>) -> bool {
        !self.intervals.is_empty() && self.intervals.iter().all(|zone| pred.can_skip(zone))
    }
}

fn compare_zone_values(
    left: &Value,
    left_key: Option<&CanonicalKey>,
    right: &Value,
    right_key: Option<&CanonicalKey>,
) -> Option<std::cmp::Ordering> {
    partial_compare_values(left, right).or_else(|| {
        let left_key = left_key.cloned().or_else(|| value_to_canonical_key(left))?;
        let right_key = right_key
            .cloned()
            .or_else(|| value_to_canonical_key(right))?;
        (left_key.family() == right_key.family()).then(|| left_key.cmp(&right_key))
    })
}

/// Tag-only variant of `ZoneColPred` — used where the Value is stored separately.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ZoneColPredKind {
    Eq,
    Gt,
    Gte,
    Lt,
    Lte,
}

/// A predicate on a single column that can be checked against a `ColZone`.
#[derive(Debug, Clone)]
pub enum ZoneColPred<'a> {
    Eq(&'a Value),
    Gt(&'a Value),
    Gte(&'a Value),
    Lt(&'a Value),
    Lte(&'a Value),
}

impl<'a> ZoneColPred<'a> {
    /// Returns `true` when the entire segment can be skipped (no row can match).
    pub fn can_skip(&self, zone: &ColZone) -> bool {
        match self {
            // Equality: skip if val < min OR val > max
            ZoneColPred::Eq(val) => {
                compare_zone_values(val, None, &zone.min, zone.min_key.as_ref())
                    .map(|o| o == std::cmp::Ordering::Less)
                    .unwrap_or(false)
                    || compare_zone_values(val, None, &zone.max, zone.max_key.as_ref())
                        .map(|o| o == std::cmp::Ordering::Greater)
                        .unwrap_or(false)
            }
            // col > val: skip if max <= val (all rows have col ≤ val, none > val)
            ZoneColPred::Gt(val) => {
                compare_zone_values(&zone.max, zone.max_key.as_ref(), val, None)
                    .map(|o| o != std::cmp::Ordering::Greater)
                    .unwrap_or(false)
            }
            // col >= val: skip if max < val
            ZoneColPred::Gte(val) => {
                compare_zone_values(&zone.max, zone.max_key.as_ref(), val, None)
                    .map(|o| o == std::cmp::Ordering::Less)
                    .unwrap_or(false)
            }
            // col < val: skip if min >= val
            ZoneColPred::Lt(val) => {
                compare_zone_values(&zone.min, zone.min_key.as_ref(), val, None)
                    .map(|o| o != std::cmp::Ordering::Less)
                    .unwrap_or(false)
            }
            // col <= val: skip if min > val
            ZoneColPred::Lte(val) => {
                compare_zone_values(&zone.min, zone.min_key.as_ref(), val, None)
                    .map(|o| o == std::cmp::Ordering::Greater)
                    .unwrap_or(false)
            }
        }
    }
}

/// The two physical kinds used by graph materialization passes.
#[derive(Clone, Copy)]
pub(crate) enum GraphEntityKind {
    Node,
    Edge,
}

impl GraphEntityKind {
    fn mask(self) -> u8 {
        match self {
            Self::Node => 1,
            Self::Edge => 2,
        }
    }

    pub(crate) fn matches(self, kind: &EntityKind) -> bool {
        matches!(
            (self, kind),
            (Self::Node, EntityKind::GraphNode(_)) | (Self::Edge, EntityKind::GraphEdge(_))
        )
    }
}

/// Maximum physical positions examined under one segment read guard.
pub(crate) const SCAN_BATCH_SIZE: usize = 256;

/// Bounded ID provenance shared by segment journals and checkpoint projection.
pub(crate) const CHECKPOINT_MUTATIONS_MAX: usize = 8192;

/// Stable append-only positions, bounded when a scan opens the segment.
pub(crate) struct SegmentScanCursor {
    flat_end: usize,
    hash_end: usize,
    deleted_end: usize,
    position: usize,
}

impl SegmentScanCursor {
    pub(crate) fn remaining(&self) -> usize {
        self.flat_end + self.hash_end - self.position
    }

    pub(crate) fn finished(&self) -> bool {
        self.remaining() == 0
    }
}

/// One indexed key at a time, with an exclusive resume ID and a source-wide
/// upper bound captured before any batch. MVCC is checked during live hydration.
pub(crate) struct GraphIndexCursor {
    key_index: usize,
    after: Option<EntityId>,
    upper: EntityId,
}

impl GraphIndexCursor {
    pub(crate) fn finished(&self, keys: &[EntityId]) -> bool {
        self.key_index == keys.len()
    }
}

/// Growing segment implementation (in-memory, writable)
pub struct GrowingSegment {
    /// Segment ID
    id: SegmentId,
    /// Collection/namespace name
    collection: String,
    /// Current state
    state: SegmentState,
    /// Creation timestamp
    created_at: u64,
    /// Last write timestamp
    last_write_at: u64,

    /// Entity storage (HashMap for random access)
    entities: HashMap<EntityId, UnifiedEntity>,
    /// Append-only positions for HashMap-resident IDs. Deleted entries stay
    /// until the segment is reclaimed; flat storage already has stable positions.
    hash_scan_ids: Vec<EntityId>,
    /// Flat entity storage for bulk inserts (no HashMap overhead, O(1) by offset)
    /// Used when entity IDs are sequential from base_entity_id
    flat_entities: Vec<UnifiedEntity>,
    /// Base entity ID for flat_entities (flat_entities[0].id == base_entity_id)
    base_entity_id: u64,
    /// Whether flat storage is active (bulk insert mode)
    use_flat: bool,
    /// Deleted entity IDs (tombstones)
    // Ordinals never change or disappear during the segment lifetime. Cursors
    // skip old tombstones but keep IDs relocated after their capture boundary.
    deleted: HashMap<EntityId, usize>,
    /// Metadata storage (type-aware)
    metadata: MetadataStorage,

    /// Primary key index: (collection, pk_value) → EntityId
    pk_index: BTreeMap<(String, String), EntityId>,
    /// Lazy schema-aware unique-key lookups, independent of the legacy
    /// first-column index. Protected by the owning segment's entity lock.
    unique_key_lookup: parking_lot::RwLock<Vec<SegmentUniqueKey>>,
    unique_key_lookup_bytes: AtomicU64,
    key_lookup: Option<SegmentKeyLookup>,
    /// Type index: kind → EntityIds
    kind_index: HashMap<String, HashSet<EntityId>>,
    /// Conservative physical presence, independent of kind_index and MVCC.
    /// Bits only accumulate; unrestricted mutable access sets both bits.
    graph_kind_mask: u8,
    /// Retains every physical graph version until physical reclamation.
    graph_read_index: parking_lot::RwLock<Option<SegmentGraphIndex>>,
    graph_read_index_bytes: AtomicU64,
    /// Cross-reference index: source → Vec<(target, ref_type)>
    cross_ref_forward: HashMap<EntityId, Vec<(EntityId, RefType)>>,
    /// Reverse cross-reference index: target → Vec<(source, ref_type)>
    cross_ref_reverse: HashMap<EntityId, Vec<(EntityId, RefType)>>,

    /// Per-column zone maps: col_name → (min, max) for segment pruning
    col_zones: HashMap<String, ColZone>,
    zone_payload_bytes: usize,
    /// Sealed-only minmax-multi summaries built from canonical ordering.
    sealed_col_zones: HashMap<String, MultiColZone>,

    /// Sequence counter for ordering
    sequence: AtomicU64,
    /// Volatile checkpoint invalidation; never part of the storage format.
    checkpoint_revision: u64,
    checkpoint_mutations: std::collections::VecDeque<(u64, EntityId)>,
    /// Approximate payload bytes of the entities physically resident in this
    /// segment. Resident means "still occupying memory", which includes
    /// tombstoned entities in flat mode (their slot must stay so positional
    /// lookup by `id - base_entity_id` keeps working).
    memory_bytes: AtomicU64,
    /// Payload bytes of the tombstoned entities that are still resident.
    /// `memory_bytes - dead_entity_bytes` is the live payload.
    dead_entity_bytes: u64,
    /// Count of tombstoned entities that are still resident.
    dead_resident_count: usize,

    /// Epoch counter for lock-free reads of `flat_entities`.
    ///
    /// Updated with `Release` ordering after every flat-mode insert so that
    /// readers can safely access `flat_entities[0..published_flat_len]` by
    /// loading with `Acquire` ordering, without holding the segment RwLock.
    /// Only meaningful when `use_flat == true`; always 0 in HashMap mode.
    pub(crate) published_flat_len: AtomicUsize,
}

impl GrowingSegment {
    pub(crate) fn may_contain_graph_kind(&self, kind: GraphEntityKind) -> bool {
        self.graph_kind_mask & kind.mask() != 0
    }

    fn observe_graph_kind(&mut self, kind: &EntityKind) {
        self.graph_kind_mask |= match kind {
            EntityKind::GraphNode(_) => GraphEntityKind::Node.mask(),
            EntityKind::GraphEdge(_) => GraphEntityKind::Edge.mask(),
            _ => 0,
        };
    }

    pub(crate) fn scan_cursor(&self) -> SegmentScanCursor {
        SegmentScanCursor {
            flat_end: self.flat_entities.len(),
            hash_end: self.hash_scan_ids.len(),
            deleted_end: self.deleted.len(),
            position: 0,
        }
    }

    /// Append-only slots survive insertions, deletions and sealing. The caller
    /// owns the segment read guard and must not re-enter storage in `visit`.
    /// IDs must be hydrated through the manager: retired sources can be stale.
    pub(crate) fn scan_batch(
        &self,
        cursor: &mut SegmentScanCursor,
        before_work: &mut impl FnMut() -> bool,
        visit: &mut impl FnMut(EntityId),
    ) -> bool {
        for _ in 0..SCAN_BATCH_SIZE {
            if cursor.finished() {
                break;
            }
            if !before_work() {
                return false;
            }
            let id = if cursor.position < cursor.flat_end {
                self.flat_entities[cursor.position].id
            } else {
                self.hash_scan_ids[cursor.position - cursor.flat_end]
            };
            cursor.position += 1;
            let eligible = match self.deleted.get(&id) {
                Some(&ordinal) => ordinal >= cursor.deleted_end,
                // Unpublished consolidation eviction leaves an unused position
                // without a tombstone. It must not become a candidate later.
                None => self.get(id).is_some(),
            };
            if eligible {
                visit(id);
            }
        }
        true
    }

    /// Direct iteration without Box<dyn> trait dispatch. Returns false to stop early.
    /// Uses concrete iterator types to avoid heap allocation per call.
    #[inline]
    pub fn for_each_fast<F>(&self, mut f: F) -> bool
    where
        F: FnMut(&UnifiedEntity) -> bool,
    {
        if self.use_flat {
            // Sequential Vec — best cache locality
            if self.deleted.is_empty() {
                for entity in &self.flat_entities {
                    if !f(entity) {
                        return false;
                    }
                }
                // Also walk the HashMap — entities added via per-row
                // `insert()` after flat mode was initialized land there.
                for entity in self.entities.values() {
                    if !f(entity) {
                        return false;
                    }
                }
            } else {
                for entity in &self.flat_entities {
                    if self.deleted.contains_key(&entity.id) {
                        continue;
                    }
                    if !f(entity) {
                        return false;
                    }
                }
                for entity in self.entities.values() {
                    if self.deleted.contains_key(&entity.id) {
                        continue;
                    }
                    if !f(entity) {
                        return false;
                    }
                }
            }
        } else {
            // HashMap values — random order, no boxing
            if self.deleted.is_empty() {
                for entity in self.entities.values() {
                    if !f(entity) {
                        return false;
                    }
                }
            } else {
                for entity in self.entities.values() {
                    if self.deleted.contains_key(&entity.id) {
                        continue;
                    }
                    if !f(entity) {
                        return false;
                    }
                }
            }
        }
        true
    }

    /// Create a new growing segment
    pub fn new(id: SegmentId, collection: impl Into<String>) -> Self {
        let now = current_unix_secs();

        Self {
            id,
            collection: collection.into(),
            state: SegmentState::Growing,
            created_at: now,
            last_write_at: now,
            entities: HashMap::new(),
            hash_scan_ids: Vec::new(),
            flat_entities: Vec::new(),
            base_entity_id: 0,
            use_flat: false,
            deleted: HashMap::new(),
            metadata: MetadataStorage::new(),
            pk_index: BTreeMap::new(),
            unique_key_lookup: parking_lot::RwLock::new(Vec::new()),
            unique_key_lookup_bytes: AtomicU64::new(0),
            key_lookup: Some(SegmentKeyLookup::default()),
            kind_index: HashMap::new(),
            graph_read_index: parking_lot::RwLock::new(Some(SegmentGraphIndex::default())),
            graph_read_index_bytes: AtomicU64::new(0),
            graph_kind_mask: 0,
            cross_ref_forward: HashMap::new(),
            cross_ref_reverse: HashMap::new(),
            col_zones: HashMap::new(),
            zone_payload_bytes: 0,
            sealed_col_zones: HashMap::new(),
            sequence: AtomicU64::new(0),
            checkpoint_revision: 0,
            checkpoint_mutations: std::collections::VecDeque::new(),
            memory_bytes: AtomicU64::new(0),
            dead_entity_bytes: 0,
            dead_resident_count: 0,
            published_flat_len: AtomicUsize::new(0),
        }
    }

    /// Get next sequence number
    fn next_sequence(&self) -> u64 {
        self.sequence.fetch_add(1, Ordering::SeqCst)
    }

    pub(crate) fn checkpoint_revision(&self) -> u64 {
        self.checkpoint_revision
    }

    fn mark_checkpoint_entity_dirty(&mut self, id: EntityId) {
        self.checkpoint_revision = self
            .checkpoint_revision
            .checked_add(1)
            .expect("segment mutation counter must not overflow");
        if self.checkpoint_mutations.len() == CHECKPOINT_MUTATIONS_MAX {
            self.checkpoint_mutations.pop_front();
        }
        self.checkpoint_mutations
            .push_back((self.checkpoint_revision, id));
    }

    pub(crate) fn checkpoint_mutations_since(&self, revision: u64) -> Option<Vec<EntityId>> {
        if revision == self.checkpoint_revision {
            return Some(Vec::new());
        }
        let mut expected = revision.checked_add(1)?;
        let mut ids = Vec::new();
        for &(sequence, id) in &self.checkpoint_mutations {
            if sequence <= revision {
                continue;
            }
            if sequence != expected {
                return None;
            }
            ids.push(id);
            expected = expected.checked_add(1)?;
        }
        (expected == self.checkpoint_revision.checked_add(1)?).then_some(ids)
    }

    pub(crate) fn checkpoint_tracking_bytes(&self) -> usize {
        self.checkpoint_mutations.capacity() * std::mem::size_of::<(u64, EntityId)>()
    }

    fn has_live_entity(&self, id: EntityId) -> bool {
        if self.deleted.contains_key(&id) {
            return false;
        }
        if self.use_flat {
            let raw = id.raw();
            if raw >= self.base_entity_id {
                let idx = (raw - self.base_entity_id) as usize;
                if self
                    .flat_entities
                    .get(idx)
                    .is_some_and(|entity| entity.id == id)
                {
                    return true;
                }
            }
            // Same fallback as `get()` — entities added via `insert()`
            // after flat mode was initialized live only in the HashMap.
            self.entities.contains_key(&id)
        } else {
            self.entities.contains_key(&id)
        }
    }

    fn update_existing_entity_in_place(
        &mut self,
        entity: &UnifiedEntity,
    ) -> Result<UpdateIndexSnapshot, SegmentError> {
        self.mark_checkpoint_entity_dirty(entity.id);
        self.invalidate_unique_key_lookup();
        if self
            .get(entity.id)
            .is_some_and(|old| SegmentKeyLookup::key(old) != SegmentKeyLookup::key(entity))
        {
            if let Some(mut index) = self.key_lookup.take() {
                index.remove(
                    self.get(entity.id)
                        .expect("invariant: existing replacement row"),
                );
                index.insert(entity);
                self.key_lookup = Some(index);
            }
        }
        if (self.graph_kind_mask != 0
            || matches!(
                entity.kind,
                EntityKind::GraphNode(_) | EntityKind::GraphEdge(_)
            ))
            && self
                .get(entity.id)
                .is_some_and(|old| !SegmentGraphIndex::same_keys(old, entity))
        {
            self.invalidate_graph_read_index();
        }
        // Record before replacement, including forced sealed updates. Failed
        // updates may leave a false positive, never a false negative.
        self.observe_graph_kind(&entity.kind);
        if self.use_flat {
            let raw = entity.id.raw();
            if raw >= self.base_entity_id {
                let idx = (raw - self.base_entity_id) as usize;
                if let Some(slot) = self
                    .flat_entities
                    .get_mut(idx)
                    .filter(|slot| slot.id == entity.id)
                {
                    let snapshot = UpdateIndexSnapshot::from_entity(slot);
                    let old_bytes = Self::estimate_entity_size(slot);
                    slot.clone_from(entity);
                    self.replace_entity_memory(old_bytes, Self::estimate_entity_size(entity));
                    return Ok(snapshot);
                }
            }
            let Some(slot) = self.entities.get_mut(&entity.id) else {
                return Err(SegmentError::NotFound(entity.id));
            };
            let snapshot = UpdateIndexSnapshot::from_entity(slot);
            let old_bytes = Self::estimate_entity_size(slot);
            slot.clone_from(entity);
            self.replace_entity_memory(old_bytes, Self::estimate_entity_size(entity));
            Ok(snapshot)
        } else {
            let Some(slot) = self.entities.get_mut(&entity.id) else {
                return Err(SegmentError::NotFound(entity.id));
            };
            let snapshot = UpdateIndexSnapshot::from_entity(slot);
            let old_bytes = Self::estimate_entity_size(slot);
            slot.clone_from(entity);
            self.replace_entity_memory(old_bytes, Self::estimate_entity_size(entity));
            Ok(snapshot)
        }
    }

    fn apply_hot_update_with_metadata(
        &mut self,
        entity: &UnifiedEntity,
        modified_columns: &[String],
        metadata: Option<&Metadata>,
    ) -> Result<(), SegmentError> {
        let old = self.update_existing_entity_in_place(entity)?;
        self.reindex_for_update(&old, entity, Some(modified_columns));
        self.update_col_zones_from_entity(entity);
        if let Some(metadata) = metadata {
            self.metadata.set_all(entity.id, metadata);
        }
        Ok(())
    }

    fn apply_update_with_metadata(
        &mut self,
        entity: &UnifiedEntity,
        metadata: Option<&Metadata>,
    ) -> Result<(), SegmentError> {
        let old = self.update_existing_entity_in_place(entity)?;
        self.reindex_for_update(&old, entity, None);
        self.update_col_zones_from_entity(entity);
        if let Some(metadata) = metadata {
            self.metadata.set_all(entity.id, metadata);
        }
        Ok(())
    }

    pub fn update_hot_batch_with_metadata<'a, I>(&mut self, items: I) -> Result<(), SegmentError>
    where
        I: IntoIterator<Item = (&'a UnifiedEntity, &'a [String], Option<&'a Metadata>)>,
    {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        let items: Vec<(&UnifiedEntity, &[String], Option<&Metadata>)> =
            items.into_iter().collect();
        if items.is_empty() {
            return Ok(());
        }

        for (entity, _, _) in &items {
            if !self.has_live_entity(entity.id) {
                return Err(SegmentError::NotFound(entity.id));
            }
        }

        for (entity, modified_columns, metadata) in items {
            self.apply_hot_update_with_metadata(entity, modified_columns, metadata)?;
        }

        self.last_write_at = current_unix_secs();
        Ok(())
    }

    /// Preserve the first removal order, including repeated flat deletes.
    fn record_tombstone(&mut self, id: EntityId) -> bool {
        self.mark_checkpoint_entity_dirty(id);
        let ordinal = self.deleted.len();
        match self.deleted.entry(id) {
            std::collections::hash_map::Entry::Vacant(entry) => {
                entry.insert(ordinal);
                true
            }
            std::collections::hash_map::Entry::Occupied(_) => false,
        }
    }

    /// Tombstone a flat-mode entity in place. Its slot stays so that positional
    /// lookup by `id - base_entity_id` keeps working, so its payload keeps
    /// costing memory until consolidation merges the segment away.
    ///
    /// Returns `false` when the entity was already tombstoned.
    fn tombstone_flat_entity(&mut self, idx: usize, id: EntityId) -> bool {
        self.invalidate_unique_key_lookup();
        if !self.record_tombstone(id) {
            return false;
        }
        if let Some(index) = self.key_lookup.as_mut() {
            index.remove(&self.flat_entities[idx]);
        }
        if let Some(index) = self.graph_read_index.get_mut() {
            index.remove(&self.flat_entities[idx]);
            self.graph_read_index_bytes
                .store(index.memory_bytes(), Ordering::Relaxed);
        }
        let size = Self::estimate_entity_size(&self.flat_entities[idx]) as u64;
        self.dead_entity_bytes += size;
        self.dead_resident_count += 1;
        self.metadata.remove_all(id);
        true
    }

    /// Remove a HashMap-resident entity. The payload is freed immediately; only
    /// the tombstone survives, and consolidation reclaims that.
    ///
    /// Returns `false` when the entity is not present.
    fn remove_hashmap_entity(&mut self, id: EntityId) -> bool {
        self.invalidate_unique_key_lookup();
        let Some(entity) = self.entities.remove(&id) else {
            return false;
        };
        self.release_memory(Self::estimate_entity_size(&entity));
        self.unindex_entity(&entity);
        self.metadata.remove_all(id);
        self.record_tombstone(id);
        true
    }

    pub fn delete_batch(&mut self, ids: &[EntityId]) -> Result<Vec<EntityId>, SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }
        if ids.is_empty() {
            return Ok(Vec::new());
        }

        let mut deleted_ids = Vec::with_capacity(ids.len());

        if self.use_flat {
            for &id in ids {
                let raw = id.raw();
                if raw < self.base_entity_id {
                    if self.remove_hashmap_entity(id) {
                        deleted_ids.push(id);
                    }
                    continue;
                }
                let idx = (raw - self.base_entity_id) as usize;
                if idx < self.flat_entities.len() && self.flat_entities[idx].id == id {
                    if self.tombstone_flat_entity(idx, id) {
                        deleted_ids.push(id);
                    }
                } else if self.remove_hashmap_entity(id) {
                    deleted_ids.push(id);
                }
            }
        } else {
            for &id in ids {
                if self.remove_hashmap_entity(id) {
                    deleted_ids.push(id);
                }
            }
        }

        if !deleted_ids.is_empty() {
            self.last_write_at = current_unix_secs();
        }

        Ok(deleted_ids)
    }

    /// Update memory estimate
    fn add_memory(&self, bytes: usize) {
        self.memory_bytes.fetch_add(bytes as u64, Ordering::Relaxed);
    }

    fn replace_entity_memory(&self, previous: usize, current: usize) {
        if current >= previous {
            self.add_memory(current - previous);
        } else {
            self.release_memory(previous - current);
        }
    }

    /// This segment's approximate resident bytes. One relaxed load — the
    /// arena's contribution to the shared accounting pool (ADR 0073 §2).
    /// Unlike `stats()` this never walks the entity map, so a stats reader
    /// does not pay `O(entities)` to observe memory.
    pub fn memory_bytes(&self) -> u64 {
        self.memory_bytes.load(Ordering::Relaxed)
            + self.zone_payload_bytes as u64
            + self.unique_key_lookup_bytes.load(Ordering::Relaxed)
            + self
                .key_lookup
                .as_ref()
                .map_or(0, SegmentKeyLookup::memory_bytes)
            + self.graph_read_index_bytes.load(Ordering::Relaxed)
            + (self.hash_scan_ids.capacity() * std::mem::size_of::<EntityId>()) as u64
    }

    /// Release a resident entity's payload from the memory estimate. Only for
    /// paths that physically drop the entity (HashMap removal, eviction).
    fn release_memory(&self, bytes: usize) {
        let mut current = self.memory_bytes.load(Ordering::Relaxed);
        loop {
            let next = current.saturating_sub(bytes as u64);
            match self.memory_bytes.compare_exchange_weak(
                current,
                next,
                Ordering::Relaxed,
                Ordering::Relaxed,
            ) {
                Ok(_) => break,
                Err(observed) => current = observed,
            }
        }
    }

    /// Entities physically present, live or tombstoned-but-resident.
    pub(crate) fn resident_entity_count(&self) -> usize {
        self.flat_entities.len() + self.entities.len()
    }

    /// Entities visible to readers.
    pub(crate) fn live_entity_count(&self) -> usize {
        self.resident_entity_count()
            .saturating_sub(self.dead_resident_count)
    }

    /// Tombstones this segment carries — including the ones whose entity body
    /// was already freed. The set itself is what leaks without consolidation.
    pub(crate) fn tombstone_count(&self) -> usize {
        self.deleted.len()
    }

    /// Approximate bytes this segment holds: resident entity payloads plus the
    /// tombstone set.
    pub(crate) fn resident_bytes(&self) -> u64 {
        self.memory_bytes() + self.deleted.len() as u64 * TOMBSTONE_ENTRY_BYTES
    }

    /// Bytes a consolidation of this segment would return to the budget: the
    /// payloads of resident tombstones plus the tombstone set that buries them.
    pub(crate) fn reclaimable_bytes(&self) -> u64 {
        self.dead_entity_bytes + self.deleted.len() as u64 * TOMBSTONE_ENTRY_BYTES
    }

    /// Snapshot the ids of every live entity, in iteration order.
    ///
    /// Consolidation copies a segment across several maintenance ticks, so it
    /// needs a stable cursor into the segment that survives concurrent
    /// tombstoning. Ids are stable; iteration position is not.
    pub(crate) fn live_entity_ids(&self) -> Vec<EntityId> {
        let mut ids = Vec::with_capacity(self.live_entity_count());
        self.for_each_fast(|entity| {
            ids.push(entity.id);
            true
        });
        ids
    }

    /// Move an existing entity into a writable segment without assigning a
    /// new sequence identity. A pending update's preimage must still match
    /// its durable checkpoint after the MVCC delete stamp is normalized.
    pub(crate) fn insert_preserving_sequence(
        &mut self,
        entity: UnifiedEntity,
    ) -> Result<EntityId, SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }
        if self.entities.contains_key(&entity.id) {
            return Err(SegmentError::AlreadyExists(entity.id));
        }
        let id = entity.id;
        self.adopt_entity(entity, None);
        self.last_write_at = current_unix_secs();
        Ok(id)
    }

    /// Take an entity from another segment, preserving its identity.
    ///
    /// Unlike `insert`, this keeps `sequence_id` — the merged segment is the
    /// same rows, not new ones, and callers (`physical_collection_roots`, MVCC
    /// ordering) read that field. All derived structures are rebuilt: kind
    /// index, primary-key index, cross-reference forward/reverse indexes, and
    /// per-column zone maps.
    pub(crate) fn adopt_entity(&mut self, entity: UnifiedEntity, metadata: Option<Metadata>) {
        let id = entity.id;

        // Keep the sequence counter ahead of every adopted entity, so a later
        // insert into this segment cannot re-issue a live sequence number.
        let next = entity.sequence_id.saturating_add(1);
        if self.sequence.load(Ordering::Relaxed) < next {
            self.sequence.store(next, Ordering::Relaxed);
        }

        self.add_memory(Self::estimate_entity_size(&entity));
        self.index_entity(&entity);
        self.update_col_zones_from_entity(&entity);
        self.entities.insert(id, entity);

        if let Some(metadata) = metadata {
            if !metadata.is_empty() {
                self.metadata.set_all(id, &metadata);
            }
        }
    }

    /// Physically remove an entity without leaving a tombstone.
    ///
    /// Only legal on a segment nobody can see yet — the half-built merged
    /// segment, when the swap discovers a copied row was deleted from its
    /// source mid-consolidation. A tombstone here would defeat the whole point.
    pub(crate) fn evict_entity(&mut self, id: EntityId) -> bool {
        self.mark_checkpoint_entity_dirty(id);
        self.invalidate_unique_key_lookup();
        let Some(entity) = self.entities.remove(&id) else {
            return false;
        };
        self.release_memory(Self::estimate_entity_size(&entity));
        self.unindex_entity(&entity);
        for cross_ref in entity.cross_refs() {
            if let Some(reverse) = self.cross_ref_reverse.get_mut(&cross_ref.target) {
                reverse.retain(|(source, _)| *source != id);
            }
        }
        self.metadata.remove_all(id);
        true
    }

    /// Estimate memory for an entity
    fn estimate_entity_size(entity: &UnifiedEntity) -> usize {
        super::memory_size::entity_bytes(entity)
    }

    /// Update per-column zone maps from a newly inserted entity's fields.
    ///
    /// Handles both insert paths:
    /// - **Named** (`row.named`): individual inserts where fields are a `HashMap<String, Value>`
    /// - **Positional** (`row.columns` + `row.schema`): bulk-inserted entities stored as `Vec<Value>`
    ///   keyed by the shared schema. Previously this path was silently skipped, meaning zone maps
    ///   were always empty for bulk-loaded tables and segment pruning never fired.
    fn update_column_zone(&mut self, column: &str, value: &Value) {
        if matches!(value, Value::Null) {
            return;
        }
        match self.col_zones.entry(column.to_string()) {
            std::collections::hash_map::Entry::Occupied(mut entry) => {
                let before = entry.get().payload_bytes();
                entry.get_mut().update(value);
                self.zone_payload_bytes = self
                    .zone_payload_bytes
                    .saturating_sub(before)
                    .saturating_add(entry.get().payload_bytes());
            }
            std::collections::hash_map::Entry::Vacant(entry) => {
                let zone = ColZone::new(value.clone());
                self.zone_payload_bytes =
                    self.zone_payload_bytes.saturating_add(zone.payload_bytes());
                entry.insert(zone);
            }
        }
    }

    fn update_col_zones_from_entity(&mut self, entity: &UnifiedEntity) {
        if let EntityData::Row(row) = &entity.data {
            if let Some(named) = &row.named {
                for (column, value) in named {
                    self.update_column_zone(column, value);
                }
            } else if let Some(schema) = &row.schema {
                for (column, value) in schema.iter().zip(&row.columns) {
                    self.update_column_zone(column, value);
                }
            }
        }
    }

    fn rebuild_sealed_col_zones(&mut self) {
        let mut values_by_col: HashMap<String, Vec<(CanonicalKey, Value)>> = HashMap::new();
        let mut family_by_col: HashMap<String, reddb_types::CanonicalKeyFamily> = HashMap::new();
        let mut mixed_family_cols = HashSet::new();
        let mut unsupported_cols = HashSet::new();

        let mut observe_row = |row: &super::entity::RowData| {
            for (col, value) in row.iter_fields() {
                if matches!(value, Value::Null) {
                    continue;
                }
                let Some(key) = value_to_canonical_key(value) else {
                    unsupported_cols.insert(col.to_string());
                    continue;
                };
                match family_by_col.get(col).copied() {
                    Some(existing) if existing != key.family() => {
                        mixed_family_cols.insert(col.to_string());
                    }
                    None => {
                        family_by_col.insert(col.to_string(), key.family());
                    }
                    _ => {}
                }
                values_by_col
                    .entry(col.to_string())
                    .or_default()
                    .push((key, value.clone()));
            }
        };

        if self.use_flat {
            for entity in &self.flat_entities {
                if self.deleted.contains_key(&entity.id) {
                    continue;
                }
                if let EntityData::Row(row) = &entity.data {
                    observe_row(row);
                }
            }
        } else {
            for entity in self.entities.values() {
                if self.deleted.contains_key(&entity.id) {
                    continue;
                }
                if let EntityData::Row(row) = &entity.data {
                    observe_row(row);
                }
            }
        }

        let mut sealed_col_zones = HashMap::new();
        for (col, mut entries) in values_by_col {
            if mixed_family_cols.contains(&col)
                || unsupported_cols.contains(&col)
                || entries.is_empty()
            {
                continue;
            }
            entries.sort_unstable_by(|left, right| left.0.cmp(&right.0));
            entries.dedup_by(|left, right| left.0 == right.0);

            let intervals = build_minmax_multi_intervals(&entries, SEALED_MULTI_ZONE_MAX_INTERVALS);
            if intervals.len() > 1 {
                sealed_col_zones.insert(col, MultiColZone { intervals });
            }
        }

        let previous = self
            .sealed_col_zones
            .values()
            .flat_map(|zones| &zones.intervals)
            .map(ColZone::payload_bytes)
            .fold(0, usize::saturating_add);
        let current = sealed_col_zones
            .values()
            .flat_map(|zones| &zones.intervals)
            .map(ColZone::payload_bytes)
            .fold(0, usize::saturating_add);
        self.zone_payload_bytes = self
            .zone_payload_bytes
            .saturating_sub(previous)
            .saturating_add(current);
        self.sealed_col_zones = sealed_col_zones;
    }

    /// Returns `true` when this segment can be entirely skipped for the given predicates.
    /// A segment is skipped only if ALL predicates say so (conservative: any non-skippable
    /// predicate forces the scan to proceed).
    pub fn can_skip_zone_preds(&self, preds: &[(&str, ZoneColPred<'_>)]) -> bool {
        if preds.is_empty() {
            return false;
        }
        for (col, pred) in preds {
            if let Some(zone) = self.sealed_col_zones.get(*col) {
                if zone.can_skip(pred) {
                    return true;
                }
                continue;
            }
            if let Some(zone) = self.col_zones.get(*col) {
                if pred.can_skip(zone) {
                    return true; // ONE predicate suffices to skip
                }
            }
        }
        false
    }

    /// Index an entity
    fn index_entity(&mut self, entity: &UnifiedEntity) {
        self.mark_checkpoint_entity_dirty(entity.id);
        if let Some(index) = self.key_lookup.as_mut() {
            index.insert(entity);
        }
        self.hash_scan_ids.push(entity.id);
        self.observe_graph_kind(&entity.kind);
        if let Some(index) = self.graph_read_index.get_mut() {
            index.insert(entity);
            self.graph_read_index_bytes
                .store(index.memory_bytes(), Ordering::Relaxed);
        }
        let indexes = self.unique_key_lookup.get_mut();
        for index in indexes.iter_mut() {
            index.insert(entity);
        }
        self.unique_key_lookup_bytes.store(
            indexes
                .iter()
                .map(|index| index.memory_bytes() as u64)
                .sum(),
            Ordering::Relaxed,
        );
        // Kind index
        let kind_key = entity.kind.storage_type().to_string();
        self.kind_index
            .entry(kind_key)
            .or_default()
            .insert(entity.id);

        // Primary key index (if applicable)
        if let EntityData::Row(row) = &entity.data {
            if let Some(first_col) = row.columns.first() {
                let pk_str = format!("{:?}", first_col);
                self.pk_index
                    .insert((entity.kind.collection().to_string(), pk_str), entity.id);
            }
        }

        // Cross-reference indices
        for cross_ref in entity.cross_refs() {
            self.cross_ref_forward
                .entry(cross_ref.source)
                .or_default()
                .push((cross_ref.target, cross_ref.ref_type));

            self.cross_ref_reverse
                .entry(cross_ref.target)
                .or_default()
                .push((cross_ref.source, cross_ref.ref_type));
        }
    }

    fn invalidate_graph_read_index(&mut self) {
        *self.graph_read_index.get_mut() = None;
        self.graph_read_index_bytes.store(0, Ordering::Relaxed);
    }

    /// The owning segment read lock keeps candidates stable. Callers must not
    /// re-enter storage in `visit`; collect IDs and evaluate RLS after unlocking.
    #[cfg(test)]
    pub(crate) fn visit_graph_index_candidates(
        &self,
        kind: GraphEntityKind,
        keys: &[EntityId],
        before_work: &mut impl FnMut() -> bool,
        visit: &mut impl FnMut(&UnifiedEntity) -> bool,
    ) -> bool {
        let Some(cached) = self.graph_read_index_for_query(before_work) else {
            return false;
        };
        let index = cached.as_ref().expect("complete graph index");
        for key in keys {
            if !before_work() {
                return false;
            }
            for id in index.candidates(kind, *key) {
                if !before_work() {
                    return false;
                }
                if let Some(entity) = self.get(id) {
                    if !visit(entity) {
                        return false;
                    }
                }
            }
        }
        true
    }

    fn graph_read_index_for_query(
        &self,
        before_work: &mut impl FnMut() -> bool,
    ) -> Option<parking_lot::RwLockReadGuard<'_, Option<SegmentGraphIndex>>> {
        let mut cached = self.graph_read_index.read();
        if cached.is_none() {
            drop(cached);
            let mut writer = self.graph_read_index.write();
            if writer.is_none() {
                let mut index = SegmentGraphIndex::default();
                // Only unrestricted structural mutation invalidates the index.
                // A stopped rebuild is discarded, never published partially.
                if !self.for_each_fast(|entity| {
                    if !before_work() {
                        return false;
                    }
                    index.insert(entity);
                    true
                }) {
                    return None;
                }
                self.graph_read_index_bytes
                    .store(index.memory_bytes(), Ordering::Relaxed);
                *writer = Some(index);
            }
            cached = parking_lot::RwLockWriteGuard::downgrade(writer);
        }
        Some(cached)
    }

    pub(crate) fn graph_index_cursor(
        &self,
        kind: GraphEntityKind,
        keys: &[EntityId],
        before_work: &mut impl FnMut() -> bool,
    ) -> Option<GraphIndexCursor> {
        let cached = self.graph_read_index_for_query(before_work)?;
        let index = cached.as_ref().expect("complete graph index");
        let mut upper = EntityId::new(0);
        for key in keys {
            if !before_work() {
                return None;
            }
            if let Some(last) = index.candidates(kind, *key).next_back() {
                upper = upper.max(last);
            }
        }
        Some(GraphIndexCursor {
            key_index: 0,
            after: None,
            upper,
        })
    }

    /// Collect at most SCAN_BATCH_SIZE physical index entries without payload
    /// copies. Resume by key seek, never by rescanning an already consumed prefix.
    pub(crate) fn graph_index_batch(
        &self,
        kind: GraphEntityKind,
        keys: &[EntityId],
        cursor: &mut GraphIndexCursor,
        before_work: &mut impl FnMut() -> bool,
        visit: &mut impl FnMut(EntityId),
    ) -> bool {
        self.graph_index_batch_with_limit(kind, keys, cursor, before_work, visit, SCAN_BATCH_SIZE)
    }

    pub(crate) fn graph_index_batch_with_limit(
        &self,
        kind: GraphEntityKind,
        keys: &[EntityId],
        cursor: &mut GraphIndexCursor,
        before_work: &mut impl FnMut() -> bool,
        visit: &mut impl FnMut(EntityId),
        limit: usize,
    ) -> bool {
        assert!(
            limit > 0 && limit <= SCAN_BATCH_SIZE,
            "bounded graph index batch"
        );
        let Some(cached) = self.graph_read_index_for_query(before_work) else {
            return false;
        };
        let index = cached.as_ref().expect("complete graph index");
        let mut count = 0;
        while !cursor.finished(keys) && count < limit {
            let key = keys[cursor.key_index];
            if !before_work() {
                return false;
            }
            for id in index
                .candidates_after(kind, key, cursor.after, cursor.upper)
                .take(limit - count)
            {
                if !before_work() {
                    return false;
                }
                cursor.after = Some(id);
                count += 1;
                visit(id);
            }
            if count == limit && cursor.after != Some(cursor.upper) {
                break;
            }
            cursor.key_index += 1;
            cursor.after = None;
        }
        true
    }

    /// Probe one declared UNIQUE/PRIMARY KEY against physical rows. Cache all
    /// current definitions together so alternating constraints never rebuilds
    /// the same segment. Schema changes replace the set; mutations invalidate it.
    /// The caller owns MVCC conflict policy and null-key admission.
    pub(crate) fn find_unique_key_conflict(
        &self,
        columns: &[Vec<String>],
        key_index: usize,
        signatures: &[String],
        exclude_id: Option<EntityId>,
        reserves_key: &impl Fn(&UnifiedEntity) -> bool,
    ) -> Option<EntityId> {
        assert!(
            key_index < columns.len(),
            "constraint index belongs to current schema"
        );
        let cached = self.unique_key_lookup.read();
        if cached.iter().map(|index| &index.columns).eq(columns.iter()) {
            return self.find_unique_key_candidate(
                &cached[key_index],
                signatures,
                exclude_id,
                reserves_key,
            );
        }
        drop(cached);
        let mut cached = self.unique_key_lookup.write();
        if !cached.iter().map(|index| &index.columns).eq(columns.iter()) {
            let mut indexes: Vec<_> = columns
                .iter()
                .map(|columns| SegmentUniqueKey::new(columns))
                .collect();
            self.for_each_fast(|entity| {
                for index in &mut indexes {
                    index.insert(entity);
                }
                true
            });
            self.unique_key_lookup_bytes.store(
                indexes
                    .iter()
                    .map(|index| index.memory_bytes() as u64)
                    .sum(),
                Ordering::Relaxed,
            );
            *cached = indexes;
        }
        self.find_unique_key_candidate(&cached[key_index], signatures, exclude_id, reserves_key)
    }

    fn find_unique_key_candidate(
        &self,
        index: &SegmentUniqueKey,
        signatures: &[String],
        exclude_id: Option<EntityId>,
        reserves_key: &impl Fn(&UnifiedEntity) -> bool,
    ) -> Option<EntityId> {
        index.candidates(signatures).iter().copied().find(|id| {
            if Some(*id) == exclude_id {
                return false;
            }
            let Some(
                entity @ UnifiedEntity {
                    data: EntityData::Row(row),
                    ..
                },
            ) = self.get(*id)
            else {
                return false;
            };
            index
                .columns
                .iter()
                .zip(signatures)
                .all(|(column, signature)| {
                    row.get_field(column)
                        .is_some_and(|value| format!("{value:?}") == *signature)
                })
                && reserves_key(entity)
        })
    }

    fn invalidate_unique_key_lookup(&mut self) {
        *self.unique_key_lookup.get_mut() = Vec::new();
        self.unique_key_lookup_bytes.store(0, Ordering::Relaxed);
    }

    /// Only matching-key candidates reach `visible`; no result vector or row clones.
    /// Raw mutable rows are rechecked alongside indexed candidates.
    pub(crate) fn find_key_entity(
        &self,
        key: &str,
        newest: bool,
        visible: &mut impl FnMut(&UnifiedEntity) -> bool,
    ) -> Option<&UnifiedEntity> {
        // Writers may temporarily take the index while updating its candidates,
        // but publish it again before releasing their exclusive segment guard.
        let index = self
            .key_lookup
            .as_ref()
            .expect("segment owns its key index");
        self.find_key_candidate(index, key, newest, visible)
    }

    fn find_key_candidate(
        &self,
        index: &SegmentKeyLookup,
        key: &str,
        newest: bool,
        visible: &mut impl FnMut(&UnifiedEntity) -> bool,
    ) -> Option<&UnifiedEntity> {
        self.select_key_entity(
            index
                .candidates(key)
                .iter()
                .chain(index.mutated_candidates())
                .filter_map(|id| self.get(*id)),
            key,
            newest,
            visible,
        )
    }

    fn select_key_entity<'a>(
        &self,
        entities: impl Iterator<Item = &'a UnifiedEntity>,
        key: &str,
        newest: bool,
        visible: &mut impl FnMut(&UnifiedEntity) -> bool,
    ) -> Option<&'a UnifiedEntity> {
        let mut best: Option<&UnifiedEntity> = None;
        for entity in entities {
            // Fingerprint collisions must never become logical-key matches.
            if SegmentKeyLookup::key(entity) != Some(key) || !visible(entity) {
                continue;
            }
            if !newest {
                return Some(entity);
            }
            if best.is_none_or(|current| entity.xmin >= current.xmin) {
                best = Some(entity);
            }
        }
        best
    }

    /// Check whether an early-exit probe key exists in this segment.
    pub fn may_contain_exact_key(&self, key: &[u8]) -> bool {
        if let Some(id) = entity_id_from_probe_key(key) {
            return self.has_live_entity(id);
        }

        let Ok(pk_str) = std::str::from_utf8(key) else {
            return false;
        };
        let pk_key = (self.collection.clone(), pk_str.to_string());
        self.pk_index
            .get(&pk_key)
            .is_some_and(|id| self.has_live_entity(*id))
    }

    /// Remove entity from indices
    fn unindex_entity(&mut self, entity: &UnifiedEntity) {
        if let Some(index) = self.key_lookup.as_mut() {
            index.remove(entity);
        }
        if let Some(index) = self.graph_read_index.get_mut() {
            index.remove(entity);
            self.graph_read_index_bytes
                .store(index.memory_bytes(), Ordering::Relaxed);
        }
        // Kind index
        let kind_key = entity.kind.storage_type().to_string();
        if let Some(set) = self.kind_index.get_mut(&kind_key) {
            set.remove(&entity.id);
        }

        // Primary key index
        if let EntityData::Row(row) = &entity.data {
            if let Some(first_col) = row.columns.first() {
                let pk_str = format!("{:?}", first_col);
                self.pk_index
                    .remove(&(entity.kind.collection().to_string(), pk_str));
            }
        }

        // Cross-reference indices
        self.cross_ref_forward.remove(&entity.id);
        // Note: reverse refs from this entity still need cleanup
    }

    /// Selective re-index for updates.
    ///
    /// Skips index work that is not needed:
    /// - `kind_index`: entity kind is immutable, so remove+reinsert is always
    ///   a no-op; we skip it entirely and keep the existing entry.
    /// - `pk_index`: only updated when the primary-key column (first column of
    ///   a Row entity) actually changed. When `modified_columns` is provided,
    ///   we check membership; otherwise we compare old vs new pk value.
    /// - `cross_ref`: only rebuilt when the refs actually differ.
    fn reindex_for_update(
        &mut self,
        old: &UpdateIndexSnapshot,
        new: &UnifiedEntity,
        modified_columns: Option<&[String]>,
    ) {
        // kind_index: kind is immutable — the existing entry is already correct.
        // No remove + reinsert needed.

        // pk_index: only update when pk column is touched
        let pk_changed = match &new.data {
            EntityData::Row(new_row) => {
                if let Some(cols) = modified_columns {
                    // Caller told us exactly what changed — check if first schema column modified
                    // pk is the first column; check by name against the schema or by position 0
                    let pk_col_name = old.pk_column_name.as_deref().or_else(|| {
                        new_row
                            .schema
                            .as_deref()
                            .and_then(|schema| schema.first().map(|name| name.as_str()))
                    });
                    match pk_col_name {
                        Some(pk_name) => cols.iter().any(|c| c.eq_ignore_ascii_case(pk_name)),
                        // No schema — fall back to value comparison
                        None => old.pk_value.as_ref() != new_row.columns.first(),
                    }
                } else {
                    old.pk_value.as_ref() != new_row.columns.first()
                }
            }
            // Non-row types don't use pk_index
            _ => false,
        };

        if pk_changed {
            // Remove old pk entry
            if let Some((collection, pk_str)) = &old.pk_index_key {
                self.pk_index.remove(&(collection.clone(), pk_str.clone()));
            }
            // Insert new pk entry
            if let EntityData::Row(row) = &new.data {
                if let Some(first_col) = row.columns.first() {
                    let pk_str = format!("{:?}", first_col);
                    self.pk_index
                        .insert((new.kind.collection().to_string(), pk_str), new.id);
                }
            }
        }

        // cross_ref: only rebuild when refs actually changed
        let new_refs = new.cross_refs();
        if old.cross_refs.as_slice() != new_refs {
            // Remove stale forward refs
            self.cross_ref_forward.remove(&new.id);
            // Prune stale entries from reverse index
            for cross_ref in &old.cross_refs {
                if let Some(rev) = self.cross_ref_reverse.get_mut(&cross_ref.target) {
                    rev.retain(|(src, _)| *src != new.id);
                }
            }
            // Add new refs
            for cross_ref in new_refs {
                self.cross_ref_forward
                    .entry(cross_ref.source)
                    .or_default()
                    .push((cross_ref.target, cross_ref.ref_type));
                self.cross_ref_reverse
                    .entry(cross_ref.target)
                    .or_default()
                    .push((cross_ref.source, cross_ref.ref_type));
            }
        }
    }

    /// Get entities referencing the given entity
    pub fn get_references_to(&self, id: EntityId) -> Vec<(EntityId, RefType)> {
        self.cross_ref_reverse.get(&id).cloned().unwrap_or_default()
    }

    /// Get entities referenced by the given entity
    pub fn get_references_from(&self, id: EntityId) -> Vec<(EntityId, RefType)> {
        self.cross_ref_forward.get(&id).cloned().unwrap_or_default()
    }

    /// Get age in seconds
    pub fn age_secs(&self) -> u64 {
        let now = current_unix_secs();
        now.saturating_sub(self.created_at)
    }

    /// Get time since last write
    pub fn idle_secs(&self) -> u64 {
        let now = current_unix_secs();
        now.saturating_sub(self.last_write_at)
    }

    /// Turbo bulk insert — minimal allocations per entity.
    ///
    /// Optimizations vs normal insert:
    /// - Skips cross-refs
    /// - Resolves kind_index once per consecutive run of the same storage type
    /// - Pre-allocates kind_index HashSet
    /// - Skips contains_key check (caller guarantees unique IDs)
    /// - Uses Relaxed ordering for sequence counter
    pub fn bulk_insert(
        &mut self,
        entities: Vec<UnifiedEntity>,
    ) -> Result<Vec<EntityId>, SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        for entity in &entities {
            self.mark_checkpoint_entity_dirty(entity.id);
        }

        if let Some(index) = self.key_lookup.as_mut() {
            for entity in &entities {
                index.insert(entity);
            }
        }

        let indexes = self.unique_key_lookup.get_mut();
        for index in indexes.iter_mut() {
            for entity in &entities {
                index.insert(entity);
            }
        }
        self.unique_key_lookup_bytes.store(
            indexes
                .iter()
                .map(|index| index.memory_bytes() as u64)
                .sum(),
            Ordering::Relaxed,
        );

        let n = entities.len();

        if entities.is_empty() {
            return Ok(Vec::new());
        }

        // A collection may receive a mixed-model batch. Index each item's actual
        // type while retaining batched lookup/reservation for homogeneous input.
        for run in
            entities.chunk_by(|left, right| left.kind.storage_type() == right.kind.storage_type())
        {
            let kind = run[0].kind.storage_type();
            self.observe_graph_kind(&run[0].kind);
            if matches!(
                run[0].kind,
                EntityKind::GraphNode(_) | EntityKind::GraphEdge(_)
            ) {
                if let Some(index) = self.graph_read_index.get_mut() {
                    for entity in run {
                        index.insert(entity);
                    }
                    self.graph_read_index_bytes
                        .store(index.memory_bytes(), Ordering::Relaxed);
                }
            }
            let kind_set = match self.kind_index.get_mut(kind) {
                Some(ids) => ids,
                None => self.kind_index.entry(kind.to_string()).or_default(),
            };
            kind_set.reserve(run.len());
            kind_set.extend(run.iter().map(|entity| entity.id));
        }

        let now = current_unix_secs();

        let base_seq = self.sequence.fetch_add(n as u64, Ordering::Relaxed);

        let mut ids = Vec::with_capacity(n);

        // Use flat storage (Vec) instead of HashMap — saves ~80 bytes/entity overhead
        if self.flat_entities.is_empty() && self.entities.is_empty() {
            // First bulk insert: initialize flat storage
            self.base_entity_id = entities.first().map(|e| e.id.raw()).unwrap_or(0);
            self.use_flat = true;
        }

        // Collect zone-update values per column position, not per
        // (row, col). The old code did `col.to_string()` + HashMap
        // probe per cell — 15 cols × 25k rows = 375K String
        // allocations per bulk_insert call for the typed_insert bench.
        //
        // New shape: accumulate `Vec<Value>` per column index; after
        // the scan, walk each non-empty column exactly once, resolve
        // its name via the shared schema, and apply all observations
        // under a single HashMap entry. 375K allocs → ~ncols allocs.
        let mut columnar_zone_updates: Vec<Vec<Value>> = Vec::new();
        let mut columnar_schema: Option<std::sync::Arc<Vec<String>>> = None;
        // Named-row fallback (non-prevalidated path) still uses the
        // per-cell vec — these rows don't share a schema, so there's
        // no columnar shortcut.
        let mut named_zone_updates: Vec<(String, Value)> = Vec::new();

        // Payload bytes for the whole batch, applied with one atomic add.
        // `bulk_insert` used to skip memory tracking entirely, which left the
        // budget blind to the rows that dominate a bulk-loaded store — and
        // blind to what consolidation reclaims from them.
        let mut batch_bytes: usize = 0;

        if self.use_flat {
            self.flat_entities.reserve(n);
            for (i, mut entity) in entities.into_iter().enumerate() {
                entity.sequence_id = base_seq + i as u64;
                let id = entity.id;
                ids.push(id);
                batch_bytes += Self::estimate_entity_size(&entity);
                if let EntityData::Row(row) = &entity.data {
                    if row.schema.is_some() && !row.columns.is_empty() {
                        if columnar_zone_updates.is_empty() {
                            columnar_zone_updates = vec![Vec::with_capacity(n); row.columns.len()];
                            columnar_schema = row.schema.clone();
                        }
                        for (ci, val) in row.columns.iter().enumerate() {
                            if !matches!(val, Value::Null) {
                                if let Some(bucket) = columnar_zone_updates.get_mut(ci) {
                                    bucket.push(val.clone());
                                }
                            }
                        }
                    } else {
                        for (col, val) in row.iter_fields() {
                            if !matches!(val, Value::Null) {
                                named_zone_updates.push((col.to_string(), val.clone()));
                            }
                        }
                    }
                }
                // Position-based `flat_entities` access in `get(id)`
                // assumes IDs are contiguous starting at
                // `base_entity_id`. When IDs ARE contiguous (the bulk
                // ingest hot path) this saves a HashMap probe per
                // lookup. But the global ID counter can jump between
                // bulk calls — CREATE INDEX, catalog DDL, even other
                // collections in the same store reserve IDs from the
                // same allocator. If we blindly `push` a gap entity
                // its actual position in the Vec no longer matches
                // `id - base`, and `flat_entities[id - base].id == id`
                // fails — the entity becomes invisible to `get(id)`.
                //
                // Route gap entities into the HashMap fallback
                // (`self.entities`). `get(id)` already falls through
                // there when the flat probe misses.
                let expected = self.base_entity_id + self.flat_entities.len() as u64;
                if id.raw() == expected {
                    self.flat_entities.push(entity);
                } else {
                    self.hash_scan_ids.push(id);
                    self.entities.insert(id, entity);
                }
            }
        } else {
            // Fallback to HashMap for non-sequential inserts
            self.entities.reserve(n);
            self.hash_scan_ids.reserve(n);
            let mut pairs = Vec::with_capacity(n);
            for (i, mut entity) in entities.into_iter().enumerate() {
                entity.sequence_id = base_seq + i as u64;
                let id = entity.id;
                ids.push(id);
                batch_bytes += Self::estimate_entity_size(&entity);
                if let EntityData::Row(row) = &entity.data {
                    for (col, val) in row.iter_fields() {
                        if !matches!(val, Value::Null) {
                            named_zone_updates.push((col.to_string(), val.clone()));
                        }
                    }
                }
                self.hash_scan_ids.push(id);
                pairs.push((id, entity));
            }
            self.entities.extend(pairs);
        }

        // Apply the accumulated zone updates.
        // Columnar path: one `col_zones.entry` call per column (not
        // per cell). Named-fallback path: unchanged.
        if !columnar_zone_updates.is_empty() {
            let schema = columnar_schema.as_ref();
            for (ci, values) in columnar_zone_updates.into_iter().enumerate() {
                if values.is_empty() {
                    continue;
                }
                let Some(col_name) = schema.and_then(|s| s.get(ci)) else {
                    continue;
                };
                // Enter the HashMap once per column. `raw_entry_mut`
                // avoids the String::clone when the column already
                // exists in the map — but that's nightly-only, so we
                // pay one `clone()` per call-per-column (ncols total)
                // instead of the old ncols×nrows.
                let mut iter = values.into_iter();
                if let Some(first) = iter.next() {
                    let previous = self
                        .col_zones
                        .get(col_name)
                        .map_or(0, ColZone::payload_bytes);
                    let zone = self
                        .col_zones
                        .entry(col_name.clone())
                        .and_modify(|z| z.update(&first))
                        .or_insert_with(|| ColZone::new(first));
                    for v in iter {
                        zone.update(&v);
                    }
                    self.zone_payload_bytes = self
                        .zone_payload_bytes
                        .saturating_sub(previous)
                        .saturating_add(zone.payload_bytes());
                }
            }
        }
        for (col, val) in named_zone_updates {
            self.update_column_zone(&col, &val);
        }

        self.add_memory(batch_bytes);
        self.last_write_at = now;

        // Publish the new flat length so lock-free readers can see the new entities.
        if self.use_flat {
            self.published_flat_len
                .store(self.flat_entities.len(), Ordering::Release);
        }

        Ok(ids)
    }

    /// Delete from this segment regardless of its seal state.
    /// Used to mutate sealed segments when DELETE touches bulk-inserted entities.
    pub(crate) fn force_delete(&mut self, id: EntityId) -> bool {
        if self.use_flat {
            let raw = id.raw();
            if raw >= self.base_entity_id {
                let idx = (raw - self.base_entity_id) as usize;
                if idx < self.flat_entities.len() && self.flat_entities[idx].id == id {
                    self.tombstone_flat_entity(idx, id);
                    return true;
                }
            }
            return self.remove_hashmap_entity(id);
        }

        self.remove_hashmap_entity(id)
    }

    /// Update an entity in this segment regardless of its seal state.
    /// Used to mutate sealed segments when UPDATE touches bulk-inserted entities.
    pub(crate) fn force_update_with_metadata(
        &mut self,
        entity: &UnifiedEntity,
        modified_columns: &[String],
        metadata: Option<&Metadata>,
    ) -> Result<(), SegmentError> {
        self.apply_hot_update_with_metadata(entity, modified_columns, metadata)
    }
}

impl UnifiedSegment for GrowingSegment {
    fn id(&self) -> SegmentId {
        self.id
    }

    fn state(&self) -> SegmentState {
        self.state
    }

    fn collection(&self) -> &str {
        &self.collection
    }

    fn stats(&self) -> SegmentStats {
        let mut stats = SegmentStats {
            entity_count: self.live_entity_count(),
            deleted_count: self.deleted.len(),
            memory_bytes: self.resident_bytes() as usize,
            ..Default::default()
        };

        for entity in self.entities.values() {
            match &entity.kind {
                EntityKind::TableRow { .. } => stats.row_count += 1,
                EntityKind::GraphNode(_) => stats.node_count += 1,
                EntityKind::GraphEdge(_) => stats.edge_count += 1,
                EntityKind::Vector { .. } => stats.vector_count += 1,
                EntityKind::TimeSeriesPoint(_) => stats.row_count += 1,
                EntityKind::QueueMessage { .. } => stats.row_count += 1,
            }
            stats.cross_ref_count += entity.cross_refs().len();
        }

        stats
    }

    fn entity_count(&self) -> usize {
        self.live_entity_count()
    }

    fn contains(&self, id: EntityId) -> bool {
        self.has_live_entity(id)
    }

    fn get(&self, id: EntityId) -> Option<&UnifiedEntity> {
        if self.deleted.contains_key(&id) {
            return None;
        }
        if self.use_flat {
            let raw = id.raw();
            if raw >= self.base_entity_id {
                let idx = (raw - self.base_entity_id) as usize;
                if let Some(entity) = self.flat_entities.get(idx).filter(|e| e.id == id) {
                    return Some(entity);
                }
            }
            // Fall through: once flat mode is initialized by a bulk_insert,
            // subsequent per-row `insert()` calls still write into the
            // HashMap (see the impl below) so reads must check both. Without
            // this fallback, a post-bulk single-row insert silently
            // disappears from `get()` / `query_all()`.
            self.entities.get(&id)
        } else {
            self.entities.get(&id)
        }
    }

    fn get_mut(&mut self, id: EntityId) -> Option<&mut UnifiedEntity> {
        self.mark_checkpoint_entity_dirty(id);
        self.invalidate_unique_key_lookup();
        self.invalidate_graph_read_index();
        if self.deleted.contains_key(&id) || !self.state.is_writable() {
            return None;
        }
        if let Some(mut index) = self.key_lookup.take() {
            if let Some(entity) = self.get(id) {
                index.mark_mutated(entity);
            }
            self.key_lookup = Some(index);
        }
        // The caller can replace the kind through this reference. Conservatively
        // disable both graph pruning passes until a new segment is rebuilt.
        self.graph_kind_mask = GraphEntityKind::Node.mask() | GraphEntityKind::Edge.mask();
        if self.use_flat {
            let raw = id.raw();
            if raw >= self.base_entity_id {
                let idx = (raw - self.base_entity_id) as usize;
                if self
                    .flat_entities
                    .get(idx)
                    .map(|e| e.id == id)
                    .unwrap_or(false)
                {
                    return self.flat_entities.get_mut(idx);
                }
            }
            // Fall through to HashMap for entities inserted via
            // per-row `insert()` after flat mode was activated.
            self.entities.get_mut(&id)
        } else {
            self.entities.get_mut(&id)
        }
    }

    fn insert(&mut self, mut entity: UnifiedEntity) -> Result<EntityId, SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        if self.entities.contains_key(&entity.id) {
            return Err(SegmentError::AlreadyExists(entity.id));
        }

        // Assign sequence ID
        entity.sequence_id = self.next_sequence();

        // Estimate and track memory
        let size = Self::estimate_entity_size(&entity);
        self.add_memory(size);

        // Index the entity
        self.index_entity(&entity);

        // Update column zone maps for range-based segment pruning
        self.update_col_zones_from_entity(&entity);

        // Store
        let id = entity.id;
        self.entities.insert(id, entity);

        // Update write timestamp
        self.last_write_at = current_unix_secs();

        Ok(id)
    }

    fn update(&mut self, entity: UnifiedEntity) -> Result<(), SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        self.apply_update_with_metadata(&entity, None)?;
        self.last_write_at = current_unix_secs();

        Ok(())
    }

    fn update_hot(
        &mut self,
        entity: UnifiedEntity,
        modified_columns: &[String],
    ) -> Result<(), SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        self.apply_hot_update_with_metadata(&entity, modified_columns, None)?;
        self.last_write_at = current_unix_secs();
        Ok(())
    }

    fn delete(&mut self, id: EntityId) -> Result<bool, SegmentError> {
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        // For flat storage, use tombstone (don't remove from Vec to keep indices valid)
        if self.use_flat {
            let raw = id.raw();
            if raw >= self.base_entity_id {
                let idx = (raw - self.base_entity_id) as usize;
                if idx < self.flat_entities.len() && self.flat_entities[idx].id == id {
                    self.tombstone_flat_entity(idx, id);
                    return Ok(true);
                }
            }
            return Ok(self.remove_hashmap_entity(id));
        }

        Ok(self.remove_hashmap_entity(id))
    }

    fn get_metadata(&self, id: EntityId) -> Option<Metadata> {
        if !self.has_live_entity(id) {
            return None;
        }
        Some(self.metadata.get_all(id))
    }

    fn set_metadata(&mut self, id: EntityId, metadata: Metadata) -> Result<(), SegmentError> {
        self.mark_checkpoint_entity_dirty(id);
        if !self.state.is_writable() {
            return Err(SegmentError::NotWritable);
        }

        if !self.has_live_entity(id) {
            return Err(SegmentError::NotFound(id));
        }

        self.metadata.set_all(id, &metadata);
        Ok(())
    }

    fn seal(&mut self) -> Result<(), SegmentError> {
        if self.state != SegmentState::Growing {
            return Err(SegmentError::InvalidState(self.state));
        }

        self.state = SegmentState::Sealing;

        // Build indices on the sealed data:
        // - HNSW/IVF for vectors (future)
        // - B-tree for sorted access (future)
        // - Inverted index for text search (future)
        self.rebuild_sealed_col_zones();

        self.state = SegmentState::Sealed;
        Ok(())
    }

    fn should_seal(&self, config: &SegmentConfig) -> bool {
        // Check entity count
        if self.entities.len() >= config.max_entities {
            return true;
        }

        // Check memory usage
        if self.memory_bytes.load(Ordering::Relaxed) as usize >= config.max_bytes {
            return true;
        }

        // Check age
        if self.age_secs() >= config.max_age_secs {
            return true;
        }

        false
    }

    fn iter(&self) -> Box<dyn Iterator<Item = &UnifiedEntity> + '_> {
        let base: Box<dyn Iterator<Item = &UnifiedEntity>> = if self.use_flat {
            // Chain flat entities (from bulk_insert) with any entities
            // that landed in the HashMap via per-row `insert()` after
            // flat mode was activated.
            Box::new(self.flat_entities.iter().chain(self.entities.values()))
        } else {
            Box::new(self.entities.values())
        };
        if self.deleted.is_empty() {
            base
        } else {
            Box::new(base.filter(|e| !self.deleted.contains_key(&e.id)))
        }
    }

    fn iter_kind(&self, kind_filter: &str) -> Box<dyn Iterator<Item = &UnifiedEntity> + '_> {
        // The iterator borrows the segment, so the index cannot change underneath
        // it. Borrow the ID set as well instead of copying O(kind count) entries.
        let ids = self.kind_index.get(kind_filter);
        // In flat mode entities live in `flat_entities`, not `entities` —
        // chain both so iter_kind doesn't drop bulk-inserted entities.
        let flat: Box<dyn Iterator<Item = &UnifiedEntity>> = if self.use_flat {
            Box::new(self.flat_entities.iter())
        } else {
            Box::new(std::iter::empty())
        };
        Box::new(flat.chain(self.entities.values()).filter(move |e| {
            if self.deleted.contains_key(&e.id) {
                return false;
            }
            if let Some(ids) = ids {
                ids.contains(&e.id)
            } else {
                false
            }
        }))
    }

    fn filter_metadata(
        &self,
        filters: &[(String, super::metadata::MetadataFilter)],
    ) -> Vec<EntityId> {
        // For growing segments, we iterate and filter. In flat mode the
        // IDs live in `flat_entities`, not `entities`, so chain both.
        let flat_ids: Box<dyn Iterator<Item = EntityId> + '_> = if self.use_flat {
            Box::new(self.flat_entities.iter().map(|e| e.id))
        } else {
            Box::new(std::iter::empty())
        };
        flat_ids
            .chain(self.entities.keys().copied())
            .filter(|id| {
                if self.deleted.contains_key(id) {
                    return false;
                }
                let metadata = self.metadata.get_all(*id);
                metadata.matches_all(filters)
            })
            .collect()
    }
}

fn build_minmax_multi_intervals(
    entries: &[(CanonicalKey, Value)],
    max_intervals: usize,
) -> Vec<ColZone> {
    if entries.is_empty() {
        return Vec::new();
    }
    if entries.len() == 1 || max_intervals <= 1 {
        return vec![ColZone::with_bounds(
            entries[0].1.clone(),
            entries[entries.len() - 1].1.clone(),
        )];
    }

    let mut split_points = if entries.len() <= max_intervals {
        (1..entries.len()).collect::<Vec<_>>()
    } else {
        let target_splits = max_intervals - 1;
        let mut selected = select_gap_split_points(entries, target_splits);
        if selected.len() < target_splits {
            for bucket in 1..max_intervals {
                let idx = bucket * entries.len() / max_intervals;
                if idx == 0 || idx >= entries.len() || selected.contains(&idx) {
                    continue;
                }
                selected.push(idx);
                if selected.len() >= target_splits {
                    break;
                }
            }
        }
        selected.sort_unstable();
        selected.dedup();
        selected
    };

    split_points.push(entries.len());

    let mut out = Vec::with_capacity(split_points.len());
    let mut start = 0usize;
    for end in split_points {
        if end <= start {
            continue;
        }
        out.push(ColZone::with_bounds(
            entries[start].1.clone(),
            entries[end - 1].1.clone(),
        ));
        start = end;
    }

    if out.is_empty() {
        out.push(ColZone::with_bounds(
            entries[0].1.clone(),
            entries[entries.len() - 1].1.clone(),
        ));
    }

    out
}

fn select_gap_split_points(entries: &[(CanonicalKey, Value)], max_splits: usize) -> Vec<usize> {
    let mut gaps = Vec::new();
    for idx in 1..entries.len() {
        if let Some(score) = canonical_gap_score(&entries[idx - 1].0, &entries[idx].0) {
            if score > 0.0 {
                gaps.push((score, idx));
            }
        }
    }
    gaps.sort_by(|left, right| {
        right
            .0
            .partial_cmp(&left.0)
            .unwrap_or(std::cmp::Ordering::Equal)
            .then_with(|| left.1.cmp(&right.1))
    });
    gaps.into_iter()
        .take(max_splits)
        .map(|(_, idx)| idx)
        .collect()
}

fn canonical_gap_score(left: &CanonicalKey, right: &CanonicalKey) -> Option<f64> {
    if left.family() != right.family() {
        return None;
    }
    match (left, right) {
        (CanonicalKey::Signed(_, l), CanonicalKey::Signed(_, r)) => {
            Some(r.saturating_sub(*l) as f64)
        }
        (CanonicalKey::Unsigned(_, l), CanonicalKey::Unsigned(_, r)) => {
            Some(r.saturating_sub(*l) as f64)
        }
        (CanonicalKey::Float(l), CanonicalKey::Float(r)) => {
            Some((f64::from_bits(*r) - f64::from_bits(*l)).abs())
        }
        _ => None,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::storage::unified::entity::RowData;
    use crate::storage::unified::MetadataValue;
    use reddb_types::Value;

    fn keyed_row(id: u64, key: &str, xmin: u64, xmax: u64) -> UnifiedEntity {
        let mut entity = UnifiedEntity::table_row(EntityId::new(id), "keys", id, vec![]);
        entity.data = EntityData::Row(RowData::with_names(
            vec![Value::text(key), Value::Integer(id as i64)],
            vec!["key".into(), "value".into()],
        ));
        entity.set_xmin(xmin);
        entity.set_xmax(xmax);
        entity
    }

    fn keyed_probe(segment: &GrowingSegment, key: &str) -> Option<EntityId> {
        segment
            .find_key_entity(key, true, &mut |_| true)
            .map(|entity| entity.id)
    }

    #[test]
    fn bulk_checkpoint_journal_preserves_prior_changes_and_ignores_rejected_batches() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment.insert(keyed_row(1, "one", 1, 0)).expect("row");
        segment
            .bulk_insert(vec![keyed_row(2, "two", 1, 0), keyed_row(3, "three", 1, 0)])
            .expect("batch");
        assert_eq!(
            segment.checkpoint_mutations_since(0),
            Some(vec![EntityId::new(1), EntityId::new(2), EntityId::new(3)])
        );
        let revision = segment.checkpoint_revision();
        segment.bulk_insert(Vec::new()).expect("empty batch");
        assert_eq!(segment.checkpoint_revision(), revision);
        segment.seal().expect("seal segment");
        assert!(matches!(
            segment.bulk_insert(vec![keyed_row(4, "four", 1, 0)]),
            Err(SegmentError::NotWritable)
        ));
        assert_eq!(segment.checkpoint_revision(), revision);
        assert_eq!(segment.checkpoint_mutations_since(revision), Some(vec![]));
    }

    #[test]
    fn bulk_checkpoint_journal_exhaustion_keeps_a_bounded_complete_suffix() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment
            .bulk_insert(
                (1..=CHECKPOINT_MUTATIONS_MAX as u64 + 1)
                    .map(|id| keyed_row(id, "key", 1, 0))
                    .collect(),
            )
            .expect("large batch");
        assert!(segment.checkpoint_mutations_since(0).is_none());
        assert_eq!(
            segment
                .checkpoint_mutations_since(1)
                .expect("complete suffix")
                .len(),
            CHECKPOINT_MUTATIONS_MAX
        );
        let revision = segment.checkpoint_revision();
        let id = EntityId::new(CHECKPOINT_MUTATIONS_MAX as u64 + 2);
        segment
            .bulk_insert(vec![keyed_row(id.raw(), "next", 1, 0)])
            .expect("later batch");
        assert_eq!(segment.checkpoint_mutations_since(revision), Some(vec![id]));
        assert_eq!(segment.checkpoint_mutations.len(), CHECKPOINT_MUTATIONS_MAX);
    }

    #[test]
    fn key_lookup_visits_only_requested_key_versions() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment
            .bulk_insert(
                (1..=4096)
                    .map(|id| keyed_row(id, &format!("other-{id}"), 1, 0))
                    .collect(),
            )
            .expect("unrelated rows");
        segment
            .insert(keyed_row(5000, "wanted", 2, 8))
            .expect("old version");
        segment
            .insert(keyed_row(5001, "wanted", 8, 0))
            .expect("new version");
        let mut visits = 0;
        let result = segment.find_key_entity("wanted", true, &mut |entity| {
            visits += 1;
            entity.xmin < 8
        });
        assert_eq!(result.map(|entity| entity.id), Some(EntityId::new(5000)));
        assert_eq!(
            visits, 2,
            "unrelated rows must never reach visibility evaluation"
        );
        assert_eq!(keyed_probe(&segment, "wanted"), Some(EntityId::new(5001)));
        let mut misses = 0;
        assert!(segment
            .find_key_entity("absent", true, &mut |_| {
                misses += 1;
                true
            })
            .is_none());
        assert_eq!(misses, 0);

        // A bucket collision narrows candidates, but cannot change key equality.
        let mut collision = SegmentKeyLookup::default();
        collision.insert(&keyed_row(1, "wanted", 1, 0));
        assert!(segment
            .find_key_candidate(&collision, "wanted", true, &mut |_| true)
            .is_none());
    }

    #[test]
    fn raw_mutation_does_not_disable_unrelated_key_candidates() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment
            .bulk_insert(
                (1..=4096)
                    .map(|id| keyed_row(id, &format!("key-{id}"), 1, 0))
                    .collect(),
            )
            .expect("rows");
        for _ in 0..3 {
            *segment.get_mut(EntityId::new(1)).expect("mutable") = keyed_row(1, "renamed", 2, 0);
        }
        let index = segment.key_lookup.as_ref().expect("index retained");
        assert_eq!(index.candidates("key-4096"), &[EntityId::new(4096)]);
        assert_eq!(index.mutated_candidates().count(), 1);
        assert!(index.candidates("key-1").is_empty());
        assert_eq!(keyed_probe(&segment, "renamed"), Some(EntityId::new(1)));
        assert_eq!(keyed_probe(&segment, "key-4096"), Some(EntityId::new(4096)));
        segment.delete(EntityId::new(1)).expect("delete raw row");
        assert_eq!(
            segment
                .key_lookup
                .as_ref()
                .unwrap()
                .mutated_candidates()
                .count(),
            0
        );
        assert_eq!(keyed_probe(&segment, "renamed"), None);
    }

    #[test]
    fn key_lookup_tracks_mutations_sealing_and_adoption() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment
            .bulk_insert(vec![keyed_row(1, "flat", 1, 0)])
            .expect("flat row");
        segment
            .insert(keyed_row(3, "hash", 1, 0))
            .expect("hash fallback");
        segment
            .bulk_insert(vec![keyed_row(5, "gap", 1, 0)])
            .expect("bulk gap");
        assert!(segment.key_lookup.as_ref().expect("index").memory_bytes() > 0);
        assert_eq!(keyed_probe(&segment, "gap"), Some(EntityId::new(5)));
        segment
            .update(keyed_row(3, "renamed", 2, 0))
            .expect("rename key");
        assert!(
            segment.key_lookup.is_some(),
            "key replacements maintain the index"
        );
        assert_eq!(keyed_probe(&segment, "hash"), None);
        assert_eq!(keyed_probe(&segment, "renamed"), Some(EntityId::new(3)));

        *segment.get_mut(EntityId::new(3)).expect("mutable row") = keyed_row(3, "raw", 3, 0);
        assert!(
            segment.key_lookup.is_some(),
            "raw mutation preserves unaffected key candidates"
        );
        assert_eq!(keyed_probe(&segment, "renamed"), None);
        assert_eq!(keyed_probe(&segment, "raw"), Some(EntityId::new(3)));
        segment.seal().expect("seal");
        segment
            .force_update_with_metadata(&keyed_row(5, "sealed", 4, 0), &["key".into()], None)
            .expect("sealed rename");
        assert_eq!(keyed_probe(&segment, "gap"), None);
        assert_eq!(keyed_probe(&segment, "sealed"), Some(EntityId::new(5)));
        assert!(segment.force_delete(EntityId::new(1)));
        assert!(segment.force_delete(EntityId::new(3)));
        assert_eq!(keyed_probe(&segment, "flat"), None);
        assert_eq!(keyed_probe(&segment, "raw"), None);

        let mut adopted = GrowingSegment::new(2, "keys");
        segment.for_each_fast(|entity| {
            adopted.adopt_entity(entity.clone(), None);
            true
        });
        assert_eq!(keyed_probe(&adopted, "sealed"), Some(EntityId::new(5)));
        assert!(adopted.evict_entity(EntityId::new(5)));
        assert_eq!(keyed_probe(&adopted, "sealed"), None);
    }

    #[test]
    #[ignore = "diagnostic microbenchmark; no timing assertions"]
    fn key_lookup_scaling_diagnostic() {
        use std::{hint::black_box, time::Instant};
        for count in [100, 10_000, 100_000] {
            let mut segment = GrowingSegment::new(1, "keys");
            segment
                .bulk_insert(
                    (1..=count)
                        .map(|id| keyed_row(id, &format!("key-{id}"), 1, 0))
                        .collect(),
                )
                .expect("rows");
            let key = format!("key-{count}");
            let started = Instant::now();
            for _ in 0..1000 {
                assert_eq!(
                    keyed_probe(black_box(&segment), black_box(&key)),
                    Some(EntityId::new(count))
                );
            }
            let indexed = started.elapsed();
            let started = Instant::now();
            for _ in 0..1000 {
                let mut found = None;
                segment.for_each_fast(|entity| {
                    if SegmentKeyLookup::key(entity) == Some(black_box(key.as_str())) {
                        found = Some(entity.id);
                        return false;
                    }
                    true
                });
                assert_eq!(black_box(found), Some(EntityId::new(count)));
            }
            eprintln!(
                "rows={count} reads=1000 indexed={indexed:?} scan={:?}",
                started.elapsed()
            );
        }
    }

    fn pruning_node(id: u64) -> UnifiedEntity {
        UnifiedEntity::graph_node(EntityId::new(id), "mixed", "person", Default::default())
    }

    fn pruning_edge(id: u64) -> UnifiedEntity {
        UnifiedEntity::graph_edge(
            EntityId::new(id),
            "mixed",
            "a",
            "b",
            1.0,
            Default::default(),
        )
    }

    #[test]
    fn graph_presence_covers_bulk_gaps_inserts_and_adoption() {
        let mut segment = GrowingSegment::new(1, "mixed");
        assert!(!segment.may_contain_graph_kind(GraphEntityKind::Node));
        assert!(!segment.may_contain_graph_kind(GraphEntityKind::Edge));
        segment
            .bulk_insert(vec![
                UnifiedEntity::vector(EntityId::new(1), "mixed", vec![]),
                pruning_node(2),
                pruning_edge(4),
            ])
            .expect("mixed bulk with gap");
        assert!(segment.may_contain_graph_kind(GraphEntityKind::Node));
        assert!(segment.may_contain_graph_kind(GraphEntityKind::Edge));
        segment.seal().expect("seal");
        assert!(segment.force_delete(EntityId::new(2)));
        assert!(segment.force_delete(EntityId::new(4)));
        // Deletion must not erase presence; reconstruction may safely tighten it.
        assert!(segment.may_contain_graph_kind(GraphEntityKind::Node));
        let mut rebuilt = GrowingSegment::new(2, "mixed");
        for item in segment.iter() {
            rebuilt.adopt_entity(item.clone(), None);
        }
        assert!(!rebuilt.may_contain_graph_kind(GraphEntityKind::Node));
        assert!(!rebuilt.may_contain_graph_kind(GraphEntityKind::Edge));
        rebuilt.adopt_entity(pruning_node(5), None);
        assert!(rebuilt.may_contain_graph_kind(GraphEntityKind::Node));
        rebuilt.insert(pruning_edge(6)).expect("single edge");
        assert!(rebuilt.may_contain_graph_kind(GraphEntityKind::Edge));
    }

    #[test]
    fn graph_presence_covers_replacement_and_unrestricted_mutation() {
        for flat in [false, true] {
            for operation in 0..4 {
                let mut segment = GrowingSegment::new(1, "mixed");
                let vector = UnifiedEntity::vector(EntityId::new(1), "mixed", vec![]);
                if flat {
                    segment.bulk_insert(vec![vector]).expect("flat vector");
                } else {
                    segment.insert(vector).expect("hashmap vector");
                }
                match operation {
                    0 => segment.update(pruning_node(1)).expect("replace with node"),
                    1 => segment
                        .update_hot(pruning_node(1), &[])
                        .expect("hot replacement"),
                    2 => {
                        segment.seal().expect("seal vector");
                        segment
                            .force_update_with_metadata(&pruning_node(1), &[], None)
                            .expect("sealed replacement");
                    }
                    _ => {
                        *segment.get_mut(EntityId::new(1)).expect("mutable vector") =
                            pruning_node(1)
                    }
                }
                assert!(segment.may_contain_graph_kind(GraphEntityKind::Node));
                assert!(matches!(
                    segment.get(EntityId::new(1)).expect("node").kind,
                    EntityKind::GraphNode(_)
                ));
                if operation == 3 {
                    assert!(segment.may_contain_graph_kind(GraphEntityKind::Edge));
                }
                segment
                    .force_update_with_metadata(&pruning_edge(1), &[], None)
                    .expect("replace with edge");
                assert!(segment.may_contain_graph_kind(GraphEntityKind::Edge));
                assert!(segment.may_contain_graph_kind(GraphEntityKind::Node));
            }
        }
    }

    fn primary_key_row(id: u64, key: Value) -> UnifiedEntity {
        UnifiedEntity::new(
            EntityId::new(id),
            EntityKind::TableRow {
                table: "keys".into(),
                row_id: id,
            },
            EntityData::Row(RowData::with_names(
                vec![Value::text("payload"), key],
                vec!["payload".into(), "primary".into()],
            )),
        )
    }

    fn primary_key_probe(
        segment: &GrowingSegment,
        key: Value,
        exclude: Option<EntityId>,
    ) -> Option<EntityId> {
        segment.find_unique_key_conflict(
            &[vec!["primary".into()]],
            0,
            &[format!("{key:?}")],
            exclude,
            &|_| true,
        )
    }

    #[test]
    fn unique_key_lookup_follows_segment_mutations_and_sealing() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment
            .bulk_insert(vec![primary_key_row(1, Value::Integer(10))])
            .expect("bulk row");
        let payload_bytes = segment.memory_bytes();
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(10), None),
            Some(EntityId::new(1))
        );
        assert!(segment.memory_bytes() > payload_bytes);
        segment
            .insert(primary_key_row(3, Value::Integer(30)))
            .expect("hashmap fallback row");
        segment
            .bulk_insert(vec![primary_key_row(4, Value::Integer(40))])
            .expect("bulk gap row");
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(40), None),
            Some(EntityId::new(4))
        );
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(30), None),
            Some(EntityId::new(3))
        );
        segment
            .update_hot(primary_key_row(1, Value::Integer(11)), &["primary".into()])
            .expect("replace flat key");
        assert_eq!(segment.unique_key_lookup_bytes.load(Ordering::Relaxed), 0);
        assert_eq!(primary_key_probe(&segment, Value::Integer(10), None), None);
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(11), None),
            Some(EntityId::new(1))
        );
        segment
            .delete_batch(&[EntityId::new(1), EntityId::new(3)])
            .expect("delete flat and map");
        assert_eq!(primary_key_probe(&segment, Value::Integer(11), None), None);
        assert_eq!(primary_key_probe(&segment, Value::Integer(30), None), None);
        segment.seal().expect("seal");
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(40), None),
            Some(EntityId::new(4))
        );
        segment
            .force_update_with_metadata(
                &primary_key_row(4, Value::Integer(41)),
                &["primary".into()],
                None,
            )
            .expect("sealed update");
        assert_eq!(primary_key_probe(&segment, Value::Integer(40), None), None);
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(41), None),
            Some(EntityId::new(4))
        );
        assert!(segment.force_delete(EntityId::new(4)));
        assert_eq!(primary_key_probe(&segment, Value::Integer(41), None), None);
    }

    #[test]
    fn unique_lookup_retains_all_current_definitions_and_rechecks_candidates() {
        let mut segment = GrowingSegment::new(1, "keys");
        segment
            .bulk_insert(vec![
                primary_key_row(1, Value::Integer(10)),
                primary_key_row(2, Value::Integer(20)),
            ])
            .expect("initial rows");
        let definitions = vec![
            vec!["primary".into()],
            vec!["payload".into()],
            vec!["primary".into(), "payload".into()],
        ];
        let payload = format!("{:?}", Value::text("payload"));
        assert_eq!(
            segment.find_unique_key_conflict(
                &definitions,
                1,
                &[payload.clone()],
                None,
                &|entity| entity.id != EntityId::new(1)
            ),
            Some(EntityId::new(2))
        );
        assert_eq!(segment.unique_key_lookup.read().len(), 3);
        segment
            .insert(primary_key_row(3, Value::Integer(30)))
            .expect("append");
        assert_eq!(
            segment.find_unique_key_conflict(
                &definitions,
                2,
                &[format!("{:?}", Value::Integer(30)), payload.clone()],
                None,
                &|_| true
            ),
            Some(EntityId::new(3))
        );
        let bytes = segment.unique_key_lookup_bytes.load(Ordering::Relaxed);
        assert_eq!(
            segment.find_unique_key_conflict(
                &definitions,
                0,
                &[format!("{:?}", Value::Integer(20))],
                None,
                &|_| true
            ),
            Some(EntityId::new(2))
        );
        assert_eq!(segment.unique_key_lookup.read().len(), 3);
        assert_eq!(
            segment.unique_key_lookup_bytes.load(Ordering::Relaxed),
            bytes
        );
        let reduced = vec![vec!["payload".into()]];
        assert_eq!(
            segment.find_unique_key_conflict(&reduced, 0, &[payload], None, &|_| true),
            Some(EntityId::new(1))
        );
        assert_eq!(segment.unique_key_lookup.read().len(), 1);
        assert!(segment.unique_key_lookup_bytes.load(Ordering::Relaxed) < bytes);
    }

    #[test]
    fn unique_key_lookup_preserves_signature_semantics_and_exclusion() {
        let mut segment = GrowingSegment::new(1, "keys");
        let values = [
            Value::Float(f64::NAN),
            Value::Float(-0.0),
            Value::Float(0.0),
            Value::Integer(1),
            Value::UnsignedInteger(1),
            Value::text("1"),
        ];
        for (offset, value) in values.iter().enumerate() {
            let id = offset as u64 + 1;
            segment
                .insert(primary_key_row(id, value.clone()))
                .expect("distinct physical row");
            assert_eq!(
                primary_key_probe(&segment, value.clone(), None),
                Some(EntityId::new(id))
            );
            assert_eq!(
                primary_key_probe(&segment, value.clone(), Some(EntityId::new(id))),
                None
            );
        }
        segment
            .insert(primary_key_row(20, Value::Integer(1)))
            .expect("duplicate physical key");
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(1), Some(EntityId::new(4))),
            Some(EntityId::new(20))
        );
        // Candidate fingerprints are not equality proof: simulate a stale or
        // colliding entry by changing its row without the normal invalidation.
        segment
            .entities
            .insert(EntityId::new(4), primary_key_row(4, Value::Integer(99)));
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(1), Some(EntityId::new(20))),
            None
        );
        // Public mutable access must invalidate before handing out the row.
        let entity = segment.get_mut(EntityId::new(4)).expect("mutable row");
        *entity = primary_key_row(4, Value::Integer(100));
        assert_eq!(
            primary_key_probe(&segment, Value::Integer(100), None),
            Some(EntityId::new(4))
        );
    }

    #[test]
    fn test_growing_segment_basic() {
        let mut segment = GrowingSegment::new(1, "test");

        let entity = UnifiedEntity::table_row(
            EntityId::new(1),
            "users",
            1,
            vec![Value::text("Alice".to_string())],
        );

        let id = segment.insert(entity).unwrap();
        assert_eq!(id, EntityId::new(1));
        assert!(segment.contains(id));

        let stats = segment.stats();
        assert_eq!(stats.entity_count, 1);
        assert_eq!(stats.row_count, 1);
    }

    #[test]
    fn test_segment_metadata() {
        let mut segment = GrowingSegment::new(1, "test");

        let entity = UnifiedEntity::table_row(
            EntityId::new(1),
            "users",
            1,
            vec![Value::text("Alice".to_string())],
        );
        segment.insert(entity).unwrap();

        let mut meta = Metadata::new();
        meta.set("role", MetadataValue::String("admin".to_string()));
        meta.set("level", MetadataValue::Int(5));

        segment.set_metadata(EntityId::new(1), meta).unwrap();

        let retrieved = segment.get_metadata(EntityId::new(1)).unwrap();
        assert_eq!(
            retrieved.get("role"),
            Some(&MetadataValue::String("admin".to_string()))
        );
    }

    #[test]
    fn test_segment_seal() {
        let mut segment = GrowingSegment::new(1, "test");

        let entity = UnifiedEntity::vector(EntityId::new(1), "embeddings", vec![0.1, 0.2, 0.3]);
        segment.insert(entity).unwrap();

        // Can write before sealing
        assert!(segment.state().is_writable());

        // Seal the segment
        segment.seal().unwrap();
        assert_eq!(segment.state(), SegmentState::Sealed);

        // Cannot write after sealing
        let entity2 = UnifiedEntity::vector(EntityId::new(2), "embeddings", vec![0.4, 0.5, 0.6]);
        assert!(segment.insert(entity2).is_err());
    }

    #[test]
    fn test_should_seal() {
        let mut segment = GrowingSegment::new(1, "test");

        let config = SegmentConfig {
            max_entities: 2,
            ..Default::default()
        };

        assert!(!segment.should_seal(&config));

        segment
            .insert(UnifiedEntity::vector(EntityId::new(1), "v", vec![0.1]))
            .unwrap();
        assert!(!segment.should_seal(&config));

        segment
            .insert(UnifiedEntity::vector(EntityId::new(2), "v", vec![0.2]))
            .unwrap();
        assert!(segment.should_seal(&config));
    }

    #[test]
    fn test_cross_references() {
        let mut segment = GrowingSegment::new(1, "test");

        let mut entity1 = UnifiedEntity::table_row(
            EntityId::new(1),
            "hosts",
            1,
            vec![Value::text("192.168.1.1".to_string())],
        );
        entity1.add_cross_ref(CrossRef::new(
            EntityId::new(1),
            EntityId::new(2),
            "nodes",
            RefType::RowToNode,
        ));
        segment.insert(entity1).unwrap();

        let refs_from = segment.get_references_from(EntityId::new(1));
        assert_eq!(refs_from.len(), 1);
        assert_eq!(refs_from[0], (EntityId::new(2), RefType::RowToNode));

        let refs_to = segment.get_references_to(EntityId::new(2));
        assert_eq!(refs_to.len(), 1);
        assert_eq!(refs_to[0], (EntityId::new(1), RefType::RowToNode));
    }

    #[test]
    fn test_zone_predicate_uses_canonical_fallback_for_email_values() {
        let mut zone = ColZone::new(Value::Email("bravo@example.com".to_string()));
        zone.update(&Value::Email("delta@example.com".to_string()));

        let probe = Value::Email("alpha@example.com".to_string());
        assert!(ZoneColPred::Eq(&probe).can_skip(&zone));

        let in_range = Value::Email("charlie@example.com".to_string());
        assert!(!ZoneColPred::Eq(&in_range).can_skip(&zone));
    }

    #[test]
    fn test_sealed_multi_zone_prunes_numeric_gap_outlier() {
        let mut segment = GrowingSegment::new(1, "test");

        for (row_id, age) in [(1_u64, 1_i64), (2, 2), (3, 3), (4, 1000)] {
            let entity = UnifiedEntity::new(
                EntityId::new(row_id),
                EntityKind::TableRow {
                    table: "users".into(),
                    row_id,
                },
                EntityData::Row(RowData::with_names(
                    vec![Value::Integer(age)],
                    vec!["age".to_string()],
                )),
            );
            segment.insert(entity).unwrap();
        }

        segment.seal().unwrap();

        let miss = Value::Integer(500);
        assert!(segment.can_skip_zone_preds(&[("age", ZoneColPred::Eq(&miss))]));

        let hit = Value::Integer(1000);
        assert!(!segment.can_skip_zone_preds(&[("age", ZoneColPred::Eq(&hit))]));
    }
}