uni_store/runtime/

l0.rs

// SPDX-License-Identifier: Apache-2.0
// Copyright 2024-2026 Dragonscale Team

use crate::runtime::wal::{Mutation, WriteAheadLog};
use anyhow::Result;
use std::collections::{HashMap, HashSet};
use std::sync::Arc;
use std::time::{SystemTime, UNIX_EPOCH};
use tracing::{instrument, trace};
use uni_common::core::id::{Eid, Vid};
use uni_common::graph::simple_graph::{Direction, SimpleGraph};
use uni_common::{Properties, Value};
use uni_crdt::Crdt;

/// Items a read-write transaction observed during execution, used for SSI
/// read-write antidependency detection at commit.
///
/// Shared (via `Arc<Mutex<_>>`) between the read path — which records reads
/// through the transaction's `QueryContext` — and the commit path, which checks
/// it against concurrently-committed write-sets. Item-level granularity;
/// phantoms are out of scope (handled by the `FOR UPDATE` escape hatch).
#[derive(Debug, Default)]
pub struct OccReadSet {
    /// Vertices the transaction read.
    pub vertices: HashSet<Vid>,
    /// Edges the transaction read.
    pub edges: HashSet<Eid>,
}

impl OccReadSet {
    /// `true` when nothing has been read yet. Used to decide whether a `FOR
    /// UPDATE` acquisition may safely re-pin a still-fresh transaction.
    pub fn is_empty(&self) -> bool {
        self.vertices.is_empty() && self.edges.is_empty()
    }
}

/// Returns the current timestamp in nanoseconds since Unix epoch.
fn now_nanos() -> i64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_nanos() as i64)
        .unwrap_or(0)
}

/// Returns the [`Crdt`] a property value encodes, or `None` if it is not one.
///
/// `Crdt` is `#[serde(tag = "t", content = "d")]`, so it deserializes only from a
/// JSON object, and only `Value::Map(_)` produces one — gating on `Map` avoids
/// allocating a JSON tree for large non-map values (e.g. embedding columns).
///
/// This is the single source of truth for "is this value CRDT-mergeable": both
/// the commit-time merge ([`L0Buffer::merge_crdt_properties`]) and the OCC
/// write-set carve-out ([`crate::runtime::occ::WriteSet::from_l0`]) consult it,
/// so the carve-out can never exclude an item the merge would actually overwrite
/// (which would silently lose an update).
pub(crate) fn try_as_crdt(v: &Value) -> Option<Crdt> {
    if !matches!(v, Value::Map(_)) {
        return None;
    }
    serde_json::from_value::<Crdt>(v.clone().into()).ok()
}

/// Serialize a constraint key for O(1) uniqueness checks.
/// Format: label + separator + sorted (prop_name, value) pairs.
pub fn serialize_constraint_key(label: &str, key_values: &[(String, Value)]) -> Vec<u8> {
    let mut buf = label.as_bytes().to_vec();
    buf.push(0); // separator
    let mut sorted = key_values.to_vec();
    sorted.sort_by(|a, b| a.0.cmp(&b.0));
    for (k, v) in &sorted {
        buf.extend(k.as_bytes());
        buf.push(0);
        // Use serde_json serialization for deterministic value encoding
        buf.extend(serde_json::to_vec(v).unwrap_or_default());
        buf.push(0);
    }
    buf
}

/// Per-type mutation counters accumulated by the L0 buffer.
///
/// Used to provide detailed mutation statistics (e.g., `nodes_created`,
/// `relationships_deleted`) on `ExecuteResult`. Callers snapshot before/after
/// execution and call [`diff()`](MutationStats::diff) to get the delta.
#[derive(Debug, Clone, Default)]
pub struct MutationStats {
    pub nodes_created: usize,
    pub nodes_deleted: usize,
    pub relationships_created: usize,
    pub relationships_deleted: usize,
    pub properties_set: usize,
    pub properties_removed: usize,
    pub labels_added: usize,
    pub labels_removed: usize,
}

impl MutationStats {
    /// Compute the field-wise difference `self - before`.
    pub fn diff(&self, before: &Self) -> Self {
        Self {
            nodes_created: self.nodes_created.saturating_sub(before.nodes_created),
            nodes_deleted: self.nodes_deleted.saturating_sub(before.nodes_deleted),
            relationships_created: self
                .relationships_created
                .saturating_sub(before.relationships_created),
            relationships_deleted: self
                .relationships_deleted
                .saturating_sub(before.relationships_deleted),
            properties_set: self.properties_set.saturating_sub(before.properties_set),
            properties_removed: self
                .properties_removed
                .saturating_sub(before.properties_removed),
            labels_added: self.labels_added.saturating_sub(before.labels_added),
            labels_removed: self.labels_removed.saturating_sub(before.labels_removed),
        }
    }
}

#[derive(Clone, Debug)]
pub struct TombstoneEntry {
    pub eid: Eid,
    pub src_vid: Vid,
    pub dst_vid: Vid,
    pub edge_type: u32,
}

pub struct L0Buffer {
    /// Graph topology using simple adjacency lists
    pub graph: SimpleGraph,
    /// Soft-deleted edges (tombstones for LSM-style merging)
    pub tombstones: HashMap<Eid, TombstoneEntry>,
    /// Soft-deleted vertices
    pub vertex_tombstones: HashSet<Vid>,
    /// Edge version tracking for MVCC
    pub edge_versions: HashMap<Eid, u64>,
    /// Vertex version tracking for MVCC
    pub vertex_versions: HashMap<Vid, u64>,
    /// Edge properties (stored separately from topology)
    pub edge_properties: HashMap<Eid, Properties>,
    /// Vertex properties (stored separately from topology)
    pub vertex_properties: HashMap<Vid, Properties>,
    /// Edge endpoint lookup: eid -> (src, dst, type)
    pub edge_endpoints: HashMap<Eid, (Vid, Vid, u32)>,
    /// Vertex labels (VID -> list of label names)
    /// New in storage design: vertices can have multiple labels
    pub vertex_labels: HashMap<Vid, Vec<String>>,
    /// Reverse index: label name → set of VIDs with that label. Maintained
    /// alongside `vertex_labels` for O(1) label-based vertex lookups.
    pub label_to_vids: HashMap<String, HashSet<Vid>>,
    /// Vids whose FULL label set was explicitly replaced by a label mutation
    /// (`SET n:Label` / `REMOVE n:Label`) in this buffer, via
    /// [`L0Buffer::set_vertex_labels`]. Distinguishes a deliberate label
    /// replacement from the empty `vertex_labels` entry a property-only write
    /// incidentally creates (`entry().or_default()`), so `merge` knows to REPLACE
    /// (not append) these vids' labels and `WriteSet::from_l0` knows they are
    /// conflictable writes. A transaction-buffer concept; empty on main L0.
    pub vertex_label_overwrites: HashSet<Vid>,
    /// Edge types (EID -> type name)
    pub edge_types: HashMap<Eid, String>,
    /// Current version counter
    pub current_version: u64,
    /// Mutation count for flush decisions
    pub mutation_count: usize,
    /// Per-type mutation counters for detailed statistics.
    pub mutation_stats: MutationStats,
    /// Write-ahead log for durability
    pub wal: Option<Arc<WriteAheadLog>>,
    /// WAL LSN at the time this L0 was rotated for flush.
    /// Used to ensure WAL truncation doesn't remove entries needed by pending flushes.
    pub wal_lsn_at_flush: u64,
    /// Vertex creation timestamps (nanoseconds since epoch)
    pub vertex_created_at: HashMap<Vid, i64>,
    /// Vertex update timestamps (nanoseconds since epoch)
    pub vertex_updated_at: HashMap<Vid, i64>,
    /// Edge creation timestamps (nanoseconds since epoch)
    pub edge_created_at: HashMap<Eid, i64>,
    /// Edge update timestamps (nanoseconds since epoch)
    pub edge_updated_at: HashMap<Eid, i64>,
    /// Estimated size in bytes for memory limit enforcement.
    /// Incremented O(1) on each mutation to avoid O(V+E) size_bytes() calls.
    pub estimated_size: usize,
    /// Per-constraint index for O(1) unique key checks.
    /// Key: constraint composite key (label + sorted property values serialized).
    /// Value: Vid that owns this key.
    pub constraint_index: HashMap<Vec<u8>, Vid>,
    /// Implicit MERGE-key guard for phantom-free `MERGE` *without* a declared
    /// `UNIQUE` constraint. Same key format as `constraint_index` (built by
    /// [`serialize_constraint_key`]), but populated only by a `MERGE` that
    /// *creates* a node, and re-probed at commit only against other concurrent
    /// `MERGE`-creates — so two concurrent `MERGE`s of the same key converge to
    /// one node (the loser aborts retriably) instead of silently duplicating,
    /// while a plain `CREATE` of the same properties is unaffected (it never
    /// registers a key). Tombstoned with the owning vid; transient (not rebuilt
    /// on recovery).
    pub merge_guard_index: HashMap<Vec<u8>, Vid>,
    /// Reverse index `ext_id` → owning vid for O(1) global ext_id uniqueness
    /// checks (`Writer::check_extid_globally_unique` previously scanned every
    /// `vertex_properties` map per insert — O(n²) ingest). Maintained by the
    /// vertex insert impls (synced to the post-CRDT-merge value) and by
    /// `apply_vertex_deletion`, so merge and WAL replay keep it consistent
    /// for free.
    pub extid_index: HashMap<String, Vid>,
    /// Per-VID set of property keys that should land via Lance MergeInsert
    /// (partial-column update) at flush time. Populated by
    /// `insert_vertex_partial`; cleared by full-row inserts and deletes.
    /// A VID present here at flush time is emitted to the partial batch;
    /// absent VIDs flush via the existing full-row Append.
    pub vertex_partial_keys: HashMap<Vid, HashSet<String>>,
    /// Edge analog of `vertex_partial_keys` (Round 12 §A). Populated by
    /// `insert_edge_partial_full`; cleared by full-row inserts and edge
    /// deletes. Per-edge-type delta-table flush honors these by emitting
    /// a `MergeInsertBuilder` source with only the touched schema
    /// columns plus `eid`, `op`, `_version`, `_updated_at`, and
    /// `overflow_json` (when an overflow prop was touched).
    pub edge_partial_keys: HashMap<Eid, HashSet<String>>,
    /// Phase B (UniConfig::defer_embeddings): VIDs whose auto-embedding
    /// was skipped at insert time and is owed at flush. Value = primary
    /// label name (the rest of the embedding config is looked up from the
    /// schema at flush time). Drained by `flush_stream_l1` before column
    /// extraction; entries are removed when the embedding lands in the
    /// vertex's L0 property map.
    pub pending_embeddings: HashMap<Vid, String>,
    /// Optimistic-concurrency read sequence (SSI). Stamped on a transaction's
    /// private L0 at creation with the Writer's commit-sequence at that moment,
    /// and consulted at commit to detect intervening conflicting commits. `0`
    /// for the main L0 and when SSI is disabled.
    pub occ_read_seq: u64,
    /// Optimistic-concurrency read-set (SSI). `Some` on a read-write
    /// transaction's private L0 when SSI tracking is active; the read path
    /// records observed ids here and commit checks them for antidependencies.
    /// `None` for the main L0 and read-only / SSI-disabled paths.
    pub occ_read_set: Option<Arc<parking_lot::Mutex<OccReadSet>>>,
}

impl std::fmt::Debug for L0Buffer {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("L0Buffer")
            .field("vertex_count", &self.graph.vertex_count())
            .field("edge_count", &self.graph.edge_count())
            .field("tombstones", &self.tombstones.len())
            .field("vertex_tombstones", &self.vertex_tombstones.len())
            .field("current_version", &self.current_version)
            .field("mutation_count", &self.mutation_count)
            .finish()
    }
}

impl Clone for L0Buffer {
    /// Clone the L0 buffer for fork/restore (ASSUME/ABDUCE).
    ///
    /// The cloned buffer does NOT share the WAL reference — forked L0s are
    /// ephemeral and should not write to the WAL.
    fn clone(&self) -> Self {
        Self {
            graph: self.graph.clone(),
            tombstones: self.tombstones.clone(),
            vertex_tombstones: self.vertex_tombstones.clone(),
            edge_versions: self.edge_versions.clone(),
            vertex_versions: self.vertex_versions.clone(),
            edge_properties: self.edge_properties.clone(),
            vertex_properties: self.vertex_properties.clone(),
            edge_endpoints: self.edge_endpoints.clone(),
            vertex_labels: self.vertex_labels.clone(),
            label_to_vids: self.label_to_vids.clone(),
            vertex_label_overwrites: self.vertex_label_overwrites.clone(),
            edge_types: self.edge_types.clone(),
            current_version: self.current_version,
            mutation_count: self.mutation_count,
            mutation_stats: self.mutation_stats.clone(),
            wal: None, // Forked L0s don't share the WAL
            wal_lsn_at_flush: self.wal_lsn_at_flush,
            vertex_created_at: self.vertex_created_at.clone(),
            vertex_updated_at: self.vertex_updated_at.clone(),
            edge_created_at: self.edge_created_at.clone(),
            edge_updated_at: self.edge_updated_at.clone(),
            estimated_size: self.estimated_size,
            constraint_index: self.constraint_index.clone(),
            merge_guard_index: self.merge_guard_index.clone(),
            extid_index: self.extid_index.clone(),
            vertex_partial_keys: self.vertex_partial_keys.clone(),
            edge_partial_keys: self.edge_partial_keys.clone(),
            pending_embeddings: self.pending_embeddings.clone(),
            occ_read_seq: self.occ_read_seq,
            // Forked L0s (ASSUME/ABDUCE) do not participate in OCC tracking.
            occ_read_set: None,
        }
    }
}

impl L0Buffer {
    /// Append labels to a vec, skipping duplicates.
    fn append_unique_labels(existing: &mut Vec<String>, labels: &[String]) {
        for label in labels {
            if !existing.contains(label) {
                existing.push(label.clone());
            }
        }
    }

    /// Add a VID to the reverse label index for each of the given labels.
    fn index_labels_for_vid(&mut self, vid: Vid, labels: &[String]) {
        for label in labels {
            self.label_to_vids
                .entry(label.clone())
                .or_default()
                .insert(vid);
        }
    }

    /// Read a vertex's current string `ext_id` from a property map.
    fn extid_of(props: &Properties) -> Option<String> {
        props
            .get("ext_id")
            .and_then(|v| v.as_str())
            .map(str::to_owned)
    }

    /// Sync `extid_index` for `vid` around a property write.
    ///
    /// `old` / `new` are the vertex's ext_id before / after the CRDT merge —
    /// the index always reflects the post-merge value, matching what the old
    /// full scan in `Writer::check_extid_globally_unique` observed. Only
    /// called when the incoming properties contain an `ext_id` key (the merge
    /// cannot change the value otherwise).
    fn sync_extid_index(&mut self, vid: Vid, old: Option<String>, new: Option<String>) {
        if old == new {
            return;
        }
        if let Some(old) = old
            && self.extid_index.get(&old) == Some(&vid)
        {
            self.extid_index.remove(&old);
        }
        if let Some(new) = new {
            self.extid_index.insert(new, vid);
        }
    }

    /// Remove a VID from all label entries in the reverse index.
    fn remove_vid_from_label_index(&mut self, vid: Vid) {
        if let Some(labels) = self.vertex_labels.get(&vid) {
            for label in labels {
                if let Some(set) = self.label_to_vids.get_mut(label) {
                    set.remove(&vid);
                }
            }
        }
    }

    /// Replaces a vertex's FULL label set — the semantics of `SET n:Label` /
    /// `REMOVE n:Label`, which resolve the new complete set before writing.
    ///
    /// Unlike [`add_vertex_labels`](Self::add_vertex_labels) (append), this clears
    /// the vid's existing labels from the reverse index, sets the new set, and
    /// re-indexes — so a removal actually removes. It marks the vid in
    /// `vertex_label_overwrites` so `merge` REPLACES (not appends) these labels at
    /// commit and `WriteSet::from_l0` treats the change as a conflictable write.
    /// Increments `mutation_count` (a label change is a real mutation; its sibling
    /// `remove_vertex_label` already does so).
    pub fn set_vertex_labels(&mut self, vid: Vid, labels: &[String]) {
        self.remove_vid_from_label_index(vid);
        self.vertex_labels.insert(vid, labels.to_vec());
        self.index_labels_for_vid(vid, labels);
        self.vertex_label_overwrites.insert(vid);
        self.current_version += 1;
        self.mutation_count += 1;
    }

    /// Merge CRDT properties into an existing property map.
    /// Attempts CRDT merge if both values are valid CRDTs, falls back to overwrite.
    ///
    /// When the entry is empty (new vertex insert), skips the expensive JSON
    /// round-trip and directly assigns the properties.
    ///
    /// Logs a warning when a CRDT value is overwritten by a non-CRDT scalar
    /// (limitation R1).
    fn merge_crdt_properties(entry: &mut Properties, properties: Properties) {
        // Fast path: new vertex with no existing properties — skip JSON round-trip
        if entry.is_empty() {
            *entry = properties;
            return;
        }

        for (k, v) in properties {
            // `try_as_crdt` performs the Map-gated CRDT probe (see its docs for the
            // wide-row perf rationale). Sharing it with `WriteSet::from_l0` keeps the
            // OCC carve-out consistent with this merge-versus-overwrite decision.
            if let Some(mut new_crdt) = try_as_crdt(&v)
                && let Some(existing_v) = entry.get(&k)
                && let Ok(existing_crdt) = serde_json::from_value::<Crdt>(existing_v.clone().into())
            {
                // Use try_merge to avoid panic on type mismatch.
                if new_crdt.try_merge(&existing_crdt).is_ok()
                    && let Ok(merged_json) = serde_json::to_value(new_crdt)
                {
                    entry.insert(k, uni_common::Value::from(merged_json));
                    continue;
                }
                // CRDT variant mismatch (or a failed re-serialize): fall through to
                // a last-writer-wins overwrite, discarding the existing CRDT's
                // merged state. The OCC commit-time carve-out check
                // (`occ::crdt_carveout_overwrite`) aborts a *concurrent* writer
                // before reaching here, and write-time schema enforcement rejects a
                // wrong declared variant; this warns on any residual (e.g. a
                // single-writer variant change) so the discarded state is visible.
                tracing::warn!(
                    property = %k,
                    existing_variant = existing_crdt.type_name(),
                    "overwriting CRDT property with a different CRDT variant \
                     (last-writer-wins); merged CRDT state is discarded"
                );
            } else if try_as_crdt(&v).is_none()
                && entry.get(&k).is_some_and(|e| try_as_crdt(e).is_some())
            {
                // R1: an existing CRDT value is overwritten by a non-CRDT scalar
                // (last-writer-wins), silently discarding the CRDT's merged state
                // — a property written as BOTH a CRDT and last-writer-wins. The OCC
                // write-set carve-out lets CRDT-only writers commit without
                // conflicting, so a concurrent LWW write on the same property
                // cannot be flagged as a conflict; surface it here instead.
                tracing::warn!(
                    property = %k,
                    "overwriting CRDT property with non-CRDT value (last-writer-wins); \
                     merged CRDT state is discarded"
                );
            }
            // Fallback: Overwrite (last-writer-wins).
            entry.insert(k, v);
        }
    }

    /// Helper function to estimate property map size in bytes.
    fn estimate_properties_size(props: &Properties) -> usize {
        props.keys().map(|k| k.len() + 32).sum()
    }

    /// Returns an estimate of the buffer size in bytes.
    /// Includes all fields for accurate memory accounting.
    pub fn size_bytes(&self) -> usize {
        let mut total = 0;

        // Topology
        total += self.graph.vertex_count() * 8;
        total += self.graph.edge_count() * 24;

        // Properties (rough estimate: key string + 32 bytes for value)
        for props in self.vertex_properties.values() {
            total += Self::estimate_properties_size(props);
        }
        for props in self.edge_properties.values() {
            total += Self::estimate_properties_size(props);
        }

        // Metadata
        total += self.tombstones.len() * 64;
        total += self.vertex_tombstones.len() * 8;
        total += self.edge_versions.len() * 16;
        total += self.vertex_versions.len() * 16;
        total += self.edge_endpoints.len() * 28; // (Vid, Vid, u32) = 8+8+4 + overhead

        // Vertex labels
        for labels in self.vertex_labels.values() {
            total += labels.iter().map(|l| l.len() + 24).sum::<usize>();
        }

        // Reverse label index (label_to_vids)
        for (label, vids) in &self.label_to_vids {
            total += label.len() + 24 + vids.len() * 8 + 48; // string + HashSet overhead
        }

        // Edge types
        for type_name in self.edge_types.values() {
            total += type_name.len() + 24;
        }

        // Timestamps (4 maps, each entry is 16 bytes: 8-byte key + 8-byte i64 value)
        total += self.vertex_created_at.len() * 16;
        total += self.vertex_updated_at.len() * 16;
        total += self.edge_created_at.len() * 16;
        total += self.edge_updated_at.len() * 16;

        total
    }

    pub fn new(start_version: u64, wal: Option<Arc<WriteAheadLog>>) -> Self {
        Self {
            graph: SimpleGraph::new(),
            tombstones: HashMap::new(),
            vertex_tombstones: HashSet::new(),
            edge_versions: HashMap::new(),
            vertex_versions: HashMap::new(),
            edge_properties: HashMap::new(),
            vertex_properties: HashMap::new(),
            edge_endpoints: HashMap::new(),
            vertex_labels: HashMap::new(),
            label_to_vids: HashMap::new(),
            vertex_label_overwrites: HashSet::new(),
            edge_types: HashMap::new(),
            current_version: start_version,
            mutation_count: 0,
            mutation_stats: MutationStats::default(),
            wal,
            wal_lsn_at_flush: 0,
            vertex_created_at: HashMap::new(),
            vertex_updated_at: HashMap::new(),
            edge_created_at: HashMap::new(),
            edge_updated_at: HashMap::new(),
            estimated_size: 0,
            constraint_index: HashMap::new(),
            merge_guard_index: HashMap::new(),
            extid_index: HashMap::new(),
            vertex_partial_keys: HashMap::new(),
            edge_partial_keys: HashMap::new(),
            pending_embeddings: HashMap::new(),
            occ_read_seq: 0,
            occ_read_set: None,
        }
    }

    pub fn insert_vertex(&mut self, vid: Vid, properties: Properties) {
        self.insert_vertex_with_labels(vid, properties, &[]);
    }

    /// Insert a vertex with associated labels.
    pub fn insert_vertex_with_labels(
        &mut self,
        vid: Vid,
        properties: Properties,
        labels: &[String],
    ) {
        self.insert_vertex_with_labels_impl(vid, properties, labels, false);
    }

    /// Core vertex insertion. When `skip_wal` is true, skips WAL append
    /// (used during merge where the caller already wrote to WAL).
    fn insert_vertex_with_labels_impl(
        &mut self,
        vid: Vid,
        properties: Properties,
        labels: &[String],
        skip_wal: bool,
    ) {
        self.current_version += 1;
        let version = self.current_version;
        let now = now_nanos();

        if !skip_wal && let Some(wal) = &self.wal {
            let _ = wal.append(Mutation::InsertVertex {
                vid,
                properties: properties.clone(),
                labels: labels.to_vec(),
            });
        }

        self.vertex_tombstones.remove(&vid);

        // Full-row insert supersedes any pending partial-update state for
        // this VID.
        self.vertex_partial_keys.remove(&vid);

        // Size/count computed up front so `properties` can be moved into the
        // CRDT merge below instead of deep-cloned.
        let props_size = Self::estimate_properties_size(&properties);
        let props_count = properties.len();
        let tracks_extid = properties.contains_key("ext_id");

        let entry = self.vertex_properties.entry(vid).or_default();
        let old_extid = if tracks_extid {
            Self::extid_of(entry)
        } else {
            None
        };
        Self::merge_crdt_properties(entry, properties);
        if tracks_extid {
            let new_extid =
                Self::extid_of(self.vertex_properties.get(&vid).expect("just inserted"));
            self.sync_extid_index(vid, old_extid, new_extid);
        }
        self.vertex_versions.insert(vid, version);

        // Set timestamps - created_at only set if this is a new vertex
        self.vertex_created_at.entry(vid).or_insert(now);
        self.vertex_updated_at.insert(vid, now);

        // Track labels — always create an entry so unlabeled vertices are
        // distinguishable from "not in L0" when queried via get_vertex_labels.
        let labels_size: usize = labels.iter().map(|l| l.len() + 24).sum();
        let existing = self.vertex_labels.entry(vid).or_default();
        Self::append_unique_labels(existing, labels);
        self.index_labels_for_vid(vid, labels);

        self.graph.add_vertex(vid);
        self.mutation_count += 1;
        self.mutation_stats.nodes_created += 1;
        self.mutation_stats.properties_set += props_count;
        self.mutation_stats.labels_added += labels.len();

        self.estimated_size += 8 + props_size + 16 + labels_size + 32;
    }

    /// Insert a vertex's FULL property row, tagging `touched_keys` so the
    /// flush emits exactly those columns via Lance `MergeInsertBuilder`
    /// instead of a full-row Append.
    ///
    /// `props` MUST be the fully-merged property map (storage union
    /// in-flight L0 union the new touched values, per
    /// `PropertyManager::get_all_vertex_props_with_ctx`). The caller is
    /// responsible for the union; L0 here just stores it so scans see
    /// the complete row without per-key reconciliation.
    ///
    /// `touched_keys` lists the property keys this SET statement
    /// actually assigned — the union of those across all coalesced
    /// SetItems on this VID. Lance MergeInsert sends a source batch
    /// with `_vid`, `_deleted`, `_version`, `_updated_at`, and those
    /// touched columns; non-touched columns retain their pre-merge
    /// values on the Lance side, skipping the wide-row write.
    ///
    /// A subsequent full-row `insert_vertex_with_labels` or
    /// `delete_vertex` on the same VID clears the partial-keys entry
    /// so partial state never outlives a stronger write.
    pub fn insert_vertex_partial_full(
        &mut self,
        vid: Vid,
        props: Properties,
        touched_keys: HashSet<String>,
        labels: &[String],
    ) {
        // Stage the full row through the existing partial-impl (same
        // CRDT merge / version bump / timestamps), preserving the
        // partial-keys entry so we can extend it below.
        self.insert_vertex_with_labels_partial_impl(vid, props, labels, false);
        self.vertex_partial_keys
            .entry(vid)
            .or_default()
            .extend(touched_keys);
    }

    /// Legacy partial-only variant used by some uni-store paths. Kept
    /// for source-compatibility but new uni-query callers should use
    /// `insert_vertex_partial_full` to preserve scan-side L0 visibility.
    pub fn insert_vertex_partial(&mut self, vid: Vid, touched: Properties, labels: &[String]) {
        // Record dirty keys BEFORE the full-row impl runs (which would
        // clear them). The keys come from the touched set; the values
        // are merged into L0 by the shared CRDT path below.
        let touched_keys: Vec<String> = touched.keys().cloned().collect();

        // If the VID already has a full-row pending insert (e.g., CREATE
        // earlier in the same tx), we must NOT downgrade it to partial.
        // Detected by: VID is in vertex_properties WITH a version stamp
        // AND not currently in vertex_partial_keys → it was written as
        // a full row recently. The conservative rule: only enable the
        // partial path when there's no full-row pending insert. We
        // approximate "no full-row pending" by checking that the VID's
        // current entry in vertex_partial_keys is non-empty OR the VID
        // is not in vertex_properties (fresh row, but caller asked
        // partial — let it through and the post-flush union covers it).
        let already_full = self.vertex_properties.contains_key(&vid)
            && !self.vertex_partial_keys.contains_key(&vid);

        // Stage the CRDT merge through the existing path. We bypass the
        // full-row `insert_vertex_with_labels_impl` clearing of
        // partial_keys by inlining the work, then restoring/extending
        // the partial-key set.
        self.insert_vertex_with_labels_partial_impl(vid, touched, labels, false);

        if !already_full {
            self.vertex_partial_keys
                .entry(vid)
                .or_default()
                .extend(touched_keys);
        }
    }

    /// Core partial-insert: same as `insert_vertex_with_labels_impl` but
    /// preserves any existing `vertex_partial_keys[vid]` entry so the
    /// caller can extend it after the merge.
    fn insert_vertex_with_labels_partial_impl(
        &mut self,
        vid: Vid,
        properties: Properties,
        labels: &[String],
        skip_wal: bool,
    ) {
        self.current_version += 1;
        let version = self.current_version;
        let now = now_nanos();

        if !skip_wal && let Some(wal) = &self.wal {
            // WAL records the partial as a full-row InsertVertex; on replay
            // the full-row path runs (which clears partial_keys). This is
            // semantically correct — L0 in memory always holds the union of
            // partial deltas via merge_crdt_properties; recovery doesn't
            // need to preserve partial-vs-full distinction.
            let _ = wal.append(Mutation::InsertVertex {
                vid,
                properties: properties.clone(),
                labels: labels.to_vec(),
            });
        }

        self.vertex_tombstones.remove(&vid);
        // NOTE: deliberately DOES NOT remove from vertex_partial_keys.
        // The caller (`insert_vertex_partial`) extends that set after.

        // Size/count computed up front so `properties` can be moved into the
        // CRDT merge below instead of deep-cloned.
        let props_size = Self::estimate_properties_size(&properties);
        let props_count = properties.len();
        let tracks_extid = properties.contains_key("ext_id");

        let entry = self.vertex_properties.entry(vid).or_default();
        let old_extid = if tracks_extid {
            Self::extid_of(entry)
        } else {
            None
        };
        Self::merge_crdt_properties(entry, properties);
        if tracks_extid {
            let new_extid =
                Self::extid_of(self.vertex_properties.get(&vid).expect("just inserted"));
            self.sync_extid_index(vid, old_extid, new_extid);
        }
        self.vertex_versions.insert(vid, version);

        self.vertex_created_at.entry(vid).or_insert(now);
        self.vertex_updated_at.insert(vid, now);

        let labels_size: usize = labels.iter().map(|l| l.len() + 24).sum();
        let existing = self.vertex_labels.entry(vid).or_default();
        Self::append_unique_labels(existing, labels);
        self.index_labels_for_vid(vid, labels);

        self.graph.add_vertex(vid);
        self.mutation_count += 1;
        // Partial writes don't create new nodes — they update existing ones.
        // But counting under properties_set is correct.
        self.mutation_stats.properties_set += props_count;
        self.mutation_stats.labels_added += labels.len();

        self.estimated_size += 8 + props_size + 16 + labels_size + 32;
    }

    /// Add labels to an existing vertex.
    pub fn add_vertex_labels(&mut self, vid: Vid, labels: &[String]) {
        let existing = self.vertex_labels.entry(vid).or_default();
        Self::append_unique_labels(existing, labels);
        self.index_labels_for_vid(vid, labels);
    }

    /// Remove a label from an existing vertex.
    /// Returns true if the label was found and removed, false otherwise.
    pub fn remove_vertex_label(&mut self, vid: Vid, label: &str) -> bool {
        if let Some(labels) = self.vertex_labels.get_mut(&vid)
            && let Some(pos) = labels.iter().position(|l| l == label)
        {
            labels.remove(pos);
            if let Some(set) = self.label_to_vids.get_mut(label) {
                set.remove(&vid);
            }
            self.current_version += 1;
            self.mutation_count += 1;
            self.mutation_stats.labels_removed += 1;
            // Note: WAL logging for label mutations not yet implemented
            // Currently consistent with add_vertex_labels behavior
            return true;
        }
        false
    }

    /// Set the type for an edge.
    pub fn set_edge_type(&mut self, eid: Eid, edge_type: String) {
        self.edge_types.insert(eid, edge_type);
    }

    pub fn delete_vertex(&mut self, vid: Vid) -> Result<()> {
        self.delete_vertex_impl(vid, false)
    }

    /// Core vertex deletion. When `skip_wal` is true, skips WAL append
    /// (used during merge where the caller already wrote to WAL).
    fn delete_vertex_impl(&mut self, vid: Vid, skip_wal: bool) -> Result<()> {
        self.current_version += 1;

        if !skip_wal && let Some(wal) = &mut self.wal {
            let labels = self.vertex_labels.get(&vid).cloned().unwrap_or_default();
            wal.append(Mutation::DeleteVertex { vid, labels })?;
        }

        self.apply_vertex_deletion(vid);
        Ok(())
    }

    /// Cascade-delete a vertex: tombstone all connected edges and remove the vertex.
    ///
    /// Shared between `delete_vertex` (live mutations) and `replay_mutations` (WAL recovery).
    fn apply_vertex_deletion(&mut self, vid: Vid) {
        let version = self.current_version;

        // Collect edges to delete using O(degree) neighbors() instead of O(E) scan
        let mut edges_to_remove = HashSet::new();

        // Collect outgoing edges
        for entry in self.graph.neighbors(vid, Direction::Outgoing) {
            edges_to_remove.insert(entry.eid);
        }

        // Collect incoming edges
        for entry in self.graph.neighbors(vid, Direction::Incoming) {
            edges_to_remove.insert(entry.eid); // HashSet handles self-loop deduplication
        }

        let cascaded_edges_count = edges_to_remove.len();

        // Tombstone and remove all collected edges
        for eid in edges_to_remove {
            // Retrieve edge endpoints from the map to create tombstone
            if let Some((src, dst, etype)) = self.edge_endpoints.get(&eid) {
                self.tombstones.insert(
                    eid,
                    TombstoneEntry {
                        eid,
                        src_vid: *src,
                        dst_vid: *dst,
                        edge_type: *etype,
                    },
                );
                self.edge_versions.insert(eid, version);
                self.edge_endpoints.remove(&eid);
                self.edge_properties.remove(&eid);
                self.graph.remove_edge(eid);
                self.mutation_count += 1;
                self.mutation_stats.relationships_deleted += 1;
            }
        }

        self.remove_vid_from_label_index(vid);
        self.vertex_tombstones.insert(vid);
        // Drop the vid's ext_id index entry (O(1) via its current property
        // value, read before the property map entry is removed below).
        if let Some(props) = self.vertex_properties.get(&vid)
            && let Some(ext) = Self::extid_of(props)
            && self.extid_index.get(&ext) == Some(&vid)
        {
            self.extid_index.remove(&ext);
        }
        self.vertex_properties.remove(&vid);
        // Deletion supersedes any pending partial-update state.
        self.vertex_partial_keys.remove(&vid);
        self.vertex_versions.insert(vid, version);
        self.graph.remove_vertex(vid);
        self.mutation_count += 1;
        self.mutation_stats.nodes_deleted += 1;

        // Remove constraint index entries for this vertex
        self.constraint_index.retain(|_, v| *v != vid);
        // Same for the implicit MERGE guard, so a later re-MERGE of a deleted
        // node's key does not false-conflict with the stale entry.
        self.merge_guard_index.retain(|_, v| *v != vid);

        // 64 bytes per edge tombstone + 8 for vertex tombstone
        self.estimated_size += cascaded_edges_count * 72 + 8;
    }

    pub fn insert_edge(
        &mut self,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        eid: Eid,
        properties: Properties,
        edge_type_name: Option<String>,
    ) -> Result<()> {
        self.insert_edge_impl(
            src_vid,
            dst_vid,
            edge_type,
            eid,
            properties,
            edge_type_name,
            false,
        )
    }

    /// Core edge insertion. When `skip_wal` is true, skips WAL append
    /// (used during merge where the caller already wrote to WAL).
    #[allow(clippy::too_many_arguments)]
    fn insert_edge_impl(
        &mut self,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        eid: Eid,
        properties: Properties,
        edge_type_name: Option<String>,
        skip_wal: bool,
    ) -> Result<()> {
        self.current_version += 1;
        let now = now_nanos();

        if !skip_wal && let Some(wal) = &mut self.wal {
            wal.append(Mutation::InsertEdge {
                src_vid,
                dst_vid,
                edge_type,
                eid,
                version: self.current_version,
                properties: properties.clone(),
                edge_type_name: edge_type_name.clone(),
            })?;
        }

        self.apply_edge_insertion(src_vid, dst_vid, edge_type, eid, properties)?;

        // Store edge type name in metadata if provided
        let type_name_size = if let Some(ref name) = edge_type_name {
            let size = name.len() + 24;
            self.edge_types.insert(eid, name.clone());
            size
        } else {
            0
        };

        // Set timestamps - created_at only set if this is a new edge
        self.edge_created_at.entry(eid).or_insert(now);
        self.edge_updated_at.insert(eid, now);

        // A full-row insert supersedes any pending partial-update state
        // for this EID (Round 12 §A).
        self.edge_partial_keys.remove(&eid);

        self.estimated_size += type_name_size;

        Ok(())
    }

    /// Insert an edge's FULL property row plus a touched-keys hint so the
    /// flush emits only those schema columns via Lance `MergeInsert` on
    /// the per-edge-type delta tables. Edge analog of
    /// `insert_vertex_partial_full` (Round 12 §A).
    #[allow(clippy::too_many_arguments)]
    pub fn insert_edge_partial_full(
        &mut self,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        eid: Eid,
        properties: Properties,
        edge_type_name: Option<String>,
        touched_keys: HashSet<String>,
    ) -> Result<()> {
        self.current_version += 1;
        let now = now_nanos();

        if let Some(wal) = &mut self.wal {
            wal.append(Mutation::InsertEdge {
                src_vid,
                dst_vid,
                edge_type,
                eid,
                version: self.current_version,
                properties: properties.clone(),
                edge_type_name: edge_type_name.clone(),
            })?;
        }

        self.apply_edge_insertion(src_vid, dst_vid, edge_type, eid, properties)?;

        // `apply_edge_insertion` cleared the partial-keys entry as a
        // safety measure (full-row insert supersedes partial). Re-insert
        // with the touched-keys hint so the flush emits a partial source.
        self.edge_partial_keys
            .entry(eid)
            .or_default()
            .extend(touched_keys);

        let type_name_size = if let Some(ref name) = edge_type_name {
            let size = name.len() + 24;
            self.edge_types.insert(eid, name.clone());
            size
        } else {
            0
        };

        self.edge_created_at.entry(eid).or_insert(now);
        self.edge_updated_at.insert(eid, now);

        self.estimated_size += type_name_size;

        Ok(())
    }

    /// Core edge insertion logic: add vertices, add edge, merge properties, update metadata.
    ///
    /// Shared between `insert_edge` (live mutations) and `replay_mutations` (WAL recovery).
    ///
    /// # Errors
    ///
    /// Returns error if either endpoint vertex has been deleted (exists in vertex_tombstones).
    /// This prevents "ghost vertex" resurrection via edge insertion. See issue #77.
    fn apply_edge_insertion(
        &mut self,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        eid: Eid,
        properties: Properties,
    ) -> Result<()> {
        let version = self.current_version;

        // Check if either endpoint has been deleted. Inserting an edge to a deleted
        // vertex would resurrect it as a "ghost vertex" with no properties. See issue #77.
        if self.vertex_tombstones.contains(&src_vid) {
            anyhow::bail!(
                "Cannot insert edge: source vertex {} has been deleted (issue #77)",
                src_vid
            );
        }
        if self.vertex_tombstones.contains(&dst_vid) {
            anyhow::bail!(
                "Cannot insert edge: destination vertex {} has been deleted (issue #77)",
                dst_vid
            );
        }

        // Add vertices to graph topology if they don't exist.
        // IMPORTANT: Only add to graph structure, do NOT call insert_vertex.
        // insert_vertex creates a new version with empty properties, which would
        // cause MVCC to pick the empty version as "latest", losing original properties.
        if !self.graph.contains_vertex(src_vid) {
            self.graph.add_vertex(src_vid);
        }
        if !self.graph.contains_vertex(dst_vid) {
            self.graph.add_vertex(dst_vid);
        }

        self.graph.add_edge(src_vid, dst_vid, eid, edge_type);

        // Store metadata with CRDT merge logic
        let props_size = Self::estimate_properties_size(&properties);
        let props_count = properties.len();
        if !properties.is_empty() {
            let entry = self.edge_properties.entry(eid).or_default();
            Self::merge_crdt_properties(entry, properties);
        }

        self.edge_versions.insert(eid, version);
        self.edge_endpoints
            .insert(eid, (src_vid, dst_vid, edge_type));
        self.tombstones.remove(&eid);
        self.mutation_count += 1;
        self.mutation_stats.relationships_created += 1;
        self.mutation_stats.properties_set += props_count;

        // 24 edge + props + 16 version + 28 endpoints + 32 timestamps
        self.estimated_size += 24 + props_size + 16 + 28 + 32;

        Ok(())
    }

    pub fn delete_edge(
        &mut self,
        eid: Eid,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
    ) -> Result<()> {
        self.delete_edge_impl(eid, src_vid, dst_vid, edge_type, false)
    }

    /// Core edge deletion. When `skip_wal` is true, skips WAL append
    /// (used during merge where the caller already wrote to WAL).
    fn delete_edge_impl(
        &mut self,
        eid: Eid,
        src_vid: Vid,
        dst_vid: Vid,
        edge_type: u32,
        skip_wal: bool,
    ) -> Result<()> {
        self.current_version += 1;
        let now = now_nanos();

        if !skip_wal && let Some(wal) = &mut self.wal {
            wal.append(Mutation::DeleteEdge {
                eid,
                src_vid,
                dst_vid,
                edge_type,
                version: self.current_version,
            })?;
        }

        self.apply_edge_deletion(eid, src_vid, dst_vid, edge_type);

        // Update timestamp - deletion is an update
        self.edge_updated_at.insert(eid, now);

        Ok(())
    }

    /// Core edge deletion logic: tombstone the edge, update version, remove from graph.
    ///
    /// Shared between `delete_edge` (live mutations) and `replay_mutations` (WAL recovery).
    fn apply_edge_deletion(&mut self, eid: Eid, src_vid: Vid, dst_vid: Vid, edge_type: u32) {
        let version = self.current_version;

        self.tombstones.insert(
            eid,
            TombstoneEntry {
                eid,
                src_vid,
                dst_vid,
                edge_type,
            },
        );
        self.edge_versions.insert(eid, version);
        // Deletion supersedes any pending partial-update state for this
        // EID (Round 12 §A).
        self.edge_partial_keys.remove(&eid);
        self.graph.remove_edge(eid);
        self.mutation_count += 1;
        self.mutation_stats.relationships_deleted += 1;

        // 64 bytes tombstone + 16 bytes version
        self.estimated_size += 80;
    }

    /// Returns neighbors in the specified direction.
    /// O(degree) complexity - iterates only edges connected to the vertex.
    pub fn get_neighbors(
        &self,
        vid: Vid,
        edge_type: u32,
        direction: Direction,
    ) -> Vec<(Vid, Eid, u64)> {
        let edges = self.graph.neighbors(vid, direction);

        edges
            .iter()
            .filter(|e| e.edge_type == edge_type && !self.is_tombstoned(e.eid))
            .map(|e| {
                let neighbor = match direction {
                    Direction::Outgoing => e.dst_vid,
                    Direction::Incoming => e.src_vid,
                };
                let version = self.edge_versions.get(&e.eid).copied().unwrap_or(0);
                (neighbor, e.eid, version)
            })
            .collect()
    }

    pub fn is_tombstoned(&self, eid: Eid) -> bool {
        self.tombstones.contains_key(&eid)
    }

    /// Returns all VIDs in vertex_labels that match the given label name.
    /// O(1) lookup via the reverse label index.
    pub fn vids_for_label(&self, label_name: &str) -> Vec<Vid> {
        self.label_to_vids
            .get(label_name)
            .map(|set| set.iter().copied().collect())
            .unwrap_or_default()
    }

    /// Returns all vertex VIDs in the L0 buffer.
    ///
    /// Used for schemaless scanning (MATCH (n) without label).
    pub fn all_vertex_vids(&self) -> Vec<Vid> {
        self.vertex_properties.keys().copied().collect()
    }

    /// Returns all VIDs in vertex_labels that match any of the given label names.
    /// Uses the reverse label index — O(sum of matching set sizes).
    pub fn vids_for_labels(&self, label_names: &[&str]) -> Vec<Vid> {
        let mut result = HashSet::new();
        for label_name in label_names {
            if let Some(set) = self.label_to_vids.get(*label_name) {
                result.extend(set.iter().copied());
            }
        }
        result.into_iter().collect()
    }

    /// Returns all VIDs that have ALL specified labels.
    /// Uses the reverse label index — intersects the per-label sets.
    pub fn vids_with_all_labels(&self, label_names: &[&str]) -> Vec<Vid> {
        if label_names.is_empty() {
            return Vec::new();
        }
        // Collect the per-label sets; if any label is missing from the index,
        // the intersection is empty.
        let sets: Vec<&HashSet<Vid>> = match label_names
            .iter()
            .map(|ln| self.label_to_vids.get(*ln))
            .collect::<Option<Vec<_>>>()
        {
            Some(s) => s,
            None => return Vec::new(),
        };
        // Start from the smallest set for efficiency.
        let smallest = sets.iter().min_by_key(|s| s.len()).unwrap();
        smallest
            .iter()
            .copied()
            .filter(|vid| sets.iter().all(|s| s.contains(vid)))
            .collect()
    }

    /// Gets the labels for a VID.
    pub fn get_vertex_labels(&self, vid: Vid) -> Option<&[String]> {
        self.vertex_labels.get(&vid).map(|v| v.as_slice())
    }

    /// Gets the edge type for an EID.
    pub fn get_edge_type(&self, eid: Eid) -> Option<&str> {
        self.edge_types.get(&eid).map(|s| s.as_str())
    }

    /// Returns all EIDs in edge_types that match the given type name.
    /// Used for L0 overlay during schemaless edge scanning.
    pub fn eids_for_type(&self, type_name: &str) -> Vec<Eid> {
        self.edge_types
            .iter()
            .filter(|(eid, etype)| *etype == type_name && !self.tombstones.contains_key(eid))
            .map(|(eid, _)| *eid)
            .collect()
    }

    /// Returns all edge EIDs in the L0 buffer (non-tombstoned).
    ///
    /// Used for schemaless scanning (`MATCH ()-[r]->()`) without type.
    pub fn all_edge_eids(&self) -> Vec<Eid> {
        self.edge_endpoints
            .keys()
            .filter(|eid| !self.tombstones.contains_key(eid))
            .copied()
            .collect()
    }

    /// Returns edge endpoint data (src_vid, dst_vid) for an EID.
    pub fn get_edge_endpoints(&self, eid: Eid) -> Option<(Vid, Vid)> {
        self.edge_endpoints
            .get(&eid)
            .map(|(src, dst, _)| (*src, *dst))
    }

    /// Returns full edge endpoint data (src_vid, dst_vid, edge_type_id) for an EID.
    pub fn get_edge_endpoint_full(&self, eid: Eid) -> Option<(Vid, Vid, u32)> {
        self.edge_endpoints.get(&eid).copied()
    }

    /// Insert a constraint key into the index for O(1) duplicate detection.
    pub fn insert_constraint_key(&mut self, key: Vec<u8>, vid: Vid) {
        self.constraint_index.insert(key, vid);
    }

    /// Check if a constraint key exists in the index, excluding a specific VID.
    /// Returns true if the key exists and is owned by a different vertex.
    pub fn has_constraint_key(&self, key: &[u8], exclude_vid: Vid) -> bool {
        self.constraint_index
            .get(key)
            .is_some_and(|&v| v != exclude_vid)
    }

    /// Register a MERGE-create's key into the implicit phantom guard.
    pub fn insert_merge_guard_key(&mut self, key: Vec<u8>, vid: Vid) {
        self.merge_guard_index.insert(key, vid);
    }

    /// Check if a MERGE-guard key exists, owned by a different vertex than
    /// `exclude_vid` — i.e. a concurrent MERGE already created this key.
    pub fn has_merge_guard_key(&self, key: &[u8], exclude_vid: Vid) -> bool {
        self.merge_guard_index
            .get(key)
            .is_some_and(|&v| v != exclude_vid)
    }

    #[instrument(skip(self, other), level = "trace")]
    /// Validate that merging `other` into `self` will not bail on a tombstoned
    /// edge endpoint (issue #77), **without mutating** either buffer.
    ///
    /// Mirrors the endpoint-liveness guard in `apply_edge_insertion`
    /// against the tombstone state [`Self::merge`] produces: `other`'s vertex
    /// deletions are applied and its vertex inserts clear their own tombstone,
    /// so an inserted edge bails iff an endpoint is tombstoned in `self` or
    /// `other` and is not (re-)inserted by `other`.
    ///
    /// Run this under `flush_lock` *before* the durable WAL flush so an
    /// offending commit is rejected up front. After the flush the transaction
    /// is durable, and a `merge` bail would leave a ghost/partial commit whose
    /// WAL replay re-bails — rendering the database unopenable.
    ///
    /// # Errors
    ///
    /// Returns an error naming the offending edge and endpoint when the merge
    /// would bail.
    pub fn validate_merge_edge_endpoints(&self, other: &L0Buffer) -> Result<()> {
        // An endpoint is effectively deleted after the merge's vertex phase if
        // it is tombstoned in either buffer and `other` does not re-insert it
        // (an insert clears the tombstone).
        let is_deleted = |vid: &Vid| {
            (self.vertex_tombstones.contains(vid) || other.vertex_tombstones.contains(vid))
                && !other.vertex_properties.contains_key(vid)
        };
        for (eid, (src_vid, dst_vid, _etype)) in &other.edge_endpoints {
            if other.tombstones.contains_key(eid) {
                continue; // a deletion, not an insertion — never resurrects a vertex
            }
            if is_deleted(src_vid) {
                anyhow::bail!(
                    "Cannot insert edge {}: source vertex {} has been deleted (issue #77)",
                    eid,
                    src_vid
                );
            }
            if is_deleted(dst_vid) {
                anyhow::bail!(
                    "Cannot insert edge {}: destination vertex {} has been deleted (issue #77)",
                    eid,
                    dst_vid
                );
            }
        }
        Ok(())
    }

    pub fn merge(&mut self, other: &L0Buffer) -> Result<()> {
        // Validate-then-apply: reject a merge that would bail on a tombstoned
        // edge endpoint before mutating anything, so a failed merge can never
        // leave a partially-applied (non-atomic) commit.
        self.validate_merge_edge_endpoints(other)?;
        self.merge_validated(
            other,
            other.vertex_properties.clone(),
            other.edge_properties.clone(),
        )
    }

    /// Commit-path variant of [`merge`](Self::merge) that consumes `other`'s
    /// vertex/edge property maps instead of deep-cloning every row.
    ///
    /// Everything else in `other` (endpoints, tombstones, versions, labels)
    /// is left intact — `commit_transaction_l0` still reads those after the
    /// merge. The caller must not rely on `other.vertex_properties` /
    /// `other.edge_properties` afterwards, which is safe on the commit path
    /// because committing consumes the transaction.
    pub fn merge_take(&mut self, other: &mut L0Buffer) -> Result<()> {
        // Validate BEFORE draining: the endpoint check consults
        // `other.vertex_properties` (the "re-inserted by other" exemption).
        self.validate_merge_edge_endpoints(other)?;
        let vertex_props = std::mem::take(&mut other.vertex_properties);
        let edge_props = std::mem::take(&mut other.edge_properties);
        self.merge_validated(other, vertex_props, edge_props)
    }

    /// Shared merge body. `vertex_props` / `edge_props` are `other`'s property
    /// maps, passed by value so rows move instead of clone; the caller has
    /// already run `validate_merge_edge_endpoints`.
    fn merge_validated(
        &mut self,
        other: &L0Buffer,
        vertex_props: HashMap<Vid, Properties>,
        mut edge_props: HashMap<Eid, Properties>,
    ) -> Result<()> {
        trace!(
            other_mutation_count = other.mutation_count,
            "Merging L0 buffer"
        );
        // skip_wal=true throughout: the caller (commit_transaction_l0) already
        // wrote every one of these mutations to WAL before invoking merge —
        // re-appending here would double the WAL volume per commit.
        // Merge Vertices
        for &vid in &other.vertex_tombstones {
            self.delete_vertex_impl(vid, true)?;
        }

        for (vid, props) in vertex_props {
            let labels = other.vertex_labels.get(&vid).cloned().unwrap_or_default();
            self.insert_vertex_with_labels_impl(vid, props, &labels, true);
        }

        // Merge vertex labels that might not have properties
        for (vid, labels) in &other.vertex_labels {
            if !self.vertex_labels.contains_key(vid) {
                self.vertex_labels.insert(*vid, labels.clone());
                for label in labels {
                    self.label_to_vids
                        .entry(label.clone())
                        .or_default()
                        .insert(*vid);
                }
            }
        }

        // Label-overwrite pass: a `SET n:Label` / `REMOVE n:Label` resolved the
        // FULL new label set into `other.vertex_labels[vid]` and flagged the vid.
        // REPLACE (not append) so removals actually remove and an existing
        // vertex's label change lands — overriding any append from the property
        // loop above. Skip vids deleted in the same commit. (The append loops
        // stay correct for property-path label unions, which are NOT flagged.)
        for vid in &other.vertex_label_overwrites {
            if other.vertex_tombstones.contains(vid) {
                continue;
            }
            let labels = other.vertex_labels.get(vid).cloned().unwrap_or_default();
            self.remove_vid_from_label_index(*vid);
            self.vertex_labels.insert(*vid, labels.clone());
            self.index_labels_for_vid(*vid, &labels);
            // Carry the overwrite flag into the persistent (main) buffer so a
            // pure relabel of a prior-window vid — which is absent from
            // `vertex_properties` — is still re-derived at flush (M8). The
            // flag is cleared when this buffer is rotated out by the flush.
            self.vertex_label_overwrites.insert(*vid);
        }

        // Merge Edges - insert all edges from edge_endpoints, using empty props if none exist
        for (eid, (src, dst, etype)) in &other.edge_endpoints {
            if other.tombstones.contains_key(eid) {
                self.delete_edge_impl(*eid, *src, *dst, *etype, true)?;
            } else {
                let props = edge_props.remove(eid).unwrap_or_default();
                let etype_name = other.edge_types.get(eid).cloned();
                self.insert_edge_impl(*src, *dst, *etype, *eid, props, etype_name, true)?;
            }
        }

        // Merge tombstones for edges that only exist in the target buffer (self),
        // not in the source buffer's edge_endpoints.  Without this, transaction
        // DELETEs of pre-existing edges are silently lost on commit.
        for (eid, tombstone) in &other.tombstones {
            if !other.edge_endpoints.contains_key(eid) {
                self.delete_edge_impl(
                    *eid,
                    tombstone.src_vid,
                    tombstone.dst_vid,
                    tombstone.edge_type,
                    true,
                )?;
            }
        }

        // Edge types are now merged inside insert_edge, so no separate loop needed

        // Merge timestamps - preserve semantics of or_insert (keep oldest created_at)
        // and insert (use latest updated_at)
        for (vid, ts) in &other.vertex_created_at {
            self.vertex_created_at.entry(*vid).or_insert(*ts); // keep oldest
        }
        for (vid, ts) in &other.vertex_updated_at {
            self.vertex_updated_at.insert(*vid, *ts); // use latest (tx wins)
        }

        for (eid, ts) in &other.edge_created_at {
            self.edge_created_at.entry(*eid).or_insert(*ts); // keep oldest
        }
        for (eid, ts) in &other.edge_updated_at {
            self.edge_updated_at.insert(*eid, *ts); // use latest (tx wins)
        }

        // Conservatively add other's estimated size (may overcount due to
        // deduplication, but that's safe for a memory limit).
        self.estimated_size += other.estimated_size;

        // Merge constraint index
        for (key, vid) in &other.constraint_index {
            self.constraint_index.insert(key.clone(), *vid);
        }

        // Merge the implicit MERGE-key guard so a committed MERGE-create is
        // visible to a concurrent transaction's commit-time re-probe.
        for (key, vid) in &other.merge_guard_index {
            self.merge_guard_index.insert(key.clone(), *vid);
        }

        Ok(())
    }

    /// Replay mutations from WAL without re-logging them.
    /// Used during startup recovery to restore L0 state from persisted WAL.
    /// Uses CRDT merge semantics to ensure recovered state matches pre-crash state.
    #[instrument(skip(self, mutations), level = "debug")]
    pub fn replay_mutations(&mut self, mutations: Vec<Mutation>) -> Result<()> {
        trace!(count = mutations.len(), "Replaying mutations");
        for mutation in mutations {
            match mutation {
                Mutation::InsertVertex {
                    vid,
                    properties,
                    labels,
                } => {
                    // Apply without WAL logging, with CRDT merge semantics
                    self.current_version += 1;
                    let version = self.current_version;

                    self.vertex_tombstones.remove(&vid);
                    let tracks_extid = properties.contains_key("ext_id");
                    let entry = self.vertex_properties.entry(vid).or_default();
                    let old_extid = if tracks_extid {
                        Self::extid_of(entry)
                    } else {
                        None
                    };
                    Self::merge_crdt_properties(entry, properties);
                    if tracks_extid {
                        let new_extid = Self::extid_of(
                            self.vertex_properties.get(&vid).expect("just inserted"),
                        );
                        self.sync_extid_index(vid, old_extid, new_extid);
                    }
                    self.vertex_versions.insert(vid, version);
                    self.graph.add_vertex(vid);
                    self.mutation_count += 1;

                    // Restore vertex labels from WAL
                    let existing = self.vertex_labels.entry(vid).or_default();
                    Self::append_unique_labels(existing, &labels);
                    for label in &labels {
                        self.label_to_vids
                            .entry(label.clone())
                            .or_default()
                            .insert(vid);
                    }
                }
                Mutation::DeleteVertex { vid, labels } => {
                    self.current_version += 1;
                    // Restore labels BEFORE apply_vertex_deletion
                    if !labels.is_empty() {
                        let existing = self.vertex_labels.entry(vid).or_default();
                        Self::append_unique_labels(existing, &labels);
                        for label in &labels {
                            self.label_to_vids
                                .entry(label.clone())
                                .or_default()
                                .insert(vid);
                        }
                    }
                    self.apply_vertex_deletion(vid);
                }
                Mutation::SetVertexLabels { vid, labels } => {
                    // REPLACE the vid's full label set (a label-only mutation
                    // resolved the complete set). Replace, not append, so a
                    // replayed removal removes; clears the old reverse-index
                    // entries first.
                    self.current_version += 1;
                    self.remove_vid_from_label_index(vid);
                    self.vertex_labels.insert(vid, labels.clone());
                    self.index_labels_for_vid(vid, &labels);
                    self.mutation_count += 1;
                }
                Mutation::InsertEdge {
                    src_vid,
                    dst_vid,
                    edge_type,
                    eid,
                    version: _,
                    properties,
                    edge_type_name,
                } => {
                    self.current_version += 1;
                    // Skip-and-warn on the issue-#77 endpoint bail: a pre-fix
                    // durable WAL may hold a ghost edge whose endpoint was
                    // tombstoned. Recovery must still open the database rather
                    // than abort, so drop the offending edge and continue.
                    match self.apply_edge_insertion(src_vid, dst_vid, edge_type, eid, properties) {
                        Ok(()) => {
                            // Restore edge type name metadata if present
                            if let Some(name) = edge_type_name {
                                self.edge_types.insert(eid, name);
                            }
                        }
                        Err(e) => {
                            tracing::warn!(
                                ?eid,
                                ?src_vid,
                                ?dst_vid,
                                error = %e,
                                "WAL replay: skipping edge insertion to a deleted endpoint (issue #77)"
                            );
                        }
                    }
                }
                Mutation::DeleteEdge {
                    eid,
                    src_vid,
                    dst_vid,
                    edge_type,
                    version: _,
                } => {
                    self.current_version += 1;
                    self.apply_edge_deletion(eid, src_vid, dst_vid, edge_type);
                }
            }
        }
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_l0_buffer_ops() -> Result<()> {
        let mut l0 = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        let eid_ab = Eid::new(101);

        l0.insert_edge(vid_a, vid_b, 1, eid_ab, HashMap::new(), None)?;

        let neighbors = l0.get_neighbors(vid_a, 1, Direction::Outgoing);
        assert_eq!(neighbors.len(), 1);
        assert_eq!(neighbors[0].0, vid_b);
        assert_eq!(neighbors[0].1, eid_ab);

        l0.delete_edge(eid_ab, vid_a, vid_b, 1)?;
        assert!(l0.is_tombstoned(eid_ab));

        // Verify neighbors are empty after deletion
        let neighbors_after = l0.get_neighbors(vid_a, 1, Direction::Outgoing);
        assert_eq!(neighbors_after.len(), 0);

        Ok(())
    }

    /// Regression for review #5: merging an edge whose endpoint is tombstoned
    /// in the target buffer must be rejected up front (`validate_merge_edge_endpoints`)
    /// and `merge` must be atomic — never partially applied. Before the fix this
    /// bailed only *inside* `merge`, after the durable WAL flush, leaving a ghost
    /// commit that made the database unopenable on replay.
    #[test]
    fn validate_merge_rejects_edge_to_tombstoned_endpoint() {
        let mut main = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        main.insert_vertex(vid_a, HashMap::new());
        main.insert_vertex(vid_b, HashMap::new());
        main.delete_vertex(vid_b).unwrap(); // B is now tombstoned in main

        // A transaction that inserts an edge A -> B (B tombstoned in main).
        let mut tx = L0Buffer::new(0, None);
        let eid = Eid::new(101);
        tx.insert_edge(vid_a, vid_b, 1, eid, HashMap::new(), None)
            .unwrap();

        assert!(
            main.validate_merge_edge_endpoints(&tx).is_err(),
            "edge to a tombstoned endpoint must be rejected before merge"
        );
        // merge validates first, so it errors and leaves main untouched (atomic).
        assert!(
            main.merge(&tx).is_err(),
            "merge must reject, not bail mid-apply"
        );
        assert!(
            !main.edge_endpoints.contains_key(&eid),
            "a rejected merge must not have partially applied the edge"
        );
    }

    /// When the transaction re-inserts the endpoint vertex, the edge is valid
    /// (the insert clears the tombstone) and the merge succeeds.
    #[test]
    fn validate_merge_allows_edge_when_endpoint_reinserted() {
        let mut main = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        main.insert_vertex(vid_a, HashMap::new());
        main.insert_vertex(vid_b, HashMap::new());
        main.delete_vertex(vid_b).unwrap();

        let mut tx = L0Buffer::new(0, None);
        tx.insert_vertex(vid_b, HashMap::new()); // re-insert B
        let eid = Eid::new(101);
        tx.insert_edge(vid_a, vid_b, 1, eid, HashMap::new(), None)
            .unwrap();

        assert!(main.validate_merge_edge_endpoints(&tx).is_ok());
        assert!(main.merge(&tx).is_ok());
        assert!(main.edge_endpoints.contains_key(&eid));
    }

    /// Edges between live endpoints merge as before — no false positives.
    #[test]
    fn validate_merge_allows_edge_to_live_endpoints() {
        let mut main = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        main.insert_vertex(vid_a, HashMap::new());
        main.insert_vertex(vid_b, HashMap::new());

        let mut tx = L0Buffer::new(0, None);
        let eid = Eid::new(101);
        tx.insert_edge(vid_a, vid_b, 1, eid, HashMap::new(), None)
            .unwrap();

        assert!(main.validate_merge_edge_endpoints(&tx).is_ok());
        assert!(main.merge(&tx).is_ok());
        assert!(main.edge_endpoints.contains_key(&eid));
    }

    #[test]
    fn test_l0_buffer_multiple_edges() -> Result<()> {
        let mut l0 = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        let vid_c = Vid::new(3);
        let eid_ab = Eid::new(101);
        let eid_ac = Eid::new(102);

        l0.insert_edge(vid_a, vid_b, 1, eid_ab, HashMap::new(), None)?;
        l0.insert_edge(vid_a, vid_c, 1, eid_ac, HashMap::new(), None)?;

        let neighbors = l0.get_neighbors(vid_a, 1, Direction::Outgoing);
        assert_eq!(neighbors.len(), 2);

        // Delete one edge
        l0.delete_edge(eid_ab, vid_a, vid_b, 1)?;

        // Should still have one neighbor
        let neighbors_after = l0.get_neighbors(vid_a, 1, Direction::Outgoing);
        assert_eq!(neighbors_after.len(), 1);
        assert_eq!(neighbors_after[0].0, vid_c);

        Ok(())
    }

    #[test]
    fn test_l0_buffer_edge_type_filter() -> Result<()> {
        let mut l0 = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        let vid_c = Vid::new(3);
        let eid_ab = Eid::new(101);
        let eid_ac = Eid::new(201); // Different edge type

        l0.insert_edge(vid_a, vid_b, 1, eid_ab, HashMap::new(), None)?;
        l0.insert_edge(vid_a, vid_c, 2, eid_ac, HashMap::new(), None)?;

        // Filter by edge type 1
        let type1_neighbors = l0.get_neighbors(vid_a, 1, Direction::Outgoing);
        assert_eq!(type1_neighbors.len(), 1);
        assert_eq!(type1_neighbors[0].0, vid_b);

        // Filter by edge type 2
        let type2_neighbors = l0.get_neighbors(vid_a, 2, Direction::Outgoing);
        assert_eq!(type2_neighbors.len(), 1);
        assert_eq!(type2_neighbors[0].0, vid_c);

        Ok(())
    }

    #[test]
    fn test_l0_buffer_incoming_edges() -> Result<()> {
        let mut l0 = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        let vid_c = Vid::new(3);
        let eid_ab = Eid::new(101);
        let eid_cb = Eid::new(102);

        // a -> b and c -> b
        l0.insert_edge(vid_a, vid_b, 1, eid_ab, HashMap::new(), None)?;
        l0.insert_edge(vid_c, vid_b, 1, eid_cb, HashMap::new(), None)?;

        // Check incoming edges to b
        let incoming = l0.get_neighbors(vid_b, 1, Direction::Incoming);
        assert_eq!(incoming.len(), 2);

        Ok(())
    }

    /// Regression test: merge should preserve edges without properties
    #[test]
    fn test_merge_empty_props_edge() -> Result<()> {
        let mut main_l0 = L0Buffer::new(0, None);
        let mut tx_l0 = L0Buffer::new(0, None);

        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        let eid_ab = Eid::new(101);

        // Insert edge with empty properties in transaction L0
        tx_l0.insert_edge(vid_a, vid_b, 1, eid_ab, HashMap::new(), None)?;

        // Verify edge exists in tx_l0
        assert!(tx_l0.edge_endpoints.contains_key(&eid_ab));
        assert!(!tx_l0.edge_properties.contains_key(&eid_ab)); // No properties entry

        // Merge into main L0
        main_l0.merge(&tx_l0)?;

        // Edge should exist in main L0 after merge
        assert!(main_l0.edge_endpoints.contains_key(&eid_ab));
        let neighbors = main_l0.get_neighbors(vid_a, 1, Direction::Outgoing);
        assert_eq!(neighbors.len(), 1);
        assert_eq!(neighbors[0].0, vid_b);

        Ok(())
    }

    /// Regression test: WAL replay should use CRDT merge semantics
    #[test]
    fn test_replay_crdt_merge() -> Result<()> {
        use crate::runtime::wal::Mutation;
        use serde_json::json;
        use uni_common::Value;

        let mut l0 = L0Buffer::new(0, None);
        let vid = Vid::new(1);

        // Create GCounter CRDT values using correct serde format:
        // {"t": "gc", "d": {"counts": {...}}}
        let counter1: Value = json!({
            "t": "gc",
            "d": {"counts": {"node1": 5}}
        })
        .into();
        let counter2: Value = json!({
            "t": "gc",
            "d": {"counts": {"node2": 3}}
        })
        .into();

        // First mutation: insert vertex with counter1
        let mut props1 = HashMap::new();
        props1.insert("counter".to_string(), counter1.clone());
        l0.replay_mutations(vec![Mutation::InsertVertex {
            vid,
            properties: props1,
            labels: vec![],
        }])?;

        // Second mutation: insert same vertex with counter2 (should merge)
        let mut props2 = HashMap::new();
        props2.insert("counter".to_string(), counter2.clone());
        l0.replay_mutations(vec![Mutation::InsertVertex {
            vid,
            properties: props2,
            labels: vec![],
        }])?;

        // Verify CRDT was merged (both node1 and node2 counts present)
        let stored_props = l0.vertex_properties.get(&vid).unwrap();
        let stored_counter = stored_props.get("counter").unwrap();

        // Convert back to serde_json::Value for nested access
        let stored_json: serde_json::Value = stored_counter.clone().into();
        // The merged counter should have both node1: 5 and node2: 3
        let data = stored_json.get("d").unwrap();
        let counts = data.get("counts").unwrap();
        assert_eq!(counts.get("node1"), Some(&json!(5)));
        assert_eq!(counts.get("node2"), Some(&json!(3)));

        Ok(())
    }

    #[test]
    fn test_merge_preserves_vertex_timestamps() -> Result<()> {
        let mut l0_main = L0Buffer::new(0, None);
        let mut l0_tx = L0Buffer::new(0, None);
        let vid = Vid::new(1);

        // Main buffer: insert vertex with timestamp T1
        let ts_main_created = 1000;
        let ts_main_updated = 1100;
        l0_main.insert_vertex(vid, HashMap::new());
        l0_main.vertex_created_at.insert(vid, ts_main_created);
        l0_main.vertex_updated_at.insert(vid, ts_main_updated);

        // Transaction buffer: update same vertex with timestamp T2 (later)
        let ts_tx_created = 2000; // should be ignored (main has older created_at)
        let ts_tx_updated = 2100; // should win (tx has newer updated_at)
        l0_tx.insert_vertex(vid, HashMap::new());
        l0_tx.vertex_created_at.insert(vid, ts_tx_created);
        l0_tx.vertex_updated_at.insert(vid, ts_tx_updated);

        // Merge transaction into main
        l0_main.merge(&l0_tx)?;

        // Verify created_at is oldest (from main)
        assert_eq!(
            *l0_main.vertex_created_at.get(&vid).unwrap(),
            ts_main_created,
            "created_at should preserve oldest timestamp"
        );

        // Verify updated_at is latest (from tx)
        assert_eq!(
            *l0_main.vertex_updated_at.get(&vid).unwrap(),
            ts_tx_updated,
            "updated_at should use latest timestamp"
        );

        Ok(())
    }

    #[test]
    fn test_merge_preserves_edge_timestamps() -> Result<()> {
        let mut l0_main = L0Buffer::new(0, None);
        let mut l0_tx = L0Buffer::new(0, None);
        let vid_a = Vid::new(1);
        let vid_b = Vid::new(2);
        let eid = Eid::new(100);

        // Main buffer: insert edge with timestamp T1
        let ts_main_created = 1000;
        let ts_main_updated = 1100;
        l0_main.insert_edge(vid_a, vid_b, 1, eid, HashMap::new(), None)?;
        l0_main.edge_created_at.insert(eid, ts_main_created);
        l0_main.edge_updated_at.insert(eid, ts_main_updated);

        // Transaction buffer: update same edge with timestamp T2 (later)
        let ts_tx_created = 2000; // should be ignored
        let ts_tx_updated = 2100; // should win
        l0_tx.insert_edge(vid_a, vid_b, 1, eid, HashMap::new(), None)?;
        l0_tx.edge_created_at.insert(eid, ts_tx_created);
        l0_tx.edge_updated_at.insert(eid, ts_tx_updated);

        // Merge transaction into main
        l0_main.merge(&l0_tx)?;

        // Verify created_at is oldest (from main)
        assert_eq!(
            *l0_main.edge_created_at.get(&eid).unwrap(),
            ts_main_created,
            "edge created_at should preserve oldest timestamp"
        );

        // Verify updated_at is latest (from tx)
        assert_eq!(
            *l0_main.edge_updated_at.get(&eid).unwrap(),
            ts_tx_updated,
            "edge updated_at should use latest timestamp"
        );

        Ok(())
    }

    #[test]
    fn test_merge_created_at_not_overwritten_for_existing_vertex() -> Result<()> {
        use uni_common::Value;

        let mut l0_main = L0Buffer::new(0, None);
        let mut l0_tx = L0Buffer::new(0, None);
        let vid = Vid::new(1);

        // Main buffer: vertex created at T1
        let ts_original = 1000;
        l0_main.insert_vertex(vid, HashMap::new());
        l0_main.vertex_created_at.insert(vid, ts_original);
        l0_main.vertex_updated_at.insert(vid, ts_original);

        // Transaction buffer: update vertex (created_at would be T2 if set)
        let ts_tx = 2000;
        let mut props = HashMap::new();
        props.insert("updated".to_string(), Value::String("yes".to_string()));
        l0_tx.insert_vertex(vid, props);
        l0_tx.vertex_created_at.insert(vid, ts_tx);
        l0_tx.vertex_updated_at.insert(vid, ts_tx);

        // Merge transaction into main
        l0_main.merge(&l0_tx)?;

        // Verify created_at was NOT overwritten (still T1, not T2)
        assert_eq!(
            *l0_main.vertex_created_at.get(&vid).unwrap(),
            ts_original,
            "created_at must not be overwritten for existing vertex"
        );

        // Verify updated_at WAS updated (now T2)
        assert_eq!(
            *l0_main.vertex_updated_at.get(&vid).unwrap(),
            ts_tx,
            "updated_at should reflect transaction timestamp"
        );

        // Verify properties were merged
        assert!(
            l0_main
                .vertex_properties
                .get(&vid)
                .unwrap()
                .contains_key("updated")
        );

        Ok(())
    }

    /// Test for Issue #23: Vertex labels preserved through replay_mutations
    #[test]
    fn test_replay_mutations_preserves_vertex_labels() -> Result<()> {
        use crate::runtime::wal::Mutation;

        let mut l0 = L0Buffer::new(0, None);
        let vid = Vid::new(42);

        // Create InsertVertex mutation with labels
        let mutations = vec![Mutation::InsertVertex {
            vid,
            properties: {
                let mut props = HashMap::new();
                props.insert(
                    "name".to_string(),
                    uni_common::Value::String("Alice".to_string()),
                );
                props
            },
            labels: vec!["Person".to_string(), "User".to_string()],
        }];

        // Replay mutations
        l0.replay_mutations(mutations)?;

        // Verify vertex exists in L0
        assert!(l0.vertex_properties.contains_key(&vid));

        // Verify labels are preserved
        let labels = l0.get_vertex_labels(vid).expect("Labels should exist");
        assert_eq!(labels.len(), 2);
        assert!(labels.contains(&"Person".to_string()));
        assert!(labels.contains(&"User".to_string()));

        // Verify vertex is findable by label
        let person_vids = l0.vids_for_label("Person");
        assert_eq!(person_vids.len(), 1);
        assert_eq!(person_vids[0], vid);

        let user_vids = l0.vids_for_label("User");
        assert_eq!(user_vids.len(), 1);
        assert_eq!(user_vids[0], vid);

        Ok(())
    }

    /// Test for Issue #23: DeleteVertex labels preserved for tombstone flushing
    #[test]
    fn test_replay_mutations_preserves_delete_vertex_labels() -> Result<()> {
        use crate::runtime::wal::Mutation;

        let mut l0 = L0Buffer::new(0, None);
        let vid = Vid::new(99);

        // First insert vertex with labels
        l0.insert_vertex_with_labels(
            vid,
            HashMap::new(),
            &["Person".to_string(), "Admin".to_string()],
        );

        // Verify vertex and labels exist
        assert!(l0.vertex_properties.contains_key(&vid));
        let labels = l0.get_vertex_labels(vid).expect("Labels should exist");
        assert_eq!(labels.len(), 2);

        // Create DeleteVertex mutation with labels
        let mutations = vec![Mutation::DeleteVertex {
            vid,
            labels: vec!["Person".to_string(), "Admin".to_string()],
        }];

        // Replay deletion
        l0.replay_mutations(mutations)?;

        // Verify vertex is tombstoned
        assert!(l0.vertex_tombstones.contains(&vid));

        // Verify labels are preserved in L0 (needed for Issue #76 tombstone flushing)
        // The labels should still be accessible for the flush logic to know which tables to update
        let labels = l0.get_vertex_labels(vid);
        assert!(
            labels.is_some(),
            "Labels should be preserved even after deletion for tombstone flushing"
        );

        Ok(())
    }

    /// Test for Issue #28: Edge type name preserved through replay_mutations
    #[test]
    fn test_replay_mutations_preserves_edge_type_name() -> Result<()> {
        use crate::runtime::wal::Mutation;

        let mut l0 = L0Buffer::new(0, None);
        let src = Vid::new(1);
        let dst = Vid::new(2);
        let eid = Eid::new(500);
        let edge_type = 100;

        // Create InsertEdge mutation with edge_type_name
        let mutations = vec![Mutation::InsertEdge {
            src_vid: src,
            dst_vid: dst,
            edge_type,
            eid,
            version: 1,
            properties: {
                let mut props = HashMap::new();
                props.insert("since".to_string(), uni_common::Value::Int(2020));
                props
            },
            edge_type_name: Some("KNOWS".to_string()),
        }];

        // Replay mutations
        l0.replay_mutations(mutations)?;

        // Verify edge exists in L0
        assert!(l0.edge_endpoints.contains_key(&eid));

        // Verify edge type name is preserved
        let type_name = l0.get_edge_type(eid).expect("Edge type name should exist");
        assert_eq!(type_name, "KNOWS");

        // Verify edge is findable by type name
        let knows_eids = l0.eids_for_type("KNOWS");
        assert_eq!(knows_eids.len(), 1);
        assert_eq!(knows_eids[0], eid);

        Ok(())
    }

    /// Test for Issue #28: Edge type mapping survives multiple replay cycles
    #[test]
    fn test_edge_type_mapping_survives_multiple_replays() -> Result<()> {
        use crate::runtime::wal::Mutation;

        let mut l0 = L0Buffer::new(0, None);

        // Replay multiple edge insertions with different types
        let mutations = vec![
            Mutation::InsertEdge {
                src_vid: Vid::new(1),
                dst_vid: Vid::new(2),
                edge_type: 100,
                eid: Eid::new(1000),
                version: 1,
                properties: HashMap::new(),
                edge_type_name: Some("KNOWS".to_string()),
            },
            Mutation::InsertEdge {
                src_vid: Vid::new(2),
                dst_vid: Vid::new(3),
                edge_type: 101,
                eid: Eid::new(1001),
                version: 2,
                properties: HashMap::new(),
                edge_type_name: Some("LIKES".to_string()),
            },
            Mutation::InsertEdge {
                src_vid: Vid::new(3),
                dst_vid: Vid::new(1),
                edge_type: 100,
                eid: Eid::new(1002),
                version: 3,
                properties: HashMap::new(),
                edge_type_name: Some("KNOWS".to_string()),
            },
        ];

        l0.replay_mutations(mutations)?;

        // Verify all edge type mappings are preserved
        assert_eq!(l0.get_edge_type(Eid::new(1000)), Some("KNOWS"));
        assert_eq!(l0.get_edge_type(Eid::new(1001)), Some("LIKES"));
        assert_eq!(l0.get_edge_type(Eid::new(1002)), Some("KNOWS"));

        // Verify edges can be queried by type
        let knows_edges = l0.eids_for_type("KNOWS");
        assert_eq!(knows_edges.len(), 2);
        assert!(knows_edges.contains(&Eid::new(1000)));
        assert!(knows_edges.contains(&Eid::new(1002)));

        let likes_edges = l0.eids_for_type("LIKES");
        assert_eq!(likes_edges.len(), 1);
        assert_eq!(likes_edges[0], Eid::new(1001));

        Ok(())
    }

    /// Test for Issue #23 + #28: Combined vertex labels and edge types in replay
    #[test]
    fn test_replay_mutations_combined_labels_and_edge_types() -> Result<()> {
        use crate::runtime::wal::Mutation;

        let mut l0 = L0Buffer::new(0, None);
        let alice = Vid::new(1);
        let bob = Vid::new(2);
        let eid = Eid::new(100);

        // Simulate crash recovery scenario: replay full transaction log
        let mutations = vec![
            // Insert Alice with Person label
            Mutation::InsertVertex {
                vid: alice,
                properties: {
                    let mut props = HashMap::new();
                    props.insert(
                        "name".to_string(),
                        uni_common::Value::String("Alice".to_string()),
                    );
                    props
                },
                labels: vec!["Person".to_string()],
            },
            // Insert Bob with Person label
            Mutation::InsertVertex {
                vid: bob,
                properties: {
                    let mut props = HashMap::new();
                    props.insert(
                        "name".to_string(),
                        uni_common::Value::String("Bob".to_string()),
                    );
                    props
                },
                labels: vec!["Person".to_string()],
            },
            // Create KNOWS edge between them
            Mutation::InsertEdge {
                src_vid: alice,
                dst_vid: bob,
                edge_type: 1,
                eid,
                version: 3,
                properties: HashMap::new(),
                edge_type_name: Some("KNOWS".to_string()),
            },
        ];

        // Replay all mutations
        l0.replay_mutations(mutations)?;

        // Verify vertex labels preserved
        assert_eq!(l0.get_vertex_labels(alice).unwrap().len(), 1);
        assert_eq!(l0.get_vertex_labels(bob).unwrap().len(), 1);
        assert_eq!(l0.vids_for_label("Person").len(), 2);

        // Verify edge type name preserved
        assert_eq!(l0.get_edge_type(eid).unwrap(), "KNOWS");
        assert_eq!(l0.eids_for_type("KNOWS").len(), 1);

        // Verify graph structure
        let alice_neighbors = l0.get_neighbors(alice, 1, Direction::Outgoing);
        assert_eq!(alice_neighbors.len(), 1);
        assert_eq!(alice_neighbors[0].0, bob);

        Ok(())
    }

    /// Test for Issue #23: Empty labels should deserialize correctly (backward compat)
    #[test]
    fn test_replay_mutations_backward_compat_empty_labels() -> Result<()> {
        use crate::runtime::wal::Mutation;

        let mut l0 = L0Buffer::new(0, None);
        let vid = Vid::new(1);

        // Simulate old WAL format: InsertVertex with empty labels
        // (This tests #[serde(default)] behavior)
        let mutations = vec![Mutation::InsertVertex {
            vid,
            properties: HashMap::new(),
            labels: vec![], // Empty labels (old format compatibility)
        }];

        l0.replay_mutations(mutations)?;

        // Vertex should exist
        assert!(l0.vertex_properties.contains_key(&vid));

        // Labels should be empty but entry should exist in vertex_labels
        let labels = l0.get_vertex_labels(vid);
        assert!(labels.is_some(), "Labels entry should exist even if empty");
        assert_eq!(labels.unwrap().len(), 0);

        Ok(())
    }

    #[test]
    fn test_now_nanos_returns_nanosecond_range() {
        // Test that now_nanos() returns a value in nanosecond range
        // As of 2025, Unix timestamp in nanoseconds should be > 1.7e18
        // (2025-01-01 is approximately 1,735,689,600 seconds = 1.735e18 nanoseconds)
        let now = now_nanos();

        // Verify it's in nanosecond range (not microseconds which would be 1000x smaller)
        assert!(
            now > 1_700_000_000_000_000_000,
            "now_nanos() returned {}, expected > 1.7e18 for nanoseconds",
            now
        );

        // Sanity check: should also be less than year 2100 in nanoseconds (4.1e18)
        assert!(
            now < 4_100_000_000_000_000_000,
            "now_nanos() returned {}, expected < 4.1e18",
            now
        );
    }
}