velesdb-core 2.0.0

//! Read-only query and traversal methods for `ConcurrentEdgeStore`.
//!
//! Extracted from the main module for single-responsibility:
//! - Edge lookups (by node, by label, by ID)
//! - BFS traversal
//! - Edge count

use super::super::csr_snapshot::{CsrSnapshot, EdgePredicate};
use super::super::traversal::{TraversalConfig, TraversalResult};
use super::super::traversal_csr::{bfs_traverse_csr, bfs_traverse_csr_filtered};
use super::{ConcurrentEdgeStore, GraphEdge};
use arc_swap::Guard;
use rustc_hash::FxHashSet;
use std::collections::VecDeque;
use std::sync::Arc;

impl ConcurrentEdgeStore {
    /// Gets all outgoing edges from a node (thread-safe).
    #[must_use]
    pub fn get_outgoing(&self, node_id: u64) -> Vec<GraphEdge> {
        let shard = &self.shards[self.shard_index(node_id)];
        let guard = shard.read();
        guard.get_outgoing(node_id).into_iter().cloned().collect()
    }

    /// Gets all incoming edges to a node (thread-safe).
    #[must_use]
    pub fn get_incoming(&self, node_id: u64) -> Vec<GraphEdge> {
        let shard = &self.shards[self.shard_index(node_id)];
        let guard = shard.read();
        guard.get_incoming(node_id).into_iter().cloned().collect()
    }

    /// Gets neighbors (target nodes) of a given node.
    ///
    /// When a CSR read snapshot is available (see
    /// [`build_read_snapshot()`](Self::build_read_snapshot)), this returns
    /// a copy from contiguous memory without resolving individual edges.
    /// Falls back to per-shard edge lookup otherwise.
    #[must_use]
    pub fn get_neighbors(&self, node_id: u64) -> Vec<u64> {
        let snapshot = self.clustered_snapshot.read();
        if let Some(idx) = snapshot.as_ref() {
            return idx.get_neighbors(node_id).to_vec();
        }
        drop(snapshot);
        self.get_outgoing(node_id)
            .iter()
            .map(GraphEdge::target)
            .collect()
    }

    /// Invokes `f` with a borrowed slice of outgoing neighbor IDs.
    ///
    /// When the CSR snapshot is available, `f` receives a zero-copy `&[u64]`
    /// from contiguous memory. Otherwise, a temporary `Vec<u64>` is built
    /// from per-shard edge lookup.
    ///
    /// Prefer this over [`get_neighbors`](Self::get_neighbors) in tight
    /// loops (BFS frontiers) where the caller processes IDs inline.
    #[inline]
    pub fn with_neighbors<F, R>(&self, node_id: u64, f: F) -> R
    where
        F: FnOnce(&[u64]) -> R,
    {
        let snapshot = self.clustered_snapshot.read();
        if let Some(idx) = snapshot.as_ref() {
            return f(idx.get_neighbors(node_id));
        }
        drop(snapshot);
        let fallback: Vec<u64> = self
            .get_outgoing(node_id)
            .iter()
            .map(GraphEdge::target)
            .collect();
        f(&fallback)
    }

    /// Gets outgoing edges filtered by label (thread-safe).
    ///
    /// # Performance Note
    ///
    /// This method delegates to the underlying `EdgeStore::get_outgoing_by_label`
    /// which uses the composite index `(source_id, label) -> edge_ids` for O(1) lookup
    /// when available (EPIC-019 US-003). Falls back to filtering if index not populated.
    #[must_use]
    pub fn get_outgoing_by_label(&self, node_id: u64, label: &str) -> Vec<GraphEdge> {
        let shard_idx = self.shard_index(node_id);
        let shard = self.shards[shard_idx].read();
        shard
            .get_outgoing_by_label(node_id, label)
            .into_iter()
            .cloned()
            .collect()
    }

    /// Gets incoming edges filtered by label (thread-safe).
    #[must_use]
    pub fn get_incoming_by_label(&self, node_id: u64, label: &str) -> Vec<GraphEdge> {
        self.get_incoming(node_id)
            .into_iter()
            .filter(|e| e.label() == label)
            .collect()
    }

    /// Gets all edges with a specific label across all shards.
    ///
    /// # Performance Warning
    ///
    /// This method iterates through ALL shards and aggregates results.
    /// For large graphs with many shards, this can be expensive.
    /// Consider using `get_outgoing_by_label(node_id, label)` if you know
    /// the source node, which is O(k) instead of O(shards × edges_per_label).
    #[must_use]
    pub fn get_edges_by_label(&self, label: &str) -> Vec<GraphEdge> {
        self.shards
            .iter()
            .flat_map(|shard| {
                shard
                    .read()
                    .get_edges_by_label(label)
                    .into_iter()
                    .cloned()
                    .collect::<Vec<_>>()
            })
            .collect()
    }

    /// Checks if an edge with the given ID exists.
    #[must_use]
    /// Returns the highest edge id in the store, if any.
    ///
    /// O(edges) over the id registry — no edge cloning.
    pub fn max_edge_id(&self) -> Option<u64> {
        self.edge_ids.read().keys().max().copied()
    }

    /// Returns `true` when an edge with `edge_id` exists.
    pub fn contains_edge(&self, edge_id: u64) -> bool {
        self.edge_ids.read().contains_key(&edge_id)
    }

    /// Gets an edge by ID using optimized source shard lookup.
    ///
    /// Returns `None` if the edge doesn't exist.
    #[must_use]
    pub fn get_edge(&self, edge_id: u64) -> Option<GraphEdge> {
        // Get source_id from registry for direct shard lookup
        let source_id = *self.edge_ids.read().get(&edge_id)?;
        let shard_idx = self.shard_index(source_id);
        self.shards[shard_idx].read().get_edge(edge_id).cloned()
    }

    /// Traverses the graph using BFS from a starting node.
    ///
    /// Returns all nodes reachable within `max_depth` hops.
    ///
    /// When a CSR read snapshot is available, neighbor lookups are zero-copy
    /// slices from contiguous memory. Otherwise uses Read-Copy-Drop pattern
    /// with per-shard locks.
    #[must_use]
    pub fn traverse_bfs(&self, start: u64, max_depth: u32) -> Vec<u64> {
        let mut visited = FxHashSet::default();
        let mut queue = VecDeque::new();
        queue.push_back((start, 0u32));

        while let Some((node, depth)) = queue.pop_front() {
            if depth > max_depth || !visited.insert(node) {
                continue;
            }

            self.with_neighbors(node, |neighbors| {
                for &neighbor in neighbors {
                    if !visited.contains(&neighbor) {
                        queue.push_back((neighbor, depth + 1));
                    }
                }
            });
        }

        visited.into_iter().collect()
    }

    /// Returns the total edge count across all shards.
    ///
    /// Uses outgoing edge count to avoid double-counting edges that span shards.
    #[must_use]
    pub fn edge_count(&self) -> usize {
        self.shards
            .iter()
            .map(|s| s.read().outgoing_edge_count())
            .sum()
    }

    /// Returns `len()` — alias for `edge_count()` for API parity with `EdgeStore`.
    #[must_use]
    pub fn len(&self) -> usize {
        self.edge_count()
    }

    /// Returns `true` if the store contains no edges.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.edge_ids.read().is_empty()
    }

    /// Returns the number of distinct edge labels in the graph.
    ///
    /// Reads from the CSR snapshot's interned label table, triggering a
    /// lazy rebuild if dirty. Returns 0 when the store has no edges.
    #[must_use]
    pub fn label_count(&self) -> usize {
        self.ensure_csr_fresh();
        let snapshot = self.csr_snapshot.load();
        snapshot.distinct_label_count()
    }

    /// Returns all edges across all shards (cloned).
    ///
    /// Uses the `edge_ids` registry to look up each edge exactly once in its
    /// source shard, avoiding double-counting for cross-shard edges.
    ///
    /// # Performance Warning
    ///
    /// Iterates all edges and clones each one. For large graphs, prefer
    /// targeted queries (`get_outgoing`, `get_edges_by_label`).
    #[must_use]
    pub fn all_edges(&self) -> Vec<GraphEdge> {
        let ids = self.edge_ids.read();
        let mut result = Vec::with_capacity(ids.len());
        for (&edge_id, &source_id) in ids.iter() {
            let shard_idx = self.shard_index(source_id);
            let guard = self.shards[shard_idx].read();
            if let Some(edge) = guard.get_edge(edge_id) {
                result.push(edge.clone());
            }
        }
        result
    }

    /// Returns the out-degree of a node without materializing edge vectors.
    ///
    /// Uses CSR snapshot when available for O(1) lookup without shard locking.
    #[must_use]
    #[inline]
    pub fn outgoing_degree(&self, node_id: u64) -> usize {
        let snapshot = self.clustered_snapshot.read();
        if let Some(idx) = snapshot.as_ref() {
            return idx.neighbor_count(node_id);
        }
        drop(snapshot);
        let shard_idx = self.shard_index(node_id);
        self.shards[shard_idx].read().outgoing_degree(node_id)
    }

    /// Returns the in-degree of a node without materializing edge vectors.
    #[must_use]
    #[inline]
    pub fn incoming_degree(&self, node_id: u64) -> usize {
        let shard_idx = self.shard_index(node_id);
        self.shards[shard_idx].read().incoming_degree(node_id)
    }

    /// Rebuilds the CSR snapshot if the dirty flag is set.
    ///
    /// Uses `swap(false, AcqRel)` to atomically clear the flag and check
    /// the previous value. Only one thread performs the rebuild; concurrent
    /// readers see the stale-but-valid snapshot until the swap completes.
    ///
    /// This is the **correctness-first** entry point used by CSR consumers
    /// that have no per-shard fallback (`traverse_bfs_csr`,
    /// `traverse_bfs_filtered`, `label_count`, `get_csr_snapshot`). It always
    /// rebuilds when dirty so the returned snapshot reflects every prior
    /// write. The write-count debounce (issue #905) is applied one level up in
    /// `traverse_bfs_config`, which prefers the per-shard path while the CSR
    /// is stale and only reaches this method once a rebuild is actually wanted.
    #[inline]
    fn ensure_csr_fresh(&self) {
        if self
            .csr_dirty
            .swap(false, std::sync::atomic::Ordering::AcqRel)
        {
            // Snapshot the write count *before* reading the shards. The
            // rebuild below reflects exactly these writes; any writer that
            // bumps the counter while we walk the shards must stay counted so
            // the next reader still rebuilds (issue #905 follow-up — a blind
            // `store(0)` after the rebuild would silently drop that increment).
            let observed = self
                .pending_writes
                .load(std::sync::atomic::Ordering::Acquire);
            if let Err(e) = self.rebuild_snapshot() {
                // Restore dirty flag so the next caller retries the rebuild.
                self.csr_dirty
                    .store(true, std::sync::atomic::Ordering::Release);
                tracing::warn!("lazy CSR snapshot rebuild failed: {e}");
                return;
            }
            // Rebuild succeeded: subtract only the writes we accounted for, so
            // a concurrent `fetch_add` between the load above and here is
            // preserved instead of being clobbered to zero. Saturating at zero
            // so two concurrent reader-triggered rebuilds cannot underflow the
            // counter — a plain `fetch_sub` could wrap to ~`u64::MAX` and wedge
            // `csr_rebuild_due` permanently true (forcing a rebuild on every read).
            let _ = self.pending_writes.fetch_update(
                std::sync::atomic::Ordering::AcqRel,
                std::sync::atomic::Ordering::Acquire,
                |cur| Some(cur.saturating_sub(observed)),
            );
        }
    }

    /// Returns `true` when the CSR snapshot reflects every committed write,
    /// i.e. it is safe to traverse without first rebuilding.
    ///
    /// A clean snapshot (no pending writes) is always authoritative. When the
    /// snapshot is dirty this returns `false` **without** triggering a
    /// rebuild, so callers with an authoritative per-shard fallback (issue
    /// #905 debounce) can avoid the O(N+E) rebuild on every read after a
    /// write.
    #[inline]
    #[must_use]
    pub(crate) fn csr_is_authoritative(&self) -> bool {
        !self.csr_dirty.load(std::sync::atomic::Ordering::Acquire)
    }

    /// Returns `true` when enough writes have accumulated that the next CSR
    /// read should pay for a rebuild rather than continue serving from the
    /// per-shard fallback (issue #905 debounce threshold reached).
    #[inline]
    #[must_use]
    pub(crate) fn csr_rebuild_due(&self) -> bool {
        self.pending_writes
            .load(std::sync::atomic::Ordering::Acquire)
            >= super::CSR_REBUILD_WRITE_THRESHOLD
    }

    /// Returns the current CSR snapshot (lock-free read).
    ///
    /// The returned `Guard` dereferences to `Arc<CsrSnapshot>` and keeps
    /// the snapshot alive for the duration of the borrow. No locks are
    /// acquired — this is a single atomic load.
    ///
    /// If the snapshot is dirty (mutation occurred since last rebuild),
    /// triggers a lazy rebuild before returning.
    #[must_use]
    pub fn get_csr_snapshot(&self) -> Guard<Arc<CsrSnapshot>> {
        self.ensure_csr_fresh();
        self.csr_snapshot.load()
    }

    /// BFS traversal on the CSR snapshot (lock-free, zero-copy).
    ///
    /// Loads the current snapshot atomically and delegates to
    /// [`bfs_traverse_csr`] for the actual traversal.
    /// Triggers a lazy CSR rebuild if dirty.
    #[must_use]
    pub fn traverse_bfs_csr(&self, source: u64, config: &TraversalConfig) -> Vec<TraversalResult> {
        self.ensure_csr_fresh();
        let snapshot = self.csr_snapshot.load();
        bfs_traverse_csr(&snapshot, source, config)
    }

    /// BFS traversal with predicate pushdown on the CSR snapshot.
    ///
    /// Loads the current snapshot atomically and delegates to
    /// [`bfs_traverse_csr_filtered`] which applies the predicate at the
    /// CSR level, avoiding materialisation of non-matching edges.
    /// Triggers a lazy CSR rebuild if dirty.
    #[must_use]
    pub fn traverse_bfs_filtered<P: EdgePredicate>(
        &self,
        source: u64,
        config: &TraversalConfig,
        predicate: &P,
    ) -> Vec<TraversalResult> {
        self.ensure_csr_fresh();
        let snapshot = self.csr_snapshot.load();
        bfs_traverse_csr_filtered(&snapshot, source, config, predicate)
    }
}