arete_server/

cache.rs

//! Entity cache for snapshot-on-subscribe functionality.
//!
//! This module provides an `EntityCache` that maintains full projected entities
//! in memory with LRU eviction. When a new client subscribes, they receive
//! cached snapshots immediately rather than waiting for the next live mutation.

use crate::mutation_batch::SlotIndexDomain;
use crate::shared_entity::{
    split_version, with_version, EntityFields, Fields, SharedEntity, VERSION_FIELD,
};
use arete_interpreter::AccountPosition;
use lru::LruCache;
use serde::{Deserialize, Serialize};
use serde_json::Value;
use std::collections::BTreeMap;
use std::collections::HashMap;
use std::sync::Arc;
use tokio::sync::RwLock;

const DEFAULT_MAX_ENTITIES_PER_VIEW: usize = 500;
const DEFAULT_MAX_ARRAY_LENGTH: usize = 100;
const DEFAULT_INITIAL_SNAPSHOT_BATCH_SIZE: usize = 50;
const DEFAULT_SUBSEQUENT_SNAPSHOT_BATCH_SIZE: usize = 100;
/// Evicted keys remembered per cached entity, for a source that does not mark
/// creations (see [`PatchOrigin::Unknown`]). Such a source can keep far more
/// entities than a view caches (a VM keeps 2,500 per state table by default
/// against 500 here), so a key the cache evicted can keep receiving patches
/// long afterwards; eight times the cache bound covers the VM's default table
/// with room to spare. A key evicted longer ago than that is taken for new.
///
/// A source that marks creations needs no memory: an unmarked patch for a key
/// a view does not hold is refused however long ago the key left, or whether
/// it was ever there.
const EVICTED_KEYS_PER_CACHED_ENTITY: usize = 8;

/// Whether a patch creates its entity, as far as its source says.
///
/// Every patch carries only the fields that changed. For a key a view holds
/// that is all it needs; for one it does not hold, the patch is the entity
/// only if it created it.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum PatchOrigin {
    /// The source marked the patch as its entity's creation
    /// ([`arete_interpreter::WholeEntity::Created`]), so it is all of the
    /// entity.
    Creation,
    /// The source marks every creation and did not mark this patch, so it
    /// changes an entity created earlier and is only part of it. A VM linked
    /// through [`crate::EntityResync`] is such a source.
    Change,
    /// The source does not mark creations. A patch for a key the view does not
    /// hold is taken for a new entity unless the view remembers evicting it.
    Unknown,
}

/// What [`EntityCache::upsert_with_append`] did with a patch.
#[derive(Debug, Clone, PartialEq)]
pub enum CacheWrite {
    /// Merged into the entity the cache already held.
    Merged,
    /// Stored as a new entity: the patch created it, or its source does not
    /// say and the cache has no record of the key.
    Created,
    /// Not stored. The cache does not hold the key and the patch carries only
    /// the fields that changed — part of an entity, not one: its source marks
    /// creations and did not mark this one, or the cache evicted the key. The
    /// patch is handed back for callers that hold a full copy elsewhere.
    Refused { patch: Value },
}

/// What one view does with a patch for a key, decided before any view
/// changes (see [`EntityCache::upsert_with_ordering`]).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Admission {
    Refuse,
    Create,
    Merge,
}

/// One view's part in a write to several views at once: the view, and the
/// `_version` its frame carries.
#[derive(Debug, Clone)]
pub(crate) struct ViewVersion<'a> {
    pub view_id: &'a str,
    pub version: Option<Value>,
}

/// One view's cached entities and the keys it evicted to stay bounded.
///
/// Both maps are unbounded `LruCache`s capped by hand: a bounded one
/// allocates its full capacity up front, which for the evicted-key memory is
/// several times the entity bound, per view, whether or not anything is ever
/// evicted from it.
struct ViewEntries {
    max_entities: usize,
    entities: LruCache<String, SharedEntity>,
    /// Keys evicted for space, most recently evicted or refused first.
    /// Allocated on the first eviction, and never for a view fed by a source
    /// that marks creations.
    evicted: Option<LruCache<String, ()>>,
    /// Whether a source that marks creations writes to this view. Such a
    /// source never consults `evicted`.
    creations_marked: bool,
    // Recent authoritative account positions and deletion barriers survive
    // snapshots. They are intentionally separate from entity `_seq`, whose
    // offset may instead be an instruction txn_index.
    lifetimes: LruCache<String, EntityLifetimeCheckpoint>,
}

impl ViewEntries {
    fn new(max_entities: usize) -> Self {
        assert!(max_entities > 0, "max_entities_per_view must be > 0");
        Self {
            max_entities,
            entities: LruCache::unbounded(),
            evicted: None,
            creations_marked: false,
            lifetimes: LruCache::unbounded(),
        }
    }

    /// Note a patch from a source that marks creations: this view no longer
    /// needs to remember its evictions.
    fn mark_creations(&mut self) {
        self.creations_marked = true;
        self.evicted = None;
    }

    /// Store `entity` under `key`, remembering whatever it pushes out.
    fn store(&mut self, key: String, entity: SharedEntity) {
        self.forget_evicted(&key);
        self.entities.put(key, entity);
        while self.entities.len() > self.max_entities {
            match self.entities.pop_lru() {
                Some((evicted, _)) => self.remember_evicted(evicted),
                None => break,
            }
        }
    }

    fn remember_evicted(&mut self, key: String) {
        if self.creations_marked {
            return;
        }
        let capacity = self
            .max_entities
            .saturating_mul(EVICTED_KEYS_PER_CACHED_ENTITY);
        let evicted = self.evicted.get_or_insert_with(LruCache::unbounded);
        evicted.put(key, ());
        while evicted.len() > capacity {
            evicted.pop_lru();
        }
    }

    fn forget_evicted(&mut self, key: &str) {
        if let Some(evicted) = self.evicted.as_mut() {
            evicted.pop(key);
        }
    }

    /// Whether `key` was evicted and is still remembered. Asking refreshes
    /// it, so an evicted key that keeps changing is not forgotten.
    fn was_evicted(&mut self, key: &str) -> bool {
        self.evicted
            .as_mut()
            .is_some_and(|evicted| evicted.get(key).is_some())
    }

    fn accepts(
        &self,
        key: &str,
        creation: bool,
        account_position: Option<AccountPosition>,
        source_seq: Option<&str>,
        source_domain: Option<SlotIndexDomain>,
    ) -> bool {
        if let Some(checkpoint) = self.lifetimes.peek(key) {
            match (checkpoint.account_position, account_position) {
                (Some(previous), Some(incoming)) if incoming <= previous => return false,
                // Once a producer opts into authoritative account ordering it
                // must carry it on every later account-owned lifetime change.
                (Some(_), None) if creation || checkpoint.deleted => return false,
                _ => {}
            }
            if checkpoint.deleted && !creation {
                return false;
            }
        }

        if account_position.is_none() && source_domain.is_some() && source_seq.is_some() {
            return self.accepts_source_ordering(key, source_seq, source_domain, !creation);
        }
        true
    }

    fn latest_source_seq<'a>(
        &'a self,
        key: &str,
        source_domain: Option<SlotIndexDomain>,
    ) -> Option<&'a str> {
        let checkpoint = self.lifetimes.peek(key);
        // An account cursor and an instruction/source cursor are independent.
        // In particular, do not recover the account update's `_seq` from the
        // entity and compare its write version with an instruction index.
        if checkpoint.is_some_and(|checkpoint| checkpoint.account_position.is_some()) {
            return checkpoint.and_then(|checkpoint| checkpoint.source_seq(source_domain));
        }
        let checkpoint = checkpoint.and_then(|checkpoint| checkpoint.source_seq(source_domain));
        // Only the legacy domain may recover ordering from an entity written
        // before explicit source domains were recorded. An entity `_seq`
        // alone cannot say whether its offset is an account write version,
        // instruction index or resolver counter.
        if source_domain != Some(SlotIndexDomain::Legacy) {
            return checkpoint;
        }
        let entity = self
            .entities
            .peek(key)
            .and_then(|entity| entity.field("_seq"))
            .and_then(Value::as_str);
        match (checkpoint, entity) {
            (Some(left), Some(right)) if cmp_seq(left, right).is_lt() => Some(right),
            (Some(left), _) => Some(left),
            (None, entity) => entity,
        }
    }

    fn accepts_source_ordering(
        &self,
        key: &str,
        incoming: Option<&str>,
        source_domain: Option<SlotIndexDomain>,
        allow_equal: bool,
    ) -> bool {
        match (self.latest_source_seq(key, source_domain), incoming) {
            (Some(previous), Some(incoming)) => {
                let ordering = cmp_seq(incoming, previous);
                ordering.is_gt() || (allow_equal && ordering.is_eq())
            }
            // Once a markerless source supplies ordering, an unsequenced
            // lifetime change cannot prove that it is newer.
            (Some(_), None) => false,
            (None, _) => true,
        }
    }

    /// Check a patch for `key` against the key's lifetime ordering, record
    /// its position, and decide what it does to this view: refused, stored as
    /// a new entity, or merged into the one held (see [`EntityCache`]'s "Only
    /// whole entities").
    fn admit(
        &mut self,
        key: &str,
        origin: PatchOrigin,
        ordering: LifetimeOrdering<'_>,
    ) -> Admission {
        let LifetimeOrdering {
            account_position,
            source_seq,
            source_domain,
        } = ordering;
        if origin != PatchOrigin::Unknown && !self.creations_marked {
            self.mark_creations();
        }

        let creation = origin == PatchOrigin::Creation;
        if !self.accepts(key, creation, account_position, source_seq, source_domain) {
            return Admission::Refuse;
        }

        if let Some(position) = account_position {
            // Account writes choose the lifetime, but they do not reset the
            // independent instruction/resolver high-water marks. Keeping
            // those cursors across deletion and recreation prevents a late
            // duplicate from the old lifetime from contaminating the new
            // row. A genuinely newer source event still advances its own
            // domain normally.
            let previous = self.lifetimes.peek(key).cloned();
            self.lifetimes.put(
                key.to_string(),
                EntityLifetimeCheckpoint {
                    account_position: Some(position),
                    source_seq: previous
                        .as_ref()
                        .and_then(|checkpoint| checkpoint.source_seq.clone()),
                    source_sequences: previous
                        .map(|checkpoint| checkpoint.source_sequences)
                        .unwrap_or_default(),
                    deleted: false,
                },
            );
            self.trim_lifetimes();
        } else if let (Some(source_seq), Some(source_domain)) = (source_seq, source_domain) {
            let mut checkpoint = if creation {
                EntityLifetimeCheckpoint {
                    account_position: None,
                    source_seq: None,
                    source_sequences: BTreeMap::new(),
                    deleted: false,
                }
            } else {
                self.lifetimes
                    .peek(key)
                    .cloned()
                    .unwrap_or(EntityLifetimeCheckpoint {
                        account_position: None,
                        source_seq: None,
                        source_sequences: BTreeMap::new(),
                        deleted: false,
                    })
            };
            checkpoint
                .source_sequences
                .insert(source_domain, source_seq.to_string());
            checkpoint.deleted = false;
            self.lifetimes.put(key.to_string(), checkpoint);
            self.trim_lifetimes();
        } else if creation
            && self.lifetimes.peek(key).is_some_and(|checkpoint| {
                checkpoint.source_seq.is_none() && checkpoint.source_sequences.is_empty()
            })
        {
            // An entirely unsequenced source preserves its historical
            // recreate behavior; there is no ordering evidence to retain.
            self.lifetimes.pop(key);
        }

        let whole = match origin {
            PatchOrigin::Creation => true,
            PatchOrigin::Change => self.entities.contains(key),
            PatchOrigin::Unknown => self.entities.contains(key) || !self.was_evicted(key),
        };
        if !whole {
            return Admission::Refuse;
        }
        // A creation is a complete replacement. Deep-merging it would retain
        // fields from the previous account lifetime.
        if creation || !self.entities.contains(key) {
            Admission::Create
        } else {
            Admission::Merge
        }
    }

    /// Whether a whole entity requested for `key` may be stored: it was
    /// requested for the account lifetime and source position that are still
    /// current (see [`EntityCache::store_whole_for_lifetime`]).
    fn accepts_whole(
        &self,
        key: &str,
        requested_account_position: Option<AccountPosition>,
        requested_source: Option<(Option<&str>, SlotIndexDomain)>,
    ) -> bool {
        let Some(checkpoint) = self.lifetimes.peek(key) else {
            return true;
        };
        if checkpoint.deleted || checkpoint.account_position != requested_account_position {
            return false;
        }
        match requested_source {
            Some((requested_source_seq, requested_source_domain)) => {
                !((checkpoint.account_position.is_none()
                    && checkpoint.source_seq(Some(requested_source_domain))
                        != requested_source_seq)
                    || self.entities.contains(key))
            }
            None => !(checkpoint.account_position.is_some() && self.entities.contains(key)),
        }
    }

    fn trim_lifetimes(&mut self) {
        while self.lifetimes.len()
            > self
                .max_entities
                .saturating_mul(EVICTED_KEYS_PER_CACHED_ENTITY)
        {
            self.lifetimes.pop_lru();
        }
    }
}

/// The authoritative account lifetime known for one projected entity.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct EntityLifetimeCheckpoint {
    #[serde(skip_serializing_if = "Option::is_none")]
    pub account_position: Option<AccountPosition>,
    /// Ordering for producers that do not carry an authoritative account
    /// position. This is never compared with `account_position`.
    #[serde(skip_serializing_if = "Option::is_none")]
    pub source_seq: Option<String>,
    /// Explicit producer-local cursors. Each domain is compared only with
    /// itself, so resolver counters cannot suppress later instructions.
    #[serde(default, skip_serializing_if = "BTreeMap::is_empty")]
    pub source_sequences: BTreeMap<SlotIndexDomain, String>,
    pub deleted: bool,
}

impl EntityLifetimeCheckpoint {
    fn source_seq(&self, source_domain: Option<SlotIndexDomain>) -> Option<&str> {
        match source_domain {
            Some(SlotIndexDomain::Legacy) | None => self
                .source_sequences
                .get(&SlotIndexDomain::Legacy)
                .map(String::as_str)
                .or(self.source_seq.as_deref()),
            Some(domain) => self.source_sequences.get(&domain).map(String::as_str),
        }
    }
}

/// `(view, key, checkpoint)` in most-recently-changed order.
pub type EntityLifetimes = Vec<(String, String, EntityLifetimeCheckpoint)>;

/// Independent ordering metadata attached to one projected cache write.
#[derive(Debug, Clone, Copy, Default)]
pub struct LifetimeOrdering<'a> {
    pub account_position: Option<AccountPosition>,
    pub source_seq: Option<&'a str>,
    /// The producer-local domain in which `source_seq` is comparable.
    /// `None` deliberately disables source recency checks.
    pub source_domain: Option<SlotIndexDomain>,
}

/// Legacy `(view, key, _seq)` barriers, retained only to restore snapshots
/// written before account positions had their own ordering domain.
pub type EntityTombstones = Vec<(String, String, Option<String>)>;

/// Compare two `_seq` values numerically.
/// `_seq` format is "{slot}:{offset}" where slot is not zero-padded.
/// This handles digit-count boundaries correctly (e.g., 99999999 < 100000000).
pub fn cmp_seq(a: &str, b: &str) -> std::cmp::Ordering {
    fn parse(s: &str) -> (u64, u64) {
        let mut parts = s.splitn(2, ':');
        let slot = parts.next().and_then(|p| p.parse().ok()).unwrap_or(0);
        let offset = parts.next().and_then(|p| p.parse().ok()).unwrap_or(0);
        (slot, offset)
    }
    parse(a).cmp(&parse(b))
}

fn account_position_from_seq(seq: &str) -> Option<AccountPosition> {
    let mut parts = seq.splitn(2, ':');
    Some(AccountPosition::new(
        parts.next()?.parse().ok()?,
        parts.next()?.parse().ok()?,
    ))
}

/// Configuration for the entity cache
#[derive(Debug, Clone)]
pub struct EntityCacheConfig {
    /// Maximum number of entities to cache per view
    pub max_entities_per_view: usize,
    /// Maximum array length before oldest elements are evicted
    pub max_array_length: usize,
    /// Number of entities to send in the first snapshot batch (for fast initial render)
    pub initial_snapshot_batch_size: usize,
    /// Number of entities to send in subsequent snapshot batches
    pub subsequent_snapshot_batch_size: usize,
}

impl Default for EntityCacheConfig {
    fn default() -> Self {
        Self {
            max_entities_per_view: DEFAULT_MAX_ENTITIES_PER_VIEW,
            max_array_length: DEFAULT_MAX_ARRAY_LENGTH,
            initial_snapshot_batch_size: DEFAULT_INITIAL_SNAPSHOT_BATCH_SIZE,
            subsequent_snapshot_batch_size: DEFAULT_SUBSEQUENT_SNAPSHOT_BATCH_SIZE,
        }
    }
}

/// Entity cache that maintains full projected entities with LRU eviction.
///
/// The cache is populated as mutations flow through the projector, regardless
/// of subscriber state. When a new subscriber connects, they receive snapshots
/// of all cached entities for their requested view.
///
/// # Only whole entities
///
/// Source mutations carry only the fields that changed. A patch for a key a
/// view holds merges into the whole entity. A patch for a key it does not
/// hold is stored only if it is the whole entity: storing a few changed
/// fields would serve them as though they were the entity, in snapshots and
/// in every derived view.
///
/// A VM linked through [`crate::EntityResync`] marks the mutation that creates
/// each entity ([`PatchOrigin::Creation`]), which is stored as-is. Any other
/// patch from it for a key a view does not hold ([`PatchOrigin::Change`]) is
/// refused with [`CacheWrite::Refused`] — whether the view evicted the key or
/// never held it, as after a restore. The projector then asks the VM for the
/// whole entity, which follows in the VM's next batch and is stored with
/// [`EntityCache::store_whole`]. That holds however many entities the VM
/// keeps and however long ago the view evicted the key.
///
/// A source that marks nothing ([`PatchOrigin::Unknown`]) leaves a new key and
/// an evicted one looking alike. Each view therefore remembers the keys it
/// evicted (bounded, most recent first) and refuses patches for them; a key
/// evicted so long ago that it has been forgotten is taken for new. A source
/// delete also ends an eviction.
#[derive(Clone)]
pub struct EntityCache {
    /// view_id -> LRU<entity_key, full_projected_entity>, plus evicted keys
    caches: Arc<RwLock<HashMap<String, ViewEntries>>>,
    config: EntityCacheConfig,
}

impl EntityCache {
    /// Create a new entity cache with default configuration
    pub fn new() -> Self {
        Self::with_config(EntityCacheConfig::default())
    }

    /// Create a new entity cache with custom configuration
    pub fn with_config(config: EntityCacheConfig) -> Self {
        Self {
            caches: Arc::new(RwLock::new(HashMap::new())),
            config,
        }
    }

    /// Maximum number of entities cached per view.
    ///
    /// Derived sorted view caches are bounded by the same value so they never
    /// hold more entities than their source view caches.
    pub fn max_entities_per_view(&self) -> usize {
        self.config.max_entities_per_view
    }

    /// Write a patch from a source that does not mark creations
    /// ([`PatchOrigin::Unknown`]).
    pub async fn upsert(&self, view_id: &str, key: &str, patch: Value) -> CacheWrite {
        self.upsert_with_append(view_id, key, patch, &[], PatchOrigin::Unknown)
            .await
    }

    /// Merge `patch` into the cached entity, store it as a new entity, or
    /// refuse it as part of an entity the cache does not hold (see the type
    /// docs).
    pub async fn upsert_with_append(
        &self,
        view_id: &str,
        key: &str,
        patch: Value,
        append_paths: &[String],
        origin: PatchOrigin,
    ) -> CacheWrite {
        self.upsert_with_lifetime(view_id, key, patch, append_paths, origin, None)
            .await
    }

    /// Apply a projected patch with optional authoritative account ordering.
    /// `_seq` is consulted for lifetime changes only when no account position
    /// exists; the two ordering domains are never compared.
    pub async fn upsert_with_lifetime(
        &self,
        view_id: &str,
        key: &str,
        patch: Value,
        append_paths: &[String],
        origin: PatchOrigin,
        account_position: Option<AccountPosition>,
    ) -> CacheWrite {
        let source_seq = patch.get("_seq").and_then(Value::as_str).map(str::to_owned);
        self.upsert_with_ordering(
            view_id,
            key,
            patch,
            append_paths,
            origin,
            LifetimeOrdering {
                account_position,
                source_seq: source_seq.as_deref(),
                // The convenience path supports markerless lifetime ordering,
                // but sparse legacy patches may mix account write versions
                // and instruction indices within one slot.
                source_domain: (origin == PatchOrigin::Creation).then_some(SlotIndexDomain::Legacy),
            },
        )
        .await
    }

    /// Apply a projected patch with both ordering domains supplied
    /// explicitly. Projections may omit `_seq`, so the projector passes its
    /// source cursor separately here.
    pub async fn upsert_with_ordering(
        &self,
        view_id: &str,
        key: &str,
        patch: Value,
        append_paths: &[String],
        origin: PatchOrigin,
        ordering: LifetimeOrdering<'_>,
    ) -> CacheWrite {
        let (patch, version) = split_version(patch);
        self.upsert_views(
            key,
            patch,
            &[ViewVersion { view_id, version }],
            append_paths,
            origin,
            ordering,
        )
        .await
        .pop()
        .expect("upsert_views returns one write per view")
    }

    /// Apply one patch to several views of its export at once, each with the
    /// `_version` its own frame carries. `patch` holds every other field.
    ///
    /// Each view decides what to do with the patch exactly as
    /// [`Self::upsert_with_ordering`] would on its own. Views that store it
    /// whole share one copy of it, and views that hold the same fields (see
    /// [`SharedEntity`]) merge it into them once and keep sharing the result.
    /// That merge happens in place unless something outside these views
    /// (a derived view, a subscriber) still holds the fields, in which case
    /// the merge copies only the top-level fields it changes and the other
    /// holder keeps the old ones, sharing the rest with the new.
    pub(crate) async fn upsert_views(
        &self,
        key: &str,
        patch: Value,
        views: &[ViewVersion<'_>],
        append_paths: &[String],
        origin: PatchOrigin,
        ordering: LifetimeOrdering<'_>,
    ) -> Vec<CacheWrite> {
        // A stray `_version` in the patch stands for any view without one,
        // and must not end up among the shared fields.
        let (mut patch, stray_version) = split_version(patch);
        let version_of = |index: usize| {
            views[index]
                .version
                .clone()
                .or_else(|| stray_version.clone())
        };
        let max_array_length = self.config.max_array_length;
        let mut caches = self.caches.write().await;

        let mut admissions: Vec<Admission> = views
            .iter()
            .map(|view| {
                caches
                    .entry(view.view_id.to_string())
                    .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view))
                    .admit(key, origin, ordering)
            })
            .collect();

        let mut writes = vec![CacheWrite::Merged; views.len()];
        for (index, admission) in admissions.iter().enumerate() {
            if *admission == Admission::Refuse {
                writes[index] = CacheWrite::Refused {
                    patch: with_version(patch.clone(), version_of(index)),
                };
            }
        }

        // Take the fields out of every merging view, so views that share them
        // can merge them once: in place when no other holder is left.
        let placeholder = Arc::new(Fields::Other(Value::Null));
        let mut groups: Vec<(Arc<Fields>, Vec<usize>)> = Vec::new();
        for (index, admission) in admissions.iter_mut().enumerate() {
            if *admission != Admission::Merge {
                continue;
            }
            let held = caches
                .get_mut(views[index].view_id)
                .and_then(|view| view.entities.get_mut(key));
            let Some(entity) = held else {
                // Admitted under this lock as held, so this cannot happen;
                // storing the patch whole is what a view without it does.
                *admission = Admission::Create;
                continue;
            };
            let fields = std::mem::replace(entity.fields_mut(), placeholder.clone());
            match groups
                .iter_mut()
                .find(|(shared, _)| Arc::ptr_eq(shared, &fields))
            {
                Some((_, members)) => members.push(index),
                None => groups.push((fields, vec![index])),
            }
        }

        if admissions.contains(&Admission::Create) {
            let source = if groups.is_empty() {
                std::mem::take(&mut patch)
            } else {
                patch.clone()
            };
            let created = Arc::new(Fields::from_value(truncate_arrays_if_needed(
                source,
                max_array_length,
            )));
            for (index, admission) in admissions.iter().enumerate() {
                if *admission != Admission::Create {
                    continue;
                }
                let version = version_of(index)
                    .filter(|_| created.is_object())
                    .map(|version| truncate_arrays_if_needed(version, max_array_length));
                if let Some(view) = caches.get_mut(views[index].view_id) {
                    view.store(
                        key.to_string(),
                        SharedEntity::from_parts(created.clone(), version),
                    );
                }
                writes[index] = CacheWrite::Created;
            }
        }

        let last_group = groups.len().saturating_sub(1);
        for (group, (mut fields, members)) in groups.into_iter().enumerate() {
            let mut group_patch = if group == last_group {
                std::mem::take(&mut patch)
            } else {
                patch.clone()
            };
            let merge_once = fields.is_object() && group_patch.is_object();
            if merge_once {
                merge_fields(
                    Arc::make_mut(&mut fields),
                    std::mem::take(&mut group_patch),
                    append_paths,
                    max_array_length,
                );
            }
            for index in members {
                let Some(entity) = caches
                    .get_mut(views[index].view_id)
                    .and_then(|view| view.entities.peek_mut(key))
                else {
                    continue;
                };
                *entity.fields_mut() = fields.clone();
                if merge_once {
                    let version = entity.version_mut();
                    *version = merge_version(
                        version.take(),
                        version_of(index),
                        append_paths,
                        max_array_length,
                    );
                } else {
                    // Not two objects: the patch replaces the entity, or
                    // merges into the array it is. Rare enough to do per view.
                    merge_shared(
                        entity,
                        with_version(group_patch.clone(), version_of(index)),
                        append_paths,
                        max_array_length,
                    );
                }
            }
        }
        writes
    }

    /// Store `entity` as the whole entity for `key`, replacing anything held
    /// and clearing an eviction: the source vouched for it being complete
    /// (see [`arete_interpreter::Mutation::mark_whole_entity`]).
    pub async fn store_whole(&self, view_id: &str, key: &str, entity: Value) -> bool {
        self.store_whole_for_lifetime(view_id, key, entity, None)
            .await
    }

    /// Store a requested whole entity only if the request was made for the
    /// account lifetime that is still current.
    pub async fn store_whole_for_lifetime(
        &self,
        view_id: &str,
        key: &str,
        entity: Value,
        requested_account_position: Option<AccountPosition>,
    ) -> bool {
        self.store_whole_checked(view_id, key, entity, requested_account_position, None)
            .await
    }

    pub async fn store_whole_for_ordering(
        &self,
        view_id: &str,
        key: &str,
        entity: Value,
        requested_account_position: Option<AccountPosition>,
        requested_source_seq: Option<&str>,
    ) -> bool {
        self.store_whole_for_ordering_in_domain(
            view_id,
            key,
            entity,
            requested_account_position,
            requested_source_seq,
            SlotIndexDomain::Legacy,
        )
        .await
    }

    pub async fn store_whole_for_ordering_in_domain(
        &self,
        view_id: &str,
        key: &str,
        entity: Value,
        requested_account_position: Option<AccountPosition>,
        requested_source_seq: Option<&str>,
        requested_source_domain: SlotIndexDomain,
    ) -> bool {
        self.store_whole_checked(
            view_id,
            key,
            entity,
            requested_account_position,
            Some((requested_source_seq, requested_source_domain)),
        )
        .await
    }

    async fn store_whole_checked(
        &self,
        view_id: &str,
        key: &str,
        entity: Value,
        requested_account_position: Option<AccountPosition>,
        requested_source: Option<(Option<&str>, SlotIndexDomain)>,
    ) -> bool {
        let (entity, version) = split_version(entity);
        self.store_whole_views(
            key,
            entity,
            &[ViewVersion { view_id, version }],
            requested_account_position,
            requested_source,
        )
        .await
        .pop()
        .expect("store_whole_views answers for every view")
    }

    /// Store a whole entity in several views at once, each under its own
    /// `_version`, where the checks of [`Self::store_whole_for_lifetime`]
    /// (or, with `requested_source`,
    /// [`Self::store_whole_for_ordering_in_domain`]) pass. `entity` holds every
    /// other field; the views that store it share one copy.
    pub(crate) async fn store_whole_views(
        &self,
        key: &str,
        entity: Value,
        views: &[ViewVersion<'_>],
        requested_account_position: Option<AccountPosition>,
        requested_source: Option<(Option<&str>, SlotIndexDomain)>,
    ) -> Vec<bool> {
        let max_array_length = self.config.max_array_length;
        let (entity, stray_version) = split_version(entity);
        let fields = Arc::new(Fields::from_value(truncate_arrays_if_needed(
            entity,
            max_array_length,
        )));
        let mut caches = self.caches.write().await;
        views
            .iter()
            .map(|view| {
                let entries = caches
                    .entry(view.view_id.to_string())
                    .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
                if !entries.accepts_whole(key, requested_account_position, requested_source) {
                    return false;
                }
                let version = view
                    .version
                    .clone()
                    .or_else(|| stray_version.clone())
                    .filter(|_| fields.is_object())
                    .map(|version| truncate_arrays_if_needed(version, max_array_length));
                entries.store(
                    key.to_string(),
                    SharedEntity::from_parts(fields.clone(), version),
                );
                true
            })
            .collect()
    }

    pub async fn account_position(&self, view_id: &str, key: &str) -> Option<AccountPosition> {
        self.caches
            .read()
            .await
            .get(view_id)
            .and_then(|view| view.lifetimes.peek(key))
            .and_then(|checkpoint| checkpoint.account_position)
    }

    pub async fn accepts_mutation(
        &self,
        view_id: &str,
        key: &str,
        _entity: &Value,
        creation: bool,
    ) -> bool {
        self.accepts_lifetime_mutation(view_id, key, creation, None)
            .await
    }

    pub async fn accepts_lifetime_mutation(
        &self,
        view_id: &str,
        key: &str,
        creation: bool,
        account_position: Option<AccountPosition>,
    ) -> bool {
        self.accepts_ordered_lifetime_mutation(view_id, key, creation, account_position, None)
            .await
    }

    pub async fn accepts_ordered_lifetime_mutation(
        &self,
        view_id: &str,
        key: &str,
        creation: bool,
        account_position: Option<AccountPosition>,
        source_seq: Option<&str>,
    ) -> bool {
        self.accepts_ordered_lifetime_mutation_in_domain(
            view_id,
            key,
            creation,
            account_position,
            source_seq,
            SlotIndexDomain::Legacy,
        )
        .await
    }

    pub async fn accepts_ordered_lifetime_mutation_in_domain(
        &self,
        view_id: &str,
        key: &str,
        creation: bool,
        account_position: Option<AccountPosition>,
        source_seq: Option<&str>,
        source_domain: SlotIndexDomain,
    ) -> bool {
        self.caches.read().await.get(view_id).is_none_or(|view| {
            view.accepts(
                key,
                creation,
                account_position,
                source_seq,
                Some(source_domain),
            )
        })
    }

    /// Apply an ordered source deletion. A delayed deletion never removes a
    /// newer recreation. Only a newer, explicitly marked creation can restart
    /// a deleted row; sparse patches and resends cannot resurrect it.
    pub async fn delete(
        &self,
        view_id: &str,
        key: &str,
        account_position: Option<AccountPosition>,
    ) -> bool {
        match account_position {
            Some(position) => {
                self.delete_ordered(view_id, key, Some(position), None)
                    .await
            }
            None => self.delete_current(view_id, key).await,
        }
    }

    /// Delete the lifetime currently held by the cache when the producer has
    /// no cursor to attach. This is an explicit, trusted operation: callers
    /// with ordering evidence must use [`Self::delete_ordered`] instead.
    pub async fn delete_current(&self, view_id: &str, key: &str) -> bool {
        let mut caches = self.caches.write().await;
        let view = caches
            .entry(view_id.to_string())
            .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
        let checkpoint = view
            .lifetimes
            .peek(key)
            .cloned()
            .unwrap_or(EntityLifetimeCheckpoint {
                account_position: None,
                source_seq: None,
                source_sequences: BTreeMap::new(),
                deleted: false,
            });
        if checkpoint.deleted {
            return true;
        }
        view.entities.pop(key);
        view.forget_evicted(key);
        view.lifetimes.put(
            key.to_string(),
            EntityLifetimeCheckpoint {
                deleted: true,
                ..checkpoint
            },
        );
        view.trim_lifetimes();
        true
    }

    /// Delete using the producer's own `_seq` ordering only when no
    /// authoritative account position is present.
    pub async fn delete_ordered(
        &self,
        view_id: &str,
        key: &str,
        account_position: Option<AccountPosition>,
        source_seq: Option<&str>,
    ) -> bool {
        self.delete_ordered_in_domain(
            view_id,
            key,
            account_position,
            source_seq,
            SlotIndexDomain::Legacy,
        )
        .await
    }

    pub async fn delete_ordered_in_domain(
        &self,
        view_id: &str,
        key: &str,
        account_position: Option<AccountPosition>,
        source_seq: Option<&str>,
        source_domain: SlotIndexDomain,
    ) -> bool {
        let mut caches = self.caches.write().await;
        let view = caches
            .entry(view_id.to_string())
            .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
        if let Some(checkpoint) = view.lifetimes.peek(key) {
            match (checkpoint.account_position, account_position) {
                (Some(previous), Some(incoming)) if incoming < previous => return false,
                (Some(previous), Some(incoming)) if incoming == previous => {
                    return checkpoint.deleted;
                }
                _ => {}
            }
        }
        if account_position.is_none()
            && source_seq.is_some()
            && !view.accepts_source_ordering(key, source_seq, Some(source_domain), true)
        {
            return false;
        }
        let previous = view.lifetimes.peek(key).cloned();
        view.entities.pop(key);
        view.forget_evicted(key);
        let mut checkpoint = if account_position.is_some() {
            let previous = previous.unwrap_or(EntityLifetimeCheckpoint {
                account_position: None,
                source_seq: None,
                source_sequences: BTreeMap::new(),
                deleted: false,
            });
            EntityLifetimeCheckpoint {
                account_position,
                source_seq: previous.source_seq,
                source_sequences: previous.source_sequences,
                deleted: true,
            }
        } else {
            previous.unwrap_or(EntityLifetimeCheckpoint {
                account_position: None,
                source_seq: None,
                source_sequences: BTreeMap::new(),
                deleted: false,
            })
        };
        if account_position.is_none() {
            if let Some(source_seq) = source_seq {
                checkpoint
                    .source_sequences
                    .insert(source_domain, source_seq.to_string());
            }
            checkpoint.deleted = true;
        }
        view.lifetimes.put(key.to_string(), checkpoint);
        view.trim_lifetimes();
        true
    }

    pub async fn dump_lifetimes(&self) -> EntityLifetimes {
        self.caches
            .read()
            .await
            .iter()
            .flat_map(|(id, view)| {
                view.lifetimes
                    .iter()
                    .map(|(key, checkpoint)| (id.clone(), key.clone(), checkpoint.clone()))
            })
            .collect()
    }

    pub async fn hydrate_lifetimes(&self, lifetimes: EntityLifetimes) {
        let mut caches = self.caches.write().await;
        for (id, key, checkpoint) in lifetimes.into_iter().rev() {
            let view = caches
                .entry(id)
                .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
            view.lifetimes.put(key, checkpoint);
            view.trim_lifetimes();
        }
    }

    /// Restore the old `_seq` barriers as account positions. Those barriers
    /// were written only by authoritative account deletion/recreation paths;
    /// whether the row exists distinguishes a live recreation from deletion.
    pub async fn hydrate_legacy_tombstones(&self, tombstones: EntityTombstones) {
        let mut caches = self.caches.write().await;
        for (id, key, seq) in tombstones.into_iter().rev() {
            let view = caches
                .entry(id)
                .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
            let deleted = !view.entities.contains(&key);
            let account_position = seq.as_deref().and_then(account_position_from_seq);
            view.lifetimes.put(
                key,
                EntityLifetimeCheckpoint {
                    account_position,
                    source_seq: None,
                    source_sequences: BTreeMap::new(),
                    deleted,
                },
            );
            view.trim_lifetimes();
        }
    }

    /// Whether the shared cache says a published delete still represents the
    /// latest lifetime. Subscription tasks use this without mutating the cache
    /// a second time after the projector has already applied the delete.
    pub async fn deletion_is_current(&self, view_id: &str, key: &str) -> bool {
        let caches = self.caches.read().await;
        caches.get(view_id).is_none_or(|view| {
            view.lifetimes
                .peek(key)
                .map_or(!view.entities.contains(key), |checkpoint| {
                    checkpoint.deleted
                })
        })
    }

    /// Merge a source patch into `base` with this cache's append and array
    /// rules, for callers that keep their own full copy of an entity.
    pub fn merge_patch(&self, base: &mut Value, patch: Value, append_paths: &[String]) {
        deep_merge_with_append(base, patch, append_paths, self.config.max_array_length);
    }

    /// [`Self::merge_patch`] for a [`SharedEntity`]: copies its fields first
    /// if another holder shares them, so the merge never reaches that holder.
    pub fn merge_patch_shared(
        &self,
        entity: &mut SharedEntity,
        patch: Value,
        append_paths: &[String],
    ) {
        merge_shared(entity, patch, append_paths, self.config.max_array_length);
    }

    /// Treat `keys` as evicted from `view_id` unless the view holds them, so
    /// patches for them from a source that does not mark creations are
    /// refused. Used after a restore, when the VM holds entities the restored
    /// cache does not; a VM that marks creations makes this moot, and the
    /// view drops the memory with its first patch.
    pub async fn remember_evicted(&self, view_id: &str, keys: impl IntoIterator<Item = String>) {
        let mut caches = self.caches.write().await;
        let view = caches
            .entry(view_id.to_string())
            .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
        for key in keys {
            if !view.entities.contains(&key) {
                view.remember_evicted(key);
            }
        }
    }

    /// Get all cached entities for a view.
    ///
    /// Returns a vector of (key, entity) pairs for sending as snapshots
    /// to new subscribers. Each is a copy; [`Self::get_all_shared`] returns
    /// the cached entities themselves.
    pub async fn get_all(&self, view_id: &str) -> Vec<(String, Value)> {
        self.get_all_shared(view_id)
            .await
            .into_iter()
            .map(|(key, entity)| (key, entity.into_value()))
            .collect()
    }

    /// Every entity cached for a view, most recently used first, sharing
    /// their fields with the cache rather than copying them.
    pub async fn get_all_shared(&self, view_id: &str) -> Vec<(String, SharedEntity)> {
        let caches = self.caches.read().await;

        caches
            .get(view_id)
            .map(|view| {
                view.entities
                    .iter()
                    .map(|(k, v)| (k.clone(), v.clone()))
                    .collect()
            })
            .unwrap_or_default()
    }

    /// Get entities with _seq greater than the provided cursor.
    ///
    /// Returns entities that have been updated after the given cursor,
    /// sorted by _seq in ascending order. Useful for resuming from
    /// a specific point in the stream.
    pub async fn get_after(
        &self,
        view_id: &str,
        cursor: &str,
        limit: Option<usize>,
    ) -> Vec<(String, Value)> {
        let caches = self.caches.read().await;

        if let Some(view) = caches.get(view_id) {
            let mut results: Vec<(String, &SharedEntity)> = view
                .entities
                .iter()
                .filter(|(_, entity)| {
                    entity
                        .field("_seq")
                        .and_then(|s| s.as_str())
                        .map(|seq| cmp_seq(seq, cursor) == std::cmp::Ordering::Greater)
                        .unwrap_or(false)
                })
                .map(|(k, v)| (k.clone(), v))
                .collect();

            // Sort by _seq (ascending)
            results.sort_by(|a, b| {
                let seq_a = a.1.field("_seq").and_then(|s| s.as_str()).unwrap_or("");
                let seq_b = b.1.field("_seq").and_then(|s| s.as_str()).unwrap_or("");
                cmp_seq(seq_a, seq_b)
            });

            // Apply limit if provided
            if let Some(limit) = limit {
                results.truncate(limit);
            }

            results
                .into_iter()
                .map(|(key, entity)| (key, entity.to_value()))
                .collect()
        } else {
            vec![]
        }
    }

    /// Get a copy of a specific entity from the cache.
    pub async fn get(&self, view_id: &str, key: &str) -> Option<Value> {
        self.get_shared(view_id, key)
            .await
            .map(SharedEntity::into_value)
    }

    /// A specific cached entity, sharing its fields with the cache rather
    /// than copying them.
    pub async fn get_shared(&self, view_id: &str, key: &str) -> Option<SharedEntity> {
        let caches = self.caches.read().await;
        caches
            .get(view_id)
            .and_then(|view| view.entities.peek(key).cloned())
    }

    /// Remove one entity after a source-wide delete.
    ///
    /// The delete ends the entity, so the key is also forgotten as evicted: a
    /// later patch for it creates a new entity.
    pub async fn remove(&self, view_id: &str, key: &str) -> Option<Value> {
        let removed = {
            let mut caches = self.caches.write().await;
            caches.get_mut(view_id).and_then(|view| {
                view.forget_evicted(key);
                view.entities.pop(key)
            })
        };
        removed.map(SharedEntity::into_value)
    }

    /// Get the number of cached entities for a view
    pub async fn len(&self, view_id: &str) -> usize {
        let caches = self.caches.read().await;
        caches
            .get(view_id)
            .map(|view| view.entities.len())
            .unwrap_or(0)
    }

    /// Check if the cache for a view is empty
    pub async fn is_empty(&self, view_id: &str) -> bool {
        self.len(view_id).await == 0
    }

    /// Get the snapshot batch configuration
    pub fn snapshot_config(&self) -> SnapshotBatchConfig {
        SnapshotBatchConfig {
            initial_batch_size: self.config.initial_snapshot_batch_size,
            subsequent_batch_size: self.config.subsequent_snapshot_batch_size,
        }
    }

    /// Clear all cached entities for a view
    pub async fn clear(&self, view_id: &str) {
        let mut caches = self.caches.write().await;
        if let Some(view) = caches.get_mut(view_id) {
            view.entities.clear();
            view.evicted = None;
        }
    }

    pub async fn clear_all(&self) {
        let mut caches = self.caches.write().await;
        caches.clear();
    }

    /// Dump every view's entities for a state snapshot.
    ///
    /// Entries are ordered most-recently-used first; [`Self::hydrate`] relies
    /// on that to reconstruct LRU eviction order. Each is a copy;
    /// [`Self::dump_shared`] returns the cached entities themselves.
    pub async fn dump(&self) -> Vec<(String, Vec<(String, Value)>)> {
        self.dump_shared()
            .await
            .into_iter()
            .map(|(view_id, entries)| {
                (
                    view_id,
                    entries
                        .into_iter()
                        .map(|(key, entity)| (key, entity.into_value()))
                        .collect(),
                )
            })
            .collect()
    }

    /// [`Self::dump`], sharing the entities' fields with the cache rather
    /// than copying them. They serialize exactly as the copies would.
    pub async fn dump_shared(&self) -> Vec<(String, Vec<(String, SharedEntity)>)> {
        let caches = self.caches.read().await;
        caches
            .iter()
            .map(|(view_id, view)| {
                (
                    view_id.clone(),
                    view.entities
                        .iter()
                        .map(|(key, entity)| (key.clone(), entity.clone()))
                        .collect(),
                )
            })
            .collect()
    }

    /// Restore entities dumped by [`Self::dump`], preserving LRU order.
    ///
    /// Entities are inserted as-is (no merge): a snapshot holds fully
    /// projected entities, not patches. A snapshot saves each view's copy of
    /// an entity on its own, so views of one export (`{export}/{mode}`) whose
    /// copies have the same fields (all but `_version`) share them again, as
    /// they did when saved. Only copies of one export are compared.
    pub async fn hydrate(&self, views: Vec<(String, Vec<(String, Value)>)>) {
        let mut caches = self.caches.write().await;
        caches.clear();
        let mut restored: HashMap<(String, String), Vec<Arc<Fields>>> = HashMap::new();
        for (view_id, entries) in views {
            let export = view_id.split('/').next().unwrap_or_default().to_string();
            let view = caches
                .entry(view_id)
                .or_insert_with(|| ViewEntries::new(self.config.max_entities_per_view));
            for (key, entity) in entries.into_iter().rev() {
                let (fields, version) = SharedEntity::new(entity).into_parts();
                let seen = restored.entry((export.clone(), key.clone())).or_default();
                let fields = match seen.iter().find(|held| **held == fields) {
                    Some(held) => held.clone(),
                    None => {
                        seen.push(fields.clone());
                        fields
                    }
                };
                view.store(key, SharedEntity::from_parts(fields, version));
            }
        }
    }

    pub async fn stats(&self) -> CacheStats {
        let caches = self.caches.read().await;
        let mut total_entities = 0;
        let mut views = Vec::new();

        for (view_id, view) in caches.iter() {
            let count = view.entities.len();
            total_entities += count;
            views.push((view_id.clone(), count));
        }

        views.sort_by_key(|b| std::cmp::Reverse(b.1));

        CacheStats {
            view_count: caches.len(),
            total_entities,
            top_views: views.into_iter().take(5).collect(),
        }
    }
}

/// A point-in-time count of an [`EntityCache`]'s contents.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct CacheStats {
    pub view_count: usize,
    pub total_entities: usize,
    pub top_views: Vec<(String, usize)>,
}

#[derive(Debug, Clone, Copy)]
pub struct SnapshotBatchConfig {
    pub initial_batch_size: usize,
    pub subsequent_batch_size: usize,
}

impl Default for EntityCache {
    fn default() -> Self {
        Self::new()
    }
}

fn deep_merge_with_append(
    base: &mut Value,
    patch: Value,
    append_paths: &[String],
    max_array_length: usize,
) {
    deep_merge_with_append_inner(base, patch, append_paths, "", max_array_length);
}

fn deep_merge_with_append_inner(
    base: &mut Value,
    patch: Value,
    append_paths: &[String],
    current_path: &str,
    max_array_length: usize,
) {
    match (base, patch) {
        (Value::Object(base_map), Value::Object(patch_map)) => {
            for (key, patch_value) in patch_map {
                let child_path = child_path(append_paths, current_path, &key);

                if let Some(base_value) = base_map.get_mut(&key) {
                    deep_merge_with_append_inner(
                        base_value,
                        patch_value,
                        append_paths,
                        &child_path,
                        max_array_length,
                    );
                } else {
                    base_map.insert(
                        key,
                        truncate_arrays_if_needed(patch_value, max_array_length),
                    );
                }
            }
        }

        (Value::Array(base_arr), Value::Array(patch_arr)) => {
            let should_append = append_paths.iter().any(|p| p == current_path);
            if should_append {
                base_arr.extend(patch_arr);
                if base_arr.len() > max_array_length {
                    let excess = base_arr.len() - max_array_length;
                    base_arr.drain(0..excess);
                }
            } else {
                *base_arr = patch_arr;
                if base_arr.len() > max_array_length {
                    let excess = base_arr.len() - max_array_length;
                    base_arr.drain(0..excess);
                }
            }
        }

        (base, patch_value) => {
            *base = truncate_arrays_if_needed(patch_value, max_array_length);
        }
    }
}

/// [`deep_merge_with_append`] for an object's shared fields and an object
/// patch: the same result, but a top-level field the merge would leave as it
/// is keeps sharing its value, and one another holder shares is merged into a
/// copy of what it keeps rather than a copy of the whole field.
fn merge_fields(
    fields: &mut Fields,
    patch: Value,
    append_paths: &[String],
    max_array_length: usize,
) {
    let (Fields::Object(members), Value::Object(patch)) = (fields, patch) else {
        unreachable!("merge_fields merges an object patch into an object");
    };
    for (key, patch_value) in patch {
        match members.get_mut(&key) {
            Some(member) => {
                if merge_changes_nothing(member, &patch_value, append_paths, &key, max_array_length)
                {
                    continue;
                }
                match Arc::get_mut(member) {
                    Some(value) => deep_merge_with_append_inner(
                        value,
                        patch_value,
                        append_paths,
                        &key,
                        max_array_length,
                    ),
                    None => {
                        *member = Arc::new(merged_copy(
                            member,
                            patch_value,
                            append_paths,
                            &key,
                            max_array_length,
                        ))
                    }
                }
            }
            None => members.insert(
                key,
                Arc::new(truncate_arrays_if_needed(patch_value, max_array_length)),
            ),
        }
    }
}

/// Whether [`deep_merge_with_append_inner`] would leave `base` exactly as it
/// is. Conservative: `false` whenever that is not plain.
fn merge_changes_nothing(
    base: &Value,
    patch: &Value,
    append_paths: &[String],
    current_path: &str,
    max_array_length: usize,
) -> bool {
    match (base, patch) {
        (Value::Object(base_map), Value::Object(patch_map)) => {
            patch_map.iter().all(|(key, patch_value)| {
                base_map.get(key).is_some_and(|base_value| {
                    merge_changes_nothing(
                        base_value,
                        patch_value,
                        append_paths,
                        &child_path(append_paths, current_path, key),
                        max_array_length,
                    )
                })
            })
        }
        (Value::Array(base_arr), Value::Array(patch_arr)) => {
            base_arr.len() <= max_array_length
                && if append_paths.iter().any(|path| path == current_path) {
                    patch_arr.is_empty()
                } else {
                    base_arr == patch_arr
                }
        }
        (Value::Object(_) | Value::Array(_), _) | (_, Value::Object(_) | Value::Array(_)) => false,
        (base, patch) => base == patch,
    }
}

/// What [`deep_merge_with_append_inner`] makes of a copy of `base`, without
/// copying the parts of `base` the patch replaces.
fn merged_copy(
    base: &Value,
    patch: Value,
    append_paths: &[String],
    current_path: &str,
    max_array_length: usize,
) -> Value {
    match (base, patch) {
        (Value::Object(base_map), Value::Object(patch_map)) => {
            // The keys `base` has, in its order, then those only the patch
            // has, in the patch's: where merging into `base` puts them.
            let mut merged = serde_json::Map::new();
            for (key, value) in base_map {
                let value = if patch_map.contains_key(key) {
                    Value::Null
                } else {
                    value.clone()
                };
                merged.insert(key.clone(), value);
            }
            for (key, patch_value) in patch_map {
                let child_path = child_path(append_paths, current_path, &key);
                match (merged.get_mut(&key), base_map.get(&key)) {
                    (Some(slot), Some(base_value)) => {
                        *slot = merged_copy(
                            base_value,
                            patch_value,
                            append_paths,
                            &child_path,
                            max_array_length,
                        );
                    }
                    _ => {
                        merged.insert(
                            key,
                            truncate_arrays_if_needed(patch_value, max_array_length),
                        );
                    }
                }
            }
            Value::Object(merged)
        }
        (Value::Array(base_arr), Value::Array(mut patch_arr)) => {
            if append_paths.iter().any(|path| path == current_path) {
                // Only the newest `max_array_length` of base and patch stay.
                let kept = max_array_length
                    .saturating_sub(patch_arr.len())
                    .min(base_arr.len());
                let mut merged = base_arr[base_arr.len() - kept..].to_vec();
                merged.append(&mut patch_arr);
                if merged.len() > max_array_length {
                    let excess = merged.len() - max_array_length;
                    merged.drain(0..excess);
                }
                Value::Array(merged)
            } else {
                if patch_arr.len() > max_array_length {
                    let excess = patch_arr.len() - max_array_length;
                    patch_arr.drain(0..excess);
                }
                Value::Array(patch_arr)
            }
        }
        (_, patch_value) => truncate_arrays_if_needed(patch_value, max_array_length),
    }
}

/// The dotted path of field `key` of the value at `current_path`, which only
/// the append check reads: without append paths there is nothing to build.
fn child_path(append_paths: &[String], current_path: &str, key: &str) -> String {
    if append_paths.is_empty() {
        String::new()
    } else if current_path.is_empty() {
        key.to_string()
    } else {
        format!("{current_path}.{key}")
    }
}

/// Recursively truncate any arrays in a value to the max length, keeping
/// the newest (last) elements.
fn truncate_arrays_if_needed(mut value: Value, max_array_length: usize) -> Value {
    truncate_arrays_in_place(&mut value, max_array_length);
    value
}

/// [`truncate_arrays_if_needed`] without rebuilding anything that needs no
/// truncation.
fn truncate_arrays_in_place(value: &mut Value, max_array_length: usize) {
    match value {
        Value::Array(arr) => {
            if arr.len() > max_array_length {
                let excess = arr.len() - max_array_length;
                arr.drain(0..excess);
            }
            for element in arr {
                truncate_arrays_in_place(element, max_array_length);
            }
        }
        Value::Object(map) => {
            for field in map.values_mut() {
                truncate_arrays_in_place(field, max_array_length);
            }
        }
        _ => {}
    }
}

/// Merge `patch` into `entity` exactly as [`deep_merge_with_append`] would
/// merge it into the whole entity, `_version` included. The fields are copied
/// first if another holder shares them.
fn merge_shared(
    entity: &mut SharedEntity,
    patch: Value,
    append_paths: &[String],
    max_array_length: usize,
) {
    let (patch, patch_version) = split_version(patch);
    if entity.fields().is_object() && patch.is_object() {
        merge_fields(
            Arc::make_mut(entity.fields_mut()),
            patch,
            append_paths,
            max_array_length,
        );
        let version = entity.version_mut();
        *version = merge_version(
            version.take(),
            patch_version,
            append_paths,
            max_array_length,
        );
        return;
    }
    // Not two objects: the patch replaces the entity, unless both are arrays
    // and it merges into the one held. Only an object has a `_version`, so
    // the entity's own goes with it.
    let (fields, _) = std::mem::replace(entity, SharedEntity::new(Value::Null)).into_parts();
    let mut whole = match (&*fields, &patch) {
        (Fields::Other(Value::Array(_)), Value::Array(_)) => match Arc::try_unwrap(fields) {
            Ok(fields) => fields.into_value(),
            Err(shared) => shared.to_value(),
        },
        _ => Value::Null,
    };
    deep_merge_with_append(
        &mut whole,
        with_version(patch, patch_version),
        append_paths,
        max_array_length,
    );
    *entity = SharedEntity::new(whole);
}

/// The `_version` an entity has after a patch: the patch's, merged into the
/// entity's as [`deep_merge_with_append`] merges any top-level field.
fn merge_version(
    current: Option<Value>,
    incoming: Option<Value>,
    append_paths: &[String],
    max_array_length: usize,
) -> Option<Value> {
    match (current, incoming) {
        (Some(mut current), Some(incoming)) => {
            deep_merge_with_append_inner(
                &mut current,
                incoming,
                append_paths,
                VERSION_FIELD,
                max_array_length,
            );
            Some(current)
        }
        (None, Some(incoming)) => Some(truncate_arrays_if_needed(incoming, max_array_length)),
        (current, None) => current,
    }
}

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

    #[tokio::test]
    async fn same_slot_instruction_activity_cannot_block_authoritative_deletion() {
        let cache = EntityCache::new();
        assert_eq!(
            cache
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"account":true,"_seq":"100:000000000009"}),
                    &[],
                    PatchOrigin::Creation,
                    Some(AccountPosition::new(100, 9)),
                )
                .await,
            CacheWrite::Created
        );
        assert_eq!(
            cache
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"instruction":true,"_seq":"100:000000000900"}),
                    &[],
                    PatchOrigin::Change
                )
                .await,
            CacheWrite::Merged
        );
        assert!(
            cache
                .delete("v", "a", Some(AccountPosition::new(100, 10)))
                .await
        );
        assert!(cache.get("v", "a").await.is_none());
        assert!(cache.deletion_is_current("v", "a").await);
    }

    #[tokio::test]
    async fn recreation_has_its_own_ordering_and_replaces_old_fields() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 1,
            ..Default::default()
        });
        assert!(
            cache
                .delete("v", "a", Some(AccountPosition::new(100, 1001)))
                .await
        );
        assert_eq!(
            cache
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"fresh":true, "_seq":"100:000000001002"}),
                    &[],
                    PatchOrigin::Creation,
                    Some(AccountPosition::new(100, 1002)),
                )
                .await,
            CacheWrite::Created
        );
        assert_eq!(
            cache
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"instruction":true, "_seq":"100:000000000900"}),
                    &[],
                    PatchOrigin::Change
                )
                .await,
            CacheWrite::Merged
        );

        // Neither a stale authoritative write/tombstone nor a delayed resend
        // from the deleted lifetime can contaminate the replacement.
        assert!(matches!(
            cache
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"old":true, "_seq":"100:000000001001"}),
                    &[],
                    PatchOrigin::Change,
                    Some(AccountPosition::new(100, 1001)),
                )
                .await,
            CacheWrite::Refused { .. }
        ));
        assert!(
            !cache
                .delete("v", "a", Some(AccountPosition::new(100, 1001)))
                .await
        );
        assert!(
            !cache
                .store_whole("v", "a", json!({"old":true, "_seq":"100:000000000900"}))
                .await
        );
        let row = cache.get("v", "a").await.unwrap();
        assert_eq!(row["fresh"], true);
        assert_eq!(row["instruction"], true);
        assert!(row.get("old").is_none());

        cache
            .upsert_with_append(
                "v",
                "other",
                json!({"other":true}),
                &[],
                PatchOrigin::Creation,
            )
            .await;
        assert!(cache.get("v", "a").await.is_none());
        assert!(matches!(
            cache
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"accountUpdate":true, "_seq":"100:000000001003"}),
                    &[],
                    PatchOrigin::Change,
                    Some(AccountPosition::new(100, 1003)),
                )
                .await,
            CacheWrite::Refused { .. }
        ));
        assert!(
            !cache
                .store_whole_for_lifetime(
                    "v",
                    "a",
                    row.clone(),
                    Some(AccountPosition::new(100, 1002)),
                )
                .await
        );
        let mut current = row;
        current["accountUpdate"] = Value::Bool(true);
        current["_seq"] = Value::String("100:000000001003".into());
        assert!(
            cache
                .store_whole_for_lifetime("v", "a", current, Some(AccountPosition::new(100, 1003)),)
                .await
        );

        // Current lifetime metadata survives a snapshot and continues to
        // admit instruction-domain offsets while rejecting stale account data.
        let restored = EntityCache::new();
        restored.hydrate(cache.dump().await).await;
        restored
            .hydrate_lifetimes(cache.dump_lifetimes().await)
            .await;
        assert_eq!(
            restored
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"afterRestore":true, "_seq":"100:000000000901"}),
                    &[],
                    PatchOrigin::Change,
                )
                .await,
            CacheWrite::Merged
        );
        assert!(matches!(
            restored
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"old":true, "_seq":"100:000000001001"}),
                    &[],
                    PatchOrigin::Change,
                    Some(AccountPosition::new(100, 1001)),
                )
                .await,
            CacheWrite::Refused { .. }
        ));
    }

    #[tokio::test]
    async fn recreation_retains_each_source_cursor_as_an_old_lifetime_barrier() {
        let cache = EntityCache::new();
        assert_eq!(
            cache
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"lifetime":"old"}),
                    &[],
                    PatchOrigin::Creation,
                    Some(AccountPosition::new(100, 9)),
                )
                .await,
            CacheWrite::Created
        );
        for (domain, seq, field) in [
            (
                SlotIndexDomain::Instruction,
                "100:000000000800",
                "oldInstruction",
            ),
            (SlotIndexDomain::Resolver, "100:900000000000", "oldResolver"),
        ] {
            let mut patch = json!({"_seq":seq});
            patch[field] = Value::Bool(true);
            assert_eq!(
                cache
                    .upsert_with_ordering(
                        "v",
                        "a",
                        patch,
                        &[],
                        PatchOrigin::Change,
                        LifetimeOrdering {
                            account_position: None,
                            source_seq: Some(seq),
                            source_domain: Some(domain),
                        },
                    )
                    .await,
                CacheWrite::Merged
            );
        }

        assert!(
            cache
                .delete("v", "a", Some(AccountPosition::new(100, 10)))
                .await
        );
        assert_eq!(
            cache
                .upsert_with_lifetime(
                    "v",
                    "a",
                    json!({"lifetime":"new"}),
                    &[],
                    PatchOrigin::Creation,
                    Some(AccountPosition::new(100, 11)),
                )
                .await,
            CacheWrite::Created
        );

        // Duplicates delayed across the deletion/recreation boundary retain
        // their old source positions and cannot change the replacement.
        for (domain, seq, field) in [
            (
                SlotIndexDomain::Instruction,
                "100:000000000799",
                "staleInstruction",
            ),
            (
                SlotIndexDomain::Resolver,
                "100:899999999999",
                "staleResolver",
            ),
        ] {
            let mut patch = json!({"_seq":seq});
            patch[field] = Value::Bool(true);
            assert!(matches!(
                cache
                    .upsert_with_ordering(
                        "v",
                        "a",
                        patch,
                        &[],
                        PatchOrigin::Change,
                        LifetimeOrdering {
                            account_position: None,
                            source_seq: Some(seq),
                            source_domain: Some(domain),
                        },
                    )
                    .await,
                CacheWrite::Refused { .. }
            ));
        }

        // A later instruction in the same source domain remains valid even
        // though its txn index is lower than account write versions.
        assert_eq!(
            cache
                .upsert_with_ordering(
                    "v",
                    "a",
                    json!({"newInstruction":true, "_seq":"100:000000000900"}),
                    &[],
                    PatchOrigin::Change,
                    LifetimeOrdering {
                        account_position: None,
                        source_seq: Some("100:000000000900"),
                        source_domain: Some(SlotIndexDomain::Instruction),
                    },
                )
                .await,
            CacheWrite::Merged
        );
        let row = cache.get("v", "a").await.unwrap();
        assert_eq!(row["lifetime"], "new");
        assert_eq!(row["newInstruction"], true);
        assert!(row.get("oldInstruction").is_none());
        assert!(row.get("oldResolver").is_none());
        assert!(row.get("staleInstruction").is_none());
        assert!(row.get("staleResolver").is_none());

        let restored = EntityCache::new();
        restored.hydrate(cache.dump().await).await;
        restored
            .hydrate_lifetimes(cache.dump_lifetimes().await)
            .await;
        assert!(matches!(
            restored
                .upsert_with_ordering(
                    "v",
                    "a",
                    json!({"staleAfterRestore":true}),
                    &[],
                    PatchOrigin::Change,
                    LifetimeOrdering {
                        account_position: None,
                        source_seq: Some("100:000000000799"),
                        source_domain: Some(SlotIndexDomain::Instruction),
                    },
                )
                .await,
            CacheWrite::Refused { .. }
        ));
    }

    #[tokio::test]
    async fn markerless_sources_keep_seq_ordering_across_recreation_and_restore() {
        let cache = EntityCache::new();
        assert_eq!(
            cache
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"old":true,"_seq":"100:000000000010"}),
                    &[],
                    PatchOrigin::Creation,
                )
                .await,
            CacheWrite::Created
        );
        assert!(
            !cache
                .delete_ordered("v", "a", None, Some("100:000000000009"))
                .await
        );
        assert!(cache.get("v", "a").await.is_some());
        assert!(
            cache
                .delete_ordered("v", "a", None, Some("100:000000000011"))
                .await
        );
        assert_eq!(
            cache
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"fresh":true,"_seq":"100:000000000012"}),
                    &[],
                    PatchOrigin::Creation,
                )
                .await,
            CacheWrite::Created
        );
        assert!(matches!(
            cache
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"stale":true,"_seq":"100:000000000010"}),
                    &[],
                    PatchOrigin::Creation,
                )
                .await,
            CacheWrite::Refused { .. }
        ));
        assert!(
            !cache
                .delete_ordered("v", "a", None, Some("100:000000000011"))
                .await
        );

        let restored = EntityCache::new();
        restored.hydrate(cache.dump().await).await;
        restored
            .hydrate_lifetimes(cache.dump_lifetimes().await)
            .await;
        assert!(
            !restored
                .delete_ordered("v", "a", None, Some("100:000000000011"))
                .await
        );
        let row = restored.get("v", "a").await.unwrap();
        assert_eq!(row["fresh"], true);
        assert!(row.get("old").is_none());
        assert!(row.get("stale").is_none());

        // An evicted row may be filled by a requested whole-entity resend,
        // but a response requested for the previous lifetime must not do it.
        let resync_cache = EntityCache::new();
        resync_cache
            .hydrate_lifetimes(cache.dump_lifetimes().await)
            .await;
        assert!(
            !resync_cache
                .store_whole_for_ordering(
                    "v",
                    "a",
                    json!({"stale_resend":true}),
                    None,
                    Some("100:000000000010"),
                )
                .await
        );
        assert!(
            resync_cache
                .store_whole_for_ordering(
                    "v",
                    "a",
                    json!({"fresh_resend":true}),
                    None,
                    Some("100:000000000012"),
                )
                .await
        );
    }

    #[tokio::test]
    async fn older_source_patch_cannot_replace_newer_row_or_move_its_cursor_back() {
        let cache = EntityCache::new();
        assert_eq!(
            cache
                .upsert_with_append(
                    "v",
                    "a",
                    json!({"value":"created","_seq":"100:000000000010"}),
                    &[],
                    PatchOrigin::Creation,
                )
                .await,
            CacheWrite::Created
        );
        assert_eq!(
            cache
                .upsert_with_ordering(
                    "v",
                    "a",
                    json!({"value":"newer","_seq":"100:000000000012"}),
                    &[],
                    PatchOrigin::Change,
                    LifetimeOrdering {
                        account_position: None,
                        source_seq: Some("100:000000000012"),
                        source_domain: Some(SlotIndexDomain::Legacy),
                    },
                )
                .await,
            CacheWrite::Merged
        );
        assert!(matches!(
            cache
                .upsert_with_ordering(
                    "v",
                    "a",
                    json!({"value":"older","stale":true,"_seq":"100:000000000011"}),
                    &[],
                    PatchOrigin::Change,
                    LifetimeOrdering {
                        account_position: None,
                        source_seq: Some("100:000000000011"),
                        source_domain: Some(SlotIndexDomain::Legacy),
                    },
                )
                .await,
            CacheWrite::Refused { .. }
        ));
        assert!(
            !cache
                .delete_ordered("v", "a", None, Some("100:000000000011"))
                .await
        );
        let row = cache.get("v", "a").await.unwrap();
        assert_eq!(row["value"], "newer");
        assert_eq!(row["_seq"], "100:000000000012");
        assert!(row.get("stale").is_none());
    }

    #[tokio::test]
    async fn independent_source_recency_domains_survive_snapshot_restore() {
        let cache = EntityCache::new();
        cache
            .upsert_with_lifetime(
                "v",
                "a",
                json!({"value":"created"}),
                &[],
                PatchOrigin::Creation,
                Some(AccountPosition::new(100, 9)),
            )
            .await;
        for (domain, seq, value) in [
            (SlotIndexDomain::Resolver, "100:900000000000", "resolver"),
            (
                SlotIndexDomain::Instruction,
                "100:000000000900",
                "instruction",
            ),
        ] {
            assert_eq!(
                cache
                    .upsert_with_ordering(
                        "v",
                        "a",
                        json!({"value":value, "_seq":seq}),
                        &[],
                        PatchOrigin::Change,
                        LifetimeOrdering {
                            account_position: None,
                            source_seq: Some(seq),
                            source_domain: Some(domain),
                        },
                    )
                    .await,
                CacheWrite::Merged
            );
        }

        let restored = EntityCache::new();
        restored.hydrate(cache.dump().await).await;
        restored
            .hydrate_lifetimes(cache.dump_lifetimes().await)
            .await;

        for (domain, seq) in [
            (SlotIndexDomain::Instruction, "100:000000000899"),
            (SlotIndexDomain::Resolver, "100:899999999999"),
        ] {
            assert!(matches!(
                restored
                    .upsert_with_ordering(
                        "v",
                        "a",
                        json!({"stale":true, "_seq":seq}),
                        &[],
                        PatchOrigin::Change,
                        LifetimeOrdering {
                            account_position: None,
                            source_seq: Some(seq),
                            source_domain: Some(domain),
                        },
                    )
                    .await,
                CacheWrite::Refused { .. }
            ));
        }
        assert_eq!(
            restored.get("v", "a").await.unwrap()["value"],
            "instruction"
        );
    }

    #[tokio::test]
    async fn explicit_current_delete_accepts_a_source_without_position() {
        let cache = EntityCache::new();
        cache
            .upsert_with_append(
                "v",
                "a",
                json!({"value":true,"_seq":"100:000000000010"}),
                &[],
                PatchOrigin::Creation,
            )
            .await;

        assert!(cache.delete("v", "a", None).await);
        assert!(cache.get("v", "a").await.is_none());
        assert!(cache.deletion_is_current("v", "a").await);
    }

    #[tokio::test]
    async fn evicted_live_row_does_not_make_an_earlier_delete_current() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 1,
            ..Default::default()
        });
        cache
            .upsert_with_lifetime(
                "v",
                "a",
                json!({"value":true,"_seq":"100:000000000010"}),
                &[],
                PatchOrigin::Creation,
                Some(AccountPosition::new(100, 10)),
            )
            .await;
        cache
            .upsert_with_append("v", "b", json!({"value":true}), &[], PatchOrigin::Creation)
            .await;

        assert!(cache.get("v", "a").await.is_none());
        assert!(!cache.deletion_is_current("v", "a").await);
        assert!(cache.delete_current("v", "a").await);
        assert!(cache.deletion_is_current("v", "a").await);
    }

    #[tokio::test]
    async fn test_basic_upsert_and_get() {
        let cache = EntityCache::new();

        cache
            .upsert("tokens/list", "abc123", json!({"name": "Test Token"}))
            .await;

        let entity = cache.get("tokens/list", "abc123").await;
        assert!(entity.is_some());
        assert_eq!(entity.unwrap()["name"], "Test Token");
    }

    #[tokio::test]
    async fn test_deep_merge_objects() {
        let cache = EntityCache::new();

        cache
            .upsert(
                "tokens/list",
                "abc123",
                json!({
                    "id": "abc123",
                    "metrics": {"volume": 100}
                }),
            )
            .await;

        cache
            .upsert(
                "tokens/list",
                "abc123",
                json!({
                    "metrics": {"trades": 50}
                }),
            )
            .await;

        let entity = cache.get("tokens/list", "abc123").await.unwrap();
        assert_eq!(entity["id"], "abc123");
        assert_eq!(entity["metrics"]["volume"], 100);
        assert_eq!(entity["metrics"]["trades"], 50);
    }

    #[tokio::test]
    async fn test_array_append() {
        let cache = EntityCache::new();

        cache
            .upsert(
                "tokens/list",
                "abc123",
                json!({
                    "events": [{"type": "buy", "amount": 100}]
                }),
            )
            .await;

        cache
            .upsert_with_append(
                "tokens/list",
                "abc123",
                json!({
                    "events": [{"type": "sell", "amount": 50}]
                }),
                &["events".to_string()],
                PatchOrigin::Unknown,
            )
            .await;

        let entity = cache.get("tokens/list", "abc123").await.unwrap();
        let events = entity["events"].as_array().unwrap();
        assert_eq!(events.len(), 2);
        assert_eq!(events[0]["type"], "buy");
        assert_eq!(events[1]["type"], "sell");
    }

    #[tokio::test]
    async fn test_array_lru_eviction() {
        let config = EntityCacheConfig {
            max_entities_per_view: 1000,
            max_array_length: 3,
            ..Default::default()
        };
        let cache = EntityCache::with_config(config);

        cache
            .upsert(
                "tokens/list",
                "abc123",
                json!({
                    "events": [
                        {"id": 1}, {"id": 2}, {"id": 3}, {"id": 4}, {"id": 5}
                    ]
                }),
            )
            .await;

        let entity = cache.get("tokens/list", "abc123").await.unwrap();
        let events = entity["events"].as_array().unwrap();

        assert_eq!(events.len(), 3);
        assert_eq!(events[0]["id"], 3);
        assert_eq!(events[1]["id"], 4);
        assert_eq!(events[2]["id"], 5);
    }

    #[tokio::test]
    async fn test_array_append_with_lru() {
        let config = EntityCacheConfig {
            max_entities_per_view: 1000,
            max_array_length: 3,
            ..Default::default()
        };
        let cache = EntityCache::with_config(config);

        cache
            .upsert(
                "tokens/list",
                "abc123",
                json!({
                    "events": [{"id": 1}, {"id": 2}]
                }),
            )
            .await;

        cache
            .upsert_with_append(
                "tokens/list",
                "abc123",
                json!({
                    "events": [{"id": 3}, {"id": 4}]
                }),
                &["events".to_string()],
                PatchOrigin::Unknown,
            )
            .await;

        let entity = cache.get("tokens/list", "abc123").await.unwrap();
        let events = entity["events"].as_array().unwrap();

        // [1,2] + [3,4] = [1,2,3,4] → LRU(3) = [2,3,4]
        assert_eq!(events.len(), 3);
        assert_eq!(events[0]["id"], 2);
        assert_eq!(events[1]["id"], 3);
        assert_eq!(events[2]["id"], 4);
    }

    #[tokio::test]
    async fn test_entity_lru_eviction() {
        let config = EntityCacheConfig {
            max_entities_per_view: 2,
            max_array_length: 100,
            ..Default::default()
        };
        let cache = EntityCache::with_config(config);

        cache.upsert("tokens/list", "key1", json!({"id": 1})).await;
        cache.upsert("tokens/list", "key2", json!({"id": 2})).await;
        cache.upsert("tokens/list", "key3", json!({"id": 3})).await;

        assert!(cache.get("tokens/list", "key1").await.is_none());
        assert!(cache.get("tokens/list", "key2").await.is_some());
        assert!(cache.get("tokens/list", "key3").await.is_some());
    }

    #[tokio::test]
    async fn a_patch_for_an_evicted_key_is_refused_not_stored() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 2,
            ..Default::default()
        });
        for id in 1..=3 {
            assert_eq!(
                cache
                    .upsert(
                        "tokens/list",
                        &format!("key{id}"),
                        json!({"id": id, "n": 0})
                    )
                    .await,
                CacheWrite::Created
            );
        }
        // key1 was evicted; this carries only what changed.
        assert_eq!(
            cache.upsert("tokens/list", "key1", json!({"n": 5})).await,
            CacheWrite::Refused {
                patch: json!({"n": 5})
            }
        );
        assert!(cache.get("tokens/list", "key1").await.is_none());
        assert_eq!(
            cache.upsert("tokens/list", "key3", json!({"n": 1})).await,
            CacheWrite::Merged
        );

        // A source delete ends the entity: it may then be created again.
        cache.remove("tokens/list", "key1").await;
        assert_eq!(
            cache
                .upsert("tokens/list", "key1", json!({"id": 1, "n": 6}))
                .await,
            CacheWrite::Created
        );
    }

    #[tokio::test]
    async fn a_whole_entity_ends_an_eviction() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 1,
            ..Default::default()
        });
        cache.upsert("v", "a", json!({"id": "a", "n": 0})).await;
        cache.upsert("v", "b", json!({"id": "b", "n": 0})).await;
        assert!(matches!(
            cache.upsert("v", "a", json!({"n": 1})).await,
            CacheWrite::Refused { .. }
        ));
        cache
            .store_whole("v", "a", json!({"id": "a", "n": 1}))
            .await;
        assert_eq!(cache.get("v", "a").await, Some(json!({"id": "a", "n": 1})));
        assert_eq!(cache.len("v").await, 1, "still bounded: b made room");
        assert_eq!(
            cache.upsert("v", "a", json!({"n": 2})).await,
            CacheWrite::Merged
        );
    }

    #[tokio::test]
    async fn the_evicted_key_memory_is_bounded() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 1,
            ..Default::default()
        });
        let remembered = EVICTED_KEYS_PER_CACHED_ENTITY;
        for key in 0..=remembered + 1 {
            cache
                .upsert("v", &key.to_string(), json!({"id": key}))
                .await;
        }
        // Key 0 was pushed out of the memory by the evictions after it; the
        // most recent evictions are still remembered.
        assert_eq!(
            cache.upsert("v", "0", json!({"id": 0})).await,
            CacheWrite::Created
        );
        assert!(matches!(
            cache
                .upsert("v", &remembered.to_string(), json!({"n": 1}))
                .await,
            CacheWrite::Refused { .. }
        ));
    }

    async fn write(
        cache: &EntityCache,
        key: &str,
        patch: Value,
        origin: PatchOrigin,
    ) -> CacheWrite {
        cache.upsert_with_append("v", key, patch, &[], origin).await
    }

    /// A source that marks creations says outright which patches are whole:
    /// no memory of evictions is needed, and none can run out.
    #[tokio::test]
    async fn an_unmarked_patch_for_a_key_not_held_is_refused_however_old() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 1,
            ..Default::default()
        });
        let evictions = EVICTED_KEYS_PER_CACHED_ENTITY * 4;
        for key in 0..=evictions {
            assert_eq!(
                write(
                    &cache,
                    &key.to_string(),
                    json!({"id": key}),
                    PatchOrigin::Creation
                )
                .await,
                CacheWrite::Created,
                "a creation is stored as soon as it arrives"
            );
        }
        // Key 0 was evicted far longer ago than any memory would reach.
        assert_eq!(
            write(&cache, "0", json!({"n": 1}), PatchOrigin::Change).await,
            CacheWrite::Refused {
                patch: json!({"n": 1})
            }
        );
        assert!(
            matches!(
                write(&cache, "never", json!({"n": 1}), PatchOrigin::Change).await,
                CacheWrite::Refused { .. }
            ),
            "nor does it matter whether the view ever held the key"
        );
        assert_eq!(cache.get("v", "0").await, None);
        assert_eq!(
            write(
                &cache,
                &evictions.to_string(),
                json!({"n": 1}),
                PatchOrigin::Change
            )
            .await,
            CacheWrite::Merged
        );
        // The source started key 0 again.
        assert_eq!(
            write(&cache, "0", json!({"id": 0, "n": 2}), PatchOrigin::Creation).await,
            CacheWrite::Created
        );
        assert_eq!(cache.get("v", "0").await, Some(json!({"id": 0, "n": 2})));
    }

    /// A creation for a key the view still holds starts a new lifetime and
    /// replaces fields that no longer exist.
    #[tokio::test]
    async fn a_creation_for_a_key_held_replaces() {
        let cache = EntityCache::new();
        write(
            &cache,
            "a",
            json!({"id": "a", "old": 1}),
            PatchOrigin::Creation,
        )
        .await;
        assert_eq!(
            write(
                &cache,
                "a",
                json!({"id": "a", "n": 1}),
                PatchOrigin::Creation
            )
            .await,
            CacheWrite::Created
        );
        assert_eq!(cache.get("v", "a").await, Some(json!({"id": "a", "n": 1})));
    }

    async fn remembers_evictions(cache: &EntityCache, view_id: &str) -> bool {
        cache
            .caches
            .read()
            .await
            .get(view_id)
            .is_some_and(|view| view.evicted.is_some())
    }

    /// A view fed by a source that marks creations never consults the
    /// evicted-key memory, so it drops what a restore put there and keeps no
    /// more.
    #[tokio::test]
    async fn a_view_fed_marked_creations_keeps_no_eviction_memory() {
        let cache = EntityCache::with_config(EntityCacheConfig {
            max_entities_per_view: 1,
            ..Default::default()
        });
        cache.remember_evicted("v", ["restored".to_string()]).await;
        assert!(remembers_evictions(&cache, "v").await);

        write(&cache, "a", json!({"id": "a"}), PatchOrigin::Creation).await;
        assert!(!remembers_evictions(&cache, "v").await);
        write(&cache, "b", json!({"id": "b"}), PatchOrigin::Creation).await;
        assert!(!remembers_evictions(&cache, "v").await, "a was evicted");
        assert!(matches!(
            write(&cache, "restored", json!({"n": 1}), PatchOrigin::Change).await,
            CacheWrite::Refused { .. }
        ));
        assert!(matches!(
            write(&cache, "a", json!({"n": 1}), PatchOrigin::Change).await,
            CacheWrite::Refused { .. }
        ));
    }

    #[tokio::test]
    async fn remembered_keys_are_refused_unless_held() {
        let cache = EntityCache::new();
        cache.upsert("tokens/list", "held", json!({"id": 1})).await;
        cache
            .remember_evicted("tokens/list", ["held".to_string(), "gone".to_string()])
            .await;
        assert_eq!(
            cache.upsert("tokens/list", "held", json!({"n": 1})).await,
            CacheWrite::Merged
        );
        assert!(matches!(
            cache.upsert("tokens/list", "gone", json!({"n": 1})).await,
            CacheWrite::Refused { .. }
        ));
        // Other views are unaffected.
        assert_eq!(
            cache.upsert("tokens/state", "gone", json!({"id": 2})).await,
            CacheWrite::Created
        );
    }

    #[tokio::test]
    async fn test_get_all() {
        let cache = EntityCache::new();

        cache.upsert("tokens/list", "key1", json!({"id": 1})).await;
        cache.upsert("tokens/list", "key2", json!({"id": 2})).await;

        let all = cache.get_all("tokens/list").await;
        assert_eq!(all.len(), 2);
    }

    #[tokio::test]
    async fn remove_is_scoped_to_one_entity() {
        let cache = EntityCache::new();
        cache.upsert("tokens/list", "one", json!({"id": 1})).await;
        cache.upsert("tokens/list", "two", json!({"id": 2})).await;

        assert_eq!(cache.remove("tokens/list", "one").await.unwrap()["id"], 1);
        assert!(cache.get("tokens/list", "one").await.is_none());
        assert!(cache.get("tokens/list", "two").await.is_some());
    }

    #[tokio::test]
    async fn test_separate_views() {
        let cache = EntityCache::new();

        cache
            .upsert("tokens/list", "key1", json!({"type": "token"}))
            .await;
        cache
            .upsert("games/list", "key1", json!({"type": "game"}))
            .await;

        let token = cache.get("tokens/list", "key1").await.unwrap();
        let game = cache.get("games/list", "key1").await.unwrap();

        assert_eq!(token["type"], "token");
        assert_eq!(game["type"], "game");
    }

    #[test]
    fn test_deep_merge_with_append() {
        let mut base = json!({
            "a": 1,
            "b": {"c": 2},
            "arr": [1, 2]
        });

        let patch = json!({
            "b": {"d": 3},
            "arr": [3],
            "e": 4
        });

        deep_merge_with_append(&mut base, patch, &["arr".to_string()], 100);

        assert_eq!(base["a"], 1);
        assert_eq!(base["b"]["c"], 2);
        assert_eq!(base["b"]["d"], 3);
        assert_eq!(base["arr"].as_array().unwrap().len(), 3);
        assert_eq!(base["e"], 4);
    }

    #[test]
    fn test_deep_merge_replace_array() {
        let mut base = json!({
            "arr": [1, 2, 3]
        });

        let patch = json!({
            "arr": [4, 5]
        });

        deep_merge_with_append(&mut base, patch, &[], 100);

        assert_eq!(base["arr"].as_array().unwrap().len(), 2);
        assert_eq!(base["arr"][0], 4);
        assert_eq!(base["arr"][1], 5);
    }

    #[test]
    fn test_deep_merge_nested_append() {
        let mut base = json!({
            "stats": {"events": [1, 2]}
        });

        let patch = json!({
            "stats": {"events": [3]}
        });

        deep_merge_with_append(&mut base, patch, &["stats.events".to_string()], 100);

        assert_eq!(base["stats"]["events"].as_array().unwrap().len(), 3);
    }

    #[test]
    fn test_snapshot_config_defaults() {
        let cache = EntityCache::new();
        let config = cache.snapshot_config();

        assert_eq!(config.initial_batch_size, 50);
        assert_eq!(config.subsequent_batch_size, 100);
    }

    #[test]
    fn test_snapshot_config_custom() {
        let config = EntityCacheConfig {
            initial_snapshot_batch_size: 25,
            subsequent_snapshot_batch_size: 75,
            ..Default::default()
        };
        let cache = EntityCache::with_config(config);
        let snapshot_config = cache.snapshot_config();

        assert_eq!(snapshot_config.initial_batch_size, 25);
        assert_eq!(snapshot_config.subsequent_batch_size, 75);
    }

    #[tokio::test]
    async fn test_get_after() {
        let cache = EntityCache::new();

        // Insert entities with _seq values
        cache
            .upsert(
                "tokens/list",
                "key1",
                json!({"id": 1, "_seq": "100:000000000001"}),
            )
            .await;
        cache
            .upsert(
                "tokens/list",
                "key2",
                json!({"id": 2, "_seq": "100:000000000002"}),
            )
            .await;
        cache
            .upsert(
                "tokens/list",
                "key3",
                json!({"id": 3, "_seq": "100:000000000003"}),
            )
            .await;
        cache
            .upsert(
                "tokens/list",
                "key4",
                json!({"id": 4, "_seq": "101:000000000001"}),
            )
            .await;

        // Get all entities after "100:000000000002"
        let after = cache
            .get_after("tokens/list", "100:000000000002", None)
            .await;

        // Should return key3 and key4 (sorted by _seq)
        assert_eq!(after.len(), 2);
        assert_eq!(after[0].0, "key3");
        assert_eq!(after[1].0, "key4");
    }

    #[tokio::test]
    async fn test_get_after_with_limit() {
        let cache = EntityCache::new();

        // Insert entities with _seq values
        cache
            .upsert(
                "tokens/list",
                "key1",
                json!({"id": 1, "_seq": "100:000000000001"}),
            )
            .await;
        cache
            .upsert(
                "tokens/list",
                "key2",
                json!({"id": 2, "_seq": "100:000000000002"}),
            )
            .await;
        cache
            .upsert(
                "tokens/list",
                "key3",
                json!({"id": 3, "_seq": "100:000000000003"}),
            )
            .await;

        // Get entities after "100:000000000000" with limit 2
        let after = cache
            .get_after("tokens/list", "100:000000000000", Some(2))
            .await;

        // Should return only first 2 (key1 and key2)
        assert_eq!(after.len(), 2);
        assert_eq!(after[0].0, "key1");
        assert_eq!(after[1].0, "key2");
    }

    #[tokio::test]
    async fn test_get_after_empty_result() {
        let cache = EntityCache::new();

        cache
            .upsert(
                "tokens/list",
                "key1",
                json!({"id": 1, "_seq": "100:000000000001"}),
            )
            .await;

        // Get entities after a future cursor
        let after = cache
            .get_after("tokens/list", "999:000000000000", None)
            .await;

        assert!(after.is_empty());
    }

    #[tokio::test]
    async fn test_get_after_missing_seq() {
        let cache = EntityCache::new();

        // Insert entity without _seq
        cache.upsert("tokens/list", "key1", json!({"id": 1})).await;

        // Get entities after any cursor - entity without _seq should not be included
        let after = cache.get_after("tokens/list", "0:000000000000", None).await;

        assert!(after.is_empty());
    }

    fn versions<'a>(views: &[(&'a str, &str)]) -> Vec<ViewVersion<'a>> {
        views
            .iter()
            .map(|(view_id, version)| ViewVersion {
                view_id,
                version: Some(json!(version)),
            })
            .collect()
    }

    const VIEWS: [&str; 3] = ["t/list", "t/state", "t/append"];

    async fn shared(cache: &EntityCache, key: &str) -> Vec<SharedEntity> {
        let mut entities = Vec::new();
        for view in VIEWS {
            entities.push(cache.get_shared(view, key).await.unwrap());
        }
        entities
    }

    async fn write_all(
        cache: &EntityCache,
        key: &str,
        patch: Value,
        stamps: [&str; 3],
        origin: PatchOrigin,
    ) -> Vec<CacheWrite> {
        let views: Vec<_> = VIEWS.into_iter().zip(stamps).collect();
        cache
            .upsert_views(
                key,
                patch,
                &versions(&views),
                &["events".to_string()],
                origin,
                LifetimeOrdering::default(),
            )
            .await
    }

    /// Views written together hold one copy of the entity's fields, each
    /// under its own `_version`, and every view still reads as the whole
    /// entity its frames describe.
    #[tokio::test]
    async fn views_written_together_share_one_copy() {
        let cache = EntityCache::new();
        let writes = write_all(
            &cache,
            "a",
            json!({"id": "a", "events": [1]}),
            ["e:1", "e:2", "e:3"],
            PatchOrigin::Creation,
        )
        .await;
        assert_eq!(writes, vec![CacheWrite::Created; 3]);
        let [list, state, append] =
            <[SharedEntity; 3]>::try_from(shared(&cache, "a").await).unwrap();
        assert!(list.shares_fields_with(&state) && list.shares_fields_with(&append));
        // Three views and the three copies just read.
        assert_eq!(Arc::strong_count(list.fields()), 6);
        assert_eq!(
            cache.get("t/state", "a").await,
            Some(json!({"id": "a", "events": [1], "_version": "e:2"}))
        );

        let writes = write_all(
            &cache,
            "a",
            json!({"n": 1, "events": [2]}),
            ["e:4", "e:5", "e:6"],
            PatchOrigin::Unknown,
        )
        .await;
        assert_eq!(writes, vec![CacheWrite::Merged; 3]);
        let merged = shared(&cache, "a").await;
        assert!(
            merged[0].shares_fields_with(&merged[1]) && merged[0].shares_fields_with(&merged[2])
        );
        for (view, version) in VIEWS.into_iter().zip(["e:4", "e:5", "e:6"]) {
            assert_eq!(
                cache.get(view, "a").await,
                Some(json!({"id": "a", "n": 1, "events": [1, 2], "_version": version})),
                "{view}"
            );
        }
        // The copies read before the merge still hold what they held.
        assert_eq!(
            list.to_value(),
            json!({"id": "a", "events": [1], "_version": "e:1"})
        );
        assert!(!list.shares_fields_with(&merged[0]));
    }

    /// Nothing else holds the views' shared fields, so the merge writes them
    /// in place instead of copying them.
    #[tokio::test]
    async fn a_merge_nobody_else_holds_happens_in_place() {
        let cache = EntityCache::new();
        write_all(
            &cache,
            "a",
            json!({"id": "a"}),
            ["e:1", "e:2", "e:3"],
            PatchOrigin::Creation,
        )
        .await;
        let before = Arc::as_ptr(cache.get_shared("t/list", "a").await.unwrap().fields());
        write_all(
            &cache,
            "a",
            json!({"n": 1}),
            ["e:4", "e:5", "e:6"],
            PatchOrigin::Unknown,
        )
        .await;
        let after = cache.get_shared("t/list", "a").await.unwrap();
        // Had the merge copied the fields, the copy would have been made
        // while the original was still allocated, at another address.
        assert_eq!(Arc::as_ptr(after.fields()), before);
        assert_eq!(Arc::strong_count(after.fields()), 4);
        assert_eq!(after["n"], 1);
    }

    /// A patch written to one view leaves another view's copy as it was,
    /// even though the two shared their fields until then.
    #[tokio::test]
    async fn a_patch_to_one_view_does_not_reach_another() {
        let cache = EntityCache::new();
        write_all(
            &cache,
            "a",
            json!({"id": "a", "nested": {"n": 0}}),
            ["e:1", "e:2", "e:3"],
            PatchOrigin::Creation,
        )
        .await;
        assert_eq!(
            cache
                .upsert_with_append(
                    "t/list",
                    "a",
                    json!({"nested": {"n": 1}, "_version": "e:4"}),
                    &[],
                    PatchOrigin::Unknown,
                )
                .await,
            CacheWrite::Merged
        );
        assert_eq!(
            cache.get("t/list", "a").await,
            Some(json!({"id": "a", "nested": {"n": 1}, "_version": "e:4"}))
        );
        for (view, version) in [("t/state", "e:2"), ("t/append", "e:3")] {
            assert_eq!(
                cache.get(view, "a").await,
                Some(json!({"id": "a", "nested": {"n": 0}, "_version": version})),
                "{view}"
            );
        }
        let [list, state, append] =
            <[SharedEntity; 3]>::try_from(shared(&cache, "a").await).unwrap();
        assert!(!list.shares_fields_with(&state));
        assert!(state.shares_fields_with(&append));
    }

    /// A view that refuses a patch keeps its copy while the views that take
    /// it move on to a new one.
    #[tokio::test]
    async fn a_view_that_refuses_a_patch_keeps_its_copy() {
        let cache = EntityCache::new();
        write_all(
            &cache,
            "a",
            json!({"id": "a", "n": 0}),
            ["e:1", "e:2", "e:3"],
            PatchOrigin::Creation,
        )
        .await;
        // Only the state view saw the entity's deletion.
        assert!(cache.delete_current("t/state", "a").await);
        let writes = write_all(
            &cache,
            "a",
            json!({"n": 1}),
            ["e:4", "e:5", "e:6"],
            PatchOrigin::Change,
        )
        .await;
        assert_eq!(writes[0], CacheWrite::Merged);
        assert_eq!(
            writes[1],
            CacheWrite::Refused {
                patch: json!({"n": 1, "_version": "e:5"})
            }
        );
        assert_eq!(writes[2], CacheWrite::Merged);
        assert_eq!(cache.get("t/state", "a").await, None);
        let list = cache.get_shared("t/list", "a").await.unwrap();
        let append = cache.get_shared("t/append", "a").await.unwrap();
        assert!(list.shares_fields_with(&append));
        assert_eq!(
            append.to_value(),
            json!({"id": "a", "n": 1, "_version": "e:6"})
        );
    }

    /// A snapshot saves each view's copy of an entity on its own; restoring
    /// it shares the fields again and keeps every view's `_version`.
    #[tokio::test]
    async fn a_restore_shares_the_copies_it_saved_per_view() {
        let cache = EntityCache::new();
        write_all(
            &cache,
            "a",
            json!({"id": "a", "events": [1, 2]}),
            ["e:1", "e:2", "e:3"],
            PatchOrigin::Creation,
        )
        .await;
        let mut dump = cache.dump().await;
        // Another export's entity under the same key is not compared, even
        // when it happens to have the same fields.
        dump.push((
            "other/list".to_string(),
            vec![("a".to_string(), json!({"id": "a", "events": [1, 2]}))],
        ));
        dump.sort_by(|left, right| left.0.cmp(&right.0));

        let restored = EntityCache::new();
        restored.hydrate(dump.clone()).await;
        let other = restored.get_shared("other/list", "a").await.unwrap();
        let copies = shared(&restored, "a").await;
        assert!(!other.shares_fields_with(&copies[0]));
        assert!(
            copies[0].shares_fields_with(&copies[1]) && copies[0].shares_fields_with(&copies[2])
        );
        let mut again = restored.dump().await;
        again.sort_by(|left, right| left.0.cmp(&right.0));
        assert_eq!(again, dump);
    }

    /// A whole entity stored in several views at once is one copy.
    #[tokio::test]
    async fn a_whole_entity_stored_in_several_views_is_one_copy() {
        let cache = EntityCache::new();
        let views = versions(&[("t/list", "e:1"), ("t/state", "e:2"), ("t/append", "e:3")]);
        let stored = cache
            .store_whole_views("a", json!({"id": "a"}), &views, None, None)
            .await;
        assert_eq!(stored, vec![true; 3]);
        let copies = shared(&cache, "a").await;
        assert!(
            copies[0].shares_fields_with(&copies[1]) && copies[0].shares_fields_with(&copies[2])
        );
        assert_eq!(
            cache.get("t/append", "a").await,
            Some(json!({"id": "a", "_version": "e:3"}))
        );
    }

    /// The value of top-level field `name` of `entity`'s shared fields.
    fn member<'a>(entity: &'a SharedEntity, name: &str) -> &'a Arc<Value> {
        match &**entity.fields() {
            Fields::Object(members) => members.get(name).expect("the field is there"),
            Fields::Other(other) => panic!("not an object: {other}"),
        }
    }

    /// A patch to one section of an entity another holder still shares
    /// copies that section only: the old and new versions share the rest.
    #[tokio::test]
    async fn a_patch_copies_only_the_sections_it_changes() {
        let cache = EntityCache::new();
        let row = json!({
            "id": {"address": "a", "mint": "m"},
            "balance": {"amount": "1", "state": "initialized"},
            "activity": {"transfers": 0},
        });
        cache.upsert("t/list", "a", row.clone()).await;
        // A derived view or a subscriber holding the current version.
        let held = cache.get_shared("t/list", "a").await.unwrap();

        cache
            .upsert("t/list", "a", json!({"balance": {"amount": "2"}}))
            .await;
        let after = cache.get_shared("t/list", "a").await.unwrap();
        assert!(!after.shares_fields_with(&held));
        assert!(Arc::ptr_eq(member(&after, "id"), member(&held, "id")));
        assert!(Arc::ptr_eq(
            member(&after, "activity"),
            member(&held, "activity")
        ));
        assert!(!Arc::ptr_eq(
            member(&after, "balance"),
            member(&held, "balance")
        ));
        assert_eq!(held.to_value(), row, "the other holder's copy is as it was");
        assert_eq!(
            after.to_value(),
            json!({
                "id": {"address": "a", "mint": "m"},
                "balance": {"amount": "2", "state": "initialized"},
                "activity": {"transfers": 0},
            })
        );

        // A whole row resent with one section changed keeps sharing the
        // sections it repeats.
        let held = after;
        let mut resent = held.to_value();
        resent["activity"]["transfers"] = json!(1);
        cache.upsert("t/list", "a", resent.clone()).await;
        let after = cache.get_shared("t/list", "a").await.unwrap();
        assert!(Arc::ptr_eq(member(&after, "id"), member(&held, "id")));
        assert!(Arc::ptr_eq(
            member(&after, "balance"),
            member(&held, "balance")
        ));
        assert!(!Arc::ptr_eq(
            member(&after, "activity"),
            member(&held, "activity")
        ));
        assert_eq!(after.to_value(), resent);
    }

    /// Merging into shared fields, held by another holder or not, gives
    /// exactly what merging into the whole value gives, and never reaches the
    /// other holder's copy.
    #[test]
    fn a_section_merge_matches_the_whole_value_merge() {
        let base = json!({
            "id": {"address": "a", "tags": ["x", "y"]},
            "balance": {"amount": "1", "nested": {"deep": [1, 2], "n": null}},
            "events": [1, 2, 3],
            "list": [1],
            "scalar": 5,
        });
        let patches = [
            json!({}),
            json!({"id": {"address": "a", "tags": ["x", "y"]}}),
            json!({"balance": {"amount": "2"}}),
            json!({"balance": {"nested": {"deep": [3], "extra": {"arr": [1, 2, 3, 4]}}}}),
            json!({"balance": {"nested": {"deep": [1, 2, 3, 4]}}}),
            json!({"events": [4]}),
            json!({"events": []}),
            json!({"events": [4, 5, 6, 7, 8]}),
            json!({"list": [2, 3, 4, 5]}),
            json!({"list": [1]}),
            json!({"scalar": {"now": "an object", "arr": [1, 2, 3, 4]}}),
            json!({"scalar": 5, "new": [1, 2, 3, 4, 5], "id": null}),
            json!({"balance": [1, 2, 3, 4], "events": {"replaced": true}}),
            json!({"balance": {"nested": {"n": null}}}),
            json!({"balance": {"amount": 1}}),
            json!({"id": {"tags": ["x", "y", "z", "w"]}}),
        ];
        let append = ["events".to_string(), "balance.nested.deep".to_string()];
        for max_array_length in [3, 100] {
            for patch in &patches {
                let mut expected = base.clone();
                deep_merge_with_append(&mut expected, patch.clone(), &append, max_array_length);
                for shared in [false, true] {
                    let mut entity = SharedEntity::new(base.clone());
                    let other = shared.then(|| entity.clone());
                    merge_shared(&mut entity, patch.clone(), &append, max_array_length);
                    assert_eq!(
                        serde_json::to_string(&entity).unwrap(),
                        serde_json::to_string(&expected).unwrap(),
                        "{patch} (shared: {shared}, max {max_array_length})"
                    );
                    if let Some(other) = other {
                        assert_eq!(other.to_value(), base, "the other holder's copy");
                    }
                }
            }
        }
    }

    /// Merging into a shared copy that is not an object (or with a patch that
    /// is not one) still follows the whole-value merge rules.
    #[test]
    fn a_shared_merge_matches_the_whole_value_merge() {
        let cases = [
            (
                json!({"a": 1, "_version": "e:1"}),
                json!({"b": 2, "_version": "e:2"}),
            ),
            (json!({"a": 1, "_version": "e:1"}), json!({"b": 2})),
            (json!({"a": 1, "_version": "e:1"}), json!([1, 2])),
            (json!([1, 2]), json!([3])),
            (json!([1, 2]), json!({"a": 1, "_version": "e:2"})),
            (json!("text"), json!(5)),
            (
                json!({"_version": "e:1", "list": [1]}),
                json!({"list": [2, 3, 4]}),
            ),
        ];
        for (base, patch) in cases {
            let append = ["list".to_string()];
            let mut expected = base.clone();
            deep_merge_with_append(&mut expected, patch.clone(), &append, 2);
            let mut entity = SharedEntity::new(base.clone());
            let other = entity.clone();
            merge_shared(&mut entity, patch.clone(), &append, 2);
            assert_eq!(entity.to_value(), expected, "{base} <- {patch}");
            assert_eq!(other.to_value(), base, "the other holder's copy");
        }
    }
}