// Copyright 2016 Mozilla
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
// this file except in compliance with the License. You may obtain a copy of the
// License at http://www.apache.org/licenses/LICENSE-2.0
// Unless required by applicable law or agreed to in writing, software distributed
// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
// CONDITIONS OF ANY KIND, either express or implied. See the License for the
// specific language governing permissions and limitations under the License.

#![allow(dead_code)]

//! This module implements the transaction application algorithm described at
//! https://github.com/mozilla/mentat/wiki/Transacting and its children pages.
//!
//! The implementation proceeds in four main stages, labeled "Pipeline stage 1" through "Pipeline
//! stage 4".  _Pipeline_ may be a misnomer, since the stages as written **cannot** be interleaved
//! in parallel.  That is, a single transacted entity cannot flow through all the stages without its
//! sibling entities.
//!
//! This unintuitive architectural decision was made because the second and third stages (resolving
//! lookup refs and tempids, respectively) operate _in bulk_ to minimize the number of expensive
//! SQLite queries by processing many in one SQLite invocation.  Pipeline stage 2 doesn't need to
//! operate like this: it is easy to handle each transacted entity independently of all the others
//! (and earlier, less efficient, implementations did this).  However, Pipeline stage 3 appears to
//! require processing multiple elements at the same time, since there can be arbitrarily complex
//! graph relationships between tempids.  Pipeline stage 4 (inserting elements into the SQL store)
//! could also be expressed as an independent operation per transacted entity, but there are
//! non-trivial uniqueness relationships inside a single transaction that need to enforced.
//! Therefore, some multi-entity processing is required, and a per-entity pipeline becomes less
//! attractive.
//!
//! A note on the types in the implementation.  The pipeline stages are strongly typed: each stage
//! accepts and produces a subset of the previous.  We hope this will reduce errors as data moves
//! through the system.  In contrast the Clojure implementation rewrote the fundamental entity type
//! in place and suffered bugs where particular code paths missed cases.
//!
//! The type hierarchy accepts `Entity` instances from the transaction parser and flows `Term`
//! instances through the term-rewriting transaction applier.  `Term` is a general `[:db/add e a v]`
//! with restrictions on the `e` and `v` components.  The hierarchy is expressed using `Result` to
//! model either/or, and layers of `Result` are stripped -- we might say the `Term` instances are
//! _lowered_ as they flow through the pipeline.  This type hierarchy could have been expressed by
//! combinatorially increasing `enum` cases, but this makes it difficult to handle the `e` and `v`
//! components symmetrically.  Hence, layers of `Result` type aliases.  Hopefully the explanatory
//! names -- `TermWithTempIdsAndLookupRefs`, anyone? -- and strongly typed stage functions will help
//! keep everything straight.

use std::borrow::Cow;
use std::collections::{BTreeMap, BTreeSet, VecDeque};
use std::iter::once;

use crate::db;
use crate::db::MentatStoring;
use crate::entids;
use crate::internal_types::{
    replace_lookup_ref, AEVTrie, AddAndRetract, KnownEntidOr, LookupRef, LookupRefOrTempId,
    TempIdHandle, TempIdMap, Term, TermWithTempIds, TermWithTempIdsAndLookupRefs,
    TermWithoutTempIds, TypedValueOr,
};
use db_traits::errors;
use db_traits::errors::{DbErrorKind, Result};
use edn::{InternSet, Keyword};

use mentat_core::util::Either;

use core_traits::{attribute, now, Attribute, Entid, KnownEntid, TypedValue, ValueType};

use mentat_core::{DateTime, Schema, TxReport, Utc};

use crate::metadata;
use crate::schema::SchemaBuilding;
use crate::tx_checking;
use crate::types::{AVMap, AVPair, PartitionMap, TransactableValue};
use crate::upsert_resolution::{FinalPopulations, Generation};
use crate::watcher::TransactWatcher;
use edn::entities as entmod;
use edn::entities::{AttributePlace, Entity, OpType, TempId};
use rusqlite;

/// Defines transactor's high level behaviour.
pub(crate) enum TransactorAction {
    /// Materialize transaction into 'datoms' and metadata
    /// views, but do not commit it into 'transactions' table.
    /// Use this if you need transaction's "side-effects", but
    /// don't want its by-products to end-up in the transaction log,
    /// e.g. when rewinding.
    Materialize,

    /// Materialize transaction into 'datoms' and metadata
    /// views, and also commit it into the 'transactions' table.
    /// Use this for regular transactions.
    MaterializeAndCommit,
}

/// A transaction on its way to being applied.
#[derive(Debug)]
pub struct Tx<'conn, 'a, W>
where
    W: TransactWatcher,
{
    /// The storage to apply against.  In the future, this will be a Mentat connection.
    store: &'conn rusqlite::Connection, // TODO: db::MentatStoring,

    /// The partition map to allocate entids from.
    ///
    /// The partition map is volatile in the sense that every succesful transaction updates
    /// allocates at least one tx ID, so we own and modify our own partition map.
    partition_map: PartitionMap,

    /// The schema to update from the transaction entities.
    ///
    /// Transactions only update the schema infrequently, so we borrow this schema until we need to
    /// modify it.
    schema_for_mutation: Cow<'a, Schema>,

    /// The schema to use when interpreting the transaction entities.
    ///
    /// This schema is not updated, so we just borrow it.
    schema: &'a Schema,

    watcher: W,

    /// The transaction ID of the transaction.
    tx_id: Entid,
}

/// Remove any :db/id value from the given map notation, converting the returned value into
/// something suitable for the entity position rather than something suitable for a value position.
pub fn remove_db_id<V: TransactableValue>(
    map: &mut entmod::MapNotation<V>,
) -> Result<Option<entmod::EntityPlace<V>>> {
    // TODO: extract lazy defined constant.
    let db_id_key = entmod::EntidOrIdent::Ident(Keyword::namespaced("db", "id"));

    let db_id: Option<entmod::EntityPlace<V>> = if let Some(id) = map.remove(&db_id_key) {
        match id {
            entmod::ValuePlace::Entid(e) => Some(entmod::EntityPlace::Entid(e)),
            entmod::ValuePlace::LookupRef(e) => Some(entmod::EntityPlace::LookupRef(e)),
            entmod::ValuePlace::TempId(e) => Some(entmod::EntityPlace::TempId(e)),
            entmod::ValuePlace::TxFunction(e) => Some(entmod::EntityPlace::TxFunction(e)),
            entmod::ValuePlace::Atom(v) => Some(v.into_entity_place()?),
            entmod::ValuePlace::Vector(_) | entmod::ValuePlace::MapNotation(_) => {
                bail!(DbErrorKind::InputError(errors::InputError::BadDbId))
            }
        }
    } else {
        None
    };

    Ok(db_id)
}

impl<'conn, 'a, W> Tx<'conn, 'a, W>
where
    W: TransactWatcher,
{
    pub fn new(
        store: &'conn rusqlite::Connection,
        partition_map: PartitionMap,
        schema_for_mutation: &'a Schema,
        schema: &'a Schema,
        watcher: W,
        tx_id: Entid,
    ) -> Tx<'conn, 'a, W> {
        Tx {
            store,
            partition_map,
            schema_for_mutation: Cow::Borrowed(schema_for_mutation),
            schema,
            watcher,
            tx_id,
        }
    }

    /// Given a collection of tempids and the [a v] pairs that they might upsert to, resolve exactly
    /// which [a v] pairs do upsert to entids, and map each tempid that upserts to the upserted
    /// entid.  The keys of the resulting map are exactly those tempids that upserted.
    pub(crate) fn resolve_temp_id_avs<'b>(
        &self,
        temp_id_avs: &'b [(TempIdHandle, AVPair)],
    ) -> Result<TempIdMap> {
        if temp_id_avs.is_empty() {
            return Ok(TempIdMap::default());
        }

        // Map [a v]->entid.
        let mut av_pairs: Vec<&AVPair> = vec![];
        for temp_id_av in temp_id_avs {
            av_pairs.push(&temp_id_av.1);
        }

        // Lookup in the store.
        let av_map: AVMap = self.store.resolve_avs(&av_pairs[..])?;

        debug!("looked up avs {:?}", av_map);

        // Map id->entid.
        let mut tempids: TempIdMap = TempIdMap::default();

        // Errors.  BTree* since we want deterministic results.
        let mut conflicting_upserts: BTreeMap<TempId, BTreeSet<KnownEntid>> = BTreeMap::default();

        for &(ref tempid, ref av_pair) in temp_id_avs {
            trace!(
                "tempid {:?} av_pair {:?} -> {:?}",
                tempid,
                av_pair,
                av_map.get(&av_pair)
            );
            if let Some(entid) = av_map.get(&av_pair).cloned().map(KnownEntid) {
                if let Some(previous) = tempids.insert(tempid.clone(), entid) {
                    if entid != previous {
                        conflicting_upserts
                            .entry((**tempid).clone())
                            .or_insert_with(|| once(previous).collect::<BTreeSet<_>>())
                            .insert(entid);
                    }
                }
            }
        }

        if !conflicting_upserts.is_empty() {
            bail!(DbErrorKind::SchemaConstraintViolation(
                errors::SchemaConstraintViolation::ConflictingUpserts {
                    conflicting_upserts
                }
            ));
        }

        Ok(tempids)
    }

    /// Pipeline stage 1: convert `Entity` instances into `Term` instances, ready for term
    /// rewriting.
    ///
    /// The `Term` instances produce share interned TempId and LookupRef handles, and we return the
    /// interned handle sets so that consumers can ensure all handles are used appropriately.
    fn entities_into_terms_with_temp_ids_and_lookup_refs<I, V: TransactableValue>(
        &self,
        entities: I,
    ) -> Result<(
        Vec<TermWithTempIdsAndLookupRefs>,
        InternSet<TempId>,
        InternSet<AVPair>,
    )>
    where
        I: IntoIterator<Item = Entity<V>>,
    {
        struct InProcess<'a> {
            partition_map: &'a PartitionMap,
            schema: &'a Schema,
            mentat_id_count: i64,
            tx_id: KnownEntid,
            temp_ids: InternSet<TempId>,
            lookup_refs: InternSet<AVPair>,
        }

        impl<'a> InProcess<'a> {
            fn with_schema_and_partition_map(
                schema: &'a Schema,
                partition_map: &'a PartitionMap,
                tx_id: KnownEntid,
            ) -> InProcess<'a> {
                InProcess {
                    partition_map,
                    schema,
                    mentat_id_count: 0,
                    tx_id,
                    temp_ids: InternSet::new(),
                    lookup_refs: InternSet::new(),
                }
            }

            fn ensure_entid_exists(&self, e: Entid) -> Result<KnownEntid> {
                if self.partition_map.contains_entid(e) {
                    Ok(KnownEntid(e))
                } else {
                    bail!(DbErrorKind::UnallocatedEntid(e))
                }
            }

            fn ensure_ident_exists(&self, e: &Keyword) -> Result<KnownEntid> {
                self.schema.require_entid(e)
            }

            fn intern_lookup_ref<W: TransactableValue>(
                &mut self,
                lookup_ref: &entmod::LookupRef<W>,
            ) -> Result<LookupRef> {
                let lr_a: i64 = match lookup_ref.a {
                    AttributePlace::Entid(entmod::EntidOrIdent::Entid(ref a)) => *a,
                    AttributePlace::Entid(entmod::EntidOrIdent::Ident(ref a)) => {
                        self.schema.require_entid(&a)?.into()
                    }
                };
                let lr_attribute: &Attribute = self.schema.require_attribute_for_entid(lr_a)?;

                let lr_typed_value: TypedValue = lookup_ref
                    .v
                    .clone()
                    .into_typed_value(&self.schema, lr_attribute.value_type)?;
                if lr_attribute.unique.is_none() {
                    bail!(DbErrorKind::NotYetImplemented(format!(
                        "Cannot resolve (lookup-ref {} {:?}) with attribute that is not :db/unique",
                        lr_a, lr_typed_value
                    )))
                }

                Ok(self.lookup_refs.intern((lr_a, lr_typed_value)))
            }

            /// Allocate private internal tempids reserved for Mentat.  Internal tempids just need to be
            /// unique within one transaction; they should never escape a transaction.
            fn allocate_mentat_id<W: TransactableValue>(&mut self) -> entmod::EntityPlace<W> {
                self.mentat_id_count += 1;
                entmod::EntityPlace::TempId(TempId::Internal(self.mentat_id_count).into())
            }

            fn entity_e_into_term_e<W: TransactableValue>(
                &mut self,
                x: entmod::EntityPlace<W>,
            ) -> Result<KnownEntidOr<LookupRefOrTempId>> {
                match x {
                    entmod::EntityPlace::Entid(e) => {
                        let e = match e {
                            entmod::EntidOrIdent::Entid(ref e) => self.ensure_entid_exists(*e)?,
                            entmod::EntidOrIdent::Ident(ref e) => self.ensure_ident_exists(&e)?,
                        };
                        Ok(Either::Left(e))
                    }

                    entmod::EntityPlace::TempId(e) => Ok(Either::Right(LookupRefOrTempId::TempId(
                        self.temp_ids.intern(e),
                    ))),

                    entmod::EntityPlace::LookupRef(ref lookup_ref) => Ok(Either::Right(
                        LookupRefOrTempId::LookupRef(self.intern_lookup_ref(lookup_ref)?),
                    )),

                    entmod::EntityPlace::TxFunction(ref tx_function) => {
                        match tx_function.op.0.as_str() {
                            "transaction-tx" => Ok(Either::Left(self.tx_id)),
                            unknown => bail!(DbErrorKind::NotYetImplemented(format!(
                                "Unknown transaction function {}",
                                unknown
                            ))),
                        }
                    }
                }
            }

            fn entity_a_into_term_a(&mut self, x: entmod::EntidOrIdent) -> Result<Entid> {
                let a = match x {
                    entmod::EntidOrIdent::Entid(ref a) => *a,
                    entmod::EntidOrIdent::Ident(ref a) => self.schema.require_entid(&a)?.into(),
                };
                Ok(a)
            }

            fn entity_e_into_term_v<W: TransactableValue>(
                &mut self,
                x: entmod::EntityPlace<W>,
            ) -> Result<TypedValueOr<LookupRefOrTempId>> {
                self.entity_e_into_term_e(x)
                    .map(|r| r.map_left(|ke| TypedValue::Ref(ke.0)))
            }

            fn entity_v_into_term_e<W: TransactableValue>(
                &mut self,
                x: entmod::ValuePlace<W>,
                backward_a: &entmod::EntidOrIdent,
            ) -> Result<KnownEntidOr<LookupRefOrTempId>> {
                match backward_a.unreversed() {
                    None => {
                        bail!(DbErrorKind::NotYetImplemented("Cannot explode map notation value in :attr/_reversed notation for forward attribute".to_string()));
                    }
                    Some(forward_a) => {
                        let forward_a = self.entity_a_into_term_a(forward_a)?;
                        let forward_attribute =
                            self.schema.require_attribute_for_entid(forward_a)?;
                        if forward_attribute.value_type != ValueType::Ref {
                            bail!(DbErrorKind::NotYetImplemented(format!("Cannot use :attr/_reversed notation for attribute {} that is not :db/valueType :db.type/ref", forward_a)))
                        }

                        match x {
                            entmod::ValuePlace::Atom(v) => {
                                // Here is where we do schema-aware typechecking: we either assert
                                // that the given value is in the attribute's value set, or (in
                                // limited cases) coerce the value into the attribute's value set.
                                match v.as_tempid() {
                                    Some(tempid) => Ok(Either::Right(LookupRefOrTempId::TempId(self.temp_ids.intern(tempid)))),
                                    None => {
                                        if let TypedValue::Ref(entid) = v.into_typed_value(&self.schema, ValueType::Ref)? {
                                            Ok(Either::Left(KnownEntid(entid)))
                                        } else {
                                            // The given value is expected to be :db.type/ref, so this shouldn't happen.
                                            bail!(DbErrorKind::NotYetImplemented(format!("Cannot use :attr/_reversed notation for attribute {} with value that is not :db.valueType :db.type/ref", forward_a)))
                                        }
                                    }
                                }
                            },

                            entmod::ValuePlace::Entid(entid) =>
                                Ok(Either::Left(KnownEntid(self.entity_a_into_term_a(entid)?))),

                            entmod::ValuePlace::TempId(tempid) =>
                                Ok(Either::Right(LookupRefOrTempId::TempId(self.temp_ids.intern(tempid)))),

                            entmod::ValuePlace::LookupRef(ref lookup_ref) =>
                                Ok(Either::Right(LookupRefOrTempId::LookupRef(self.intern_lookup_ref(lookup_ref)?))),

                            entmod::ValuePlace::TxFunction(ref tx_function) => {
                                match tx_function.op.0.as_str() {
                                    "transaction-tx" => Ok(Either::Left(KnownEntid(self.tx_id.0))),
                                    unknown=> bail!(DbErrorKind::NotYetImplemented(format!("Unknown transaction function {}", unknown))),
                                }
                            },

                            entmod::ValuePlace::Vector(_) =>
                                bail!(DbErrorKind::NotYetImplemented(format!("Cannot explode vector value in :attr/_reversed notation for attribute {}", forward_a))),

                            entmod::ValuePlace::MapNotation(_) =>
                                bail!(DbErrorKind::NotYetImplemented(format!("Cannot explode map notation value in :attr/_reversed notation for attribute {}", forward_a))),
                        }
                    }
                }
            }
        }

        let mut in_process = InProcess::with_schema_and_partition_map(
            &self.schema,
            &self.partition_map,
            KnownEntid(self.tx_id),
        );

        // We want to handle entities in the order they're given to us, while also "exploding" some
        // entities into many.  We therefore push the initial entities onto the back of the deque,
        // take from the front of the deque, and explode onto the front as well.
        let mut deque: VecDeque<Entity<V>> = VecDeque::default();
        deque.extend(entities);

        let mut terms: Vec<TermWithTempIdsAndLookupRefs> = Vec::with_capacity(deque.len());

        while let Some(entity) = deque.pop_front() {
            match entity {
                Entity::MapNotation(mut map_notation) => {
                    // :db/id is optional; if it's not given, we generate a special internal tempid
                    // to use for upserting.  This tempid will not be reported in the TxReport.
                    let db_id: entmod::EntityPlace<V> = remove_db_id(&mut map_notation)?
                        .unwrap_or_else(|| in_process.allocate_mentat_id());

                    // We're not nested, so :db/isComponent is not relevant.  We just explode the
                    // map notation.
                    for (a, v) in map_notation {
                        deque.push_front(Entity::AddOrRetract {
                            op: OpType::Add,
                            e: db_id.clone(),
                            a: AttributePlace::Entid(a),
                            v,
                        });
                    }
                }

                Entity::AddOrRetract { op, e, a, v } => {
                    let AttributePlace::Entid(a) = a;

                    if let Some(reversed_a) = a.unreversed() {
                        let reversed_e = in_process.entity_v_into_term_e(v, &a)?;
                        let reversed_a = in_process.entity_a_into_term_a(reversed_a)?;
                        let reversed_v = in_process.entity_e_into_term_v(e)?;
                        terms.push(Term::AddOrRetract(
                            OpType::Add,
                            reversed_e,
                            reversed_a,
                            reversed_v,
                        ));
                    } else {
                        let a = in_process.entity_a_into_term_a(a)?;
                        let attribute = self.schema.require_attribute_for_entid(a)?;

                        let v = match v {
                            entmod::ValuePlace::Atom(v) => {
                                // Here is where we do schema-aware typechecking: we either assert
                                // that the given value is in the attribute's value set, or (in
                                // limited cases) coerce the value into the attribute's value set.
                                if attribute.value_type == ValueType::Ref {
                                    match v.as_tempid() {
                                        Some(tempid) => Either::Right(LookupRefOrTempId::TempId(
                                            in_process.temp_ids.intern(tempid),
                                        )),
                                        None => v
                                            .into_typed_value(&self.schema, attribute.value_type)
                                            .map(Either::Left)?,
                                    }
                                } else {
                                    v.into_typed_value(&self.schema, attribute.value_type)
                                        .map(Either::Left)?
                                }
                            }

                            entmod::ValuePlace::Entid(entid) => Either::Left(TypedValue::Ref(
                                in_process.entity_a_into_term_a(entid)?,
                            )),

                            entmod::ValuePlace::TempId(tempid) => Either::Right(
                                LookupRefOrTempId::TempId(in_process.temp_ids.intern(tempid)),
                            ),

                            entmod::ValuePlace::LookupRef(ref lookup_ref) => {
                                if attribute.value_type != ValueType::Ref {
                                    bail!(DbErrorKind::NotYetImplemented(format!("Cannot resolve value lookup ref for attribute {} that is not :db/valueType :db.type/ref", a)))
                                }

                                Either::Right(LookupRefOrTempId::LookupRef(
                                    in_process.intern_lookup_ref(lookup_ref)?,
                                ))
                            }

                            entmod::ValuePlace::TxFunction(ref tx_function) => {
                                let typed_value = match tx_function.op.0.as_str() {
                                    "transaction-tx" => TypedValue::Ref(self.tx_id),
                                    unknown => bail!(DbErrorKind::NotYetImplemented(format!(
                                        "Unknown transaction function {}",
                                        unknown
                                    ))),
                                };

                                // Here we do schema-aware typechecking: we assert that the computed
                                // value is in the attribute's value set.  If and when we have
                                // transaction functions that produce numeric values, we'll have to
                                // be more careful here, because a function that produces an integer
                                // value can be used where a double is expected.  See also
                                // `SchemaTypeChecking.to_typed_value(...)`.
                                if attribute.value_type != typed_value.value_type() {
                                    bail!(DbErrorKind::NotYetImplemented(format!("Transaction function {} produced value of type {} but expected type {}",
                                                                               tx_function.op.0.as_str(), typed_value.value_type(), attribute.value_type)));
                                }

                                Either::Left(typed_value)
                            }

                            entmod::ValuePlace::Vector(vs) => {
                                if !attribute.multival {
                                    bail!(DbErrorKind::NotYetImplemented(format!("Cannot explode vector value for attribute {} that is not :db.cardinality :db.cardinality/many", a)));
                                }

                                for vv in vs {
                                    deque.push_front(Entity::AddOrRetract {
                                        op,
                                        e: e.clone(),
                                        a: AttributePlace::Entid(entmod::EntidOrIdent::Entid(a)),
                                        v: vv,
                                    });
                                }
                                continue;
                            }

                            entmod::ValuePlace::MapNotation(mut map_notation) => {
                                // TODO: consider handling this at the tx-parser level.  That would be
                                // more strict and expressive, but it would lead to splitting
                                // AddOrRetract, which proliferates types and code, or only handling
                                // nested maps rather than map values, like Datomic does.
                                if op != OpType::Add {
                                    bail!(DbErrorKind::NotYetImplemented(format!("Cannot explode nested map value in :db/retract for attribute {}", a)));
                                }

                                if attribute.value_type != ValueType::Ref {
                                    bail!(DbErrorKind::NotYetImplemented(format!("Cannot explode nested map value for attribute {} that is not :db/valueType :db.type/ref", a)))
                                }

                                // :db/id is optional; if it's not given, we generate a special internal tempid
                                // to use for upserting.  This tempid will not be reported in the TxReport.
                                let db_id: Option<entmod::EntityPlace<V>> =
                                    remove_db_id(&mut map_notation)?;
                                let mut dangling = db_id.is_none();
                                let db_id: entmod::EntityPlace<V> =
                                    db_id.unwrap_or_else(|| in_process.allocate_mentat_id());

                                // We're nested, so we want to ensure we're not creating "dangling"
                                // entities that can't be reached.  If we're :db/isComponent, then this
                                // is not dangling.  Otherwise, the resulting map needs to have a
                                // :db/unique :db.unique/identity [a v] pair, so that it's reachable.
                                // Per http://docs.datomic.com/transactions.html: "Either the reference
                                // to the nested map must be a component attribute, or the nested map
                                // must include a unique attribute. This constraint prevents the
                                // accidental creation of easily-orphaned entities that have no identity
                                // or relation to other entities."
                                if attribute.component {
                                    dangling = false;
                                }

                                for (inner_a, inner_v) in map_notation {
                                    if let Some(reversed_a) = inner_a.unreversed() {
                                        // We definitely have a reference.  The reference might be
                                        // dangling (a bare entid, for example), but we don't yet
                                        // support nested maps and reverse notation simultaneously
                                        // (i.e., we don't accept {:reverse/_attribute {:nested map}})
                                        // so we don't need to check that the nested map reference isn't
                                        // dangling.
                                        dangling = false;

                                        let reversed_e =
                                            in_process.entity_v_into_term_e(inner_v, &inner_a)?;
                                        let reversed_a =
                                            in_process.entity_a_into_term_a(reversed_a)?;
                                        let reversed_v =
                                            in_process.entity_e_into_term_v(db_id.clone())?;
                                        terms.push(Term::AddOrRetract(
                                            OpType::Add,
                                            reversed_e,
                                            reversed_a,
                                            reversed_v,
                                        ));
                                    } else {
                                        let inner_a = in_process.entity_a_into_term_a(inner_a)?;
                                        let inner_attribute =
                                            self.schema.require_attribute_for_entid(inner_a)?;
                                        if inner_attribute.unique
                                            == Some(attribute::Unique::Identity)
                                        {
                                            dangling = false;
                                        }

                                        deque.push_front(Entity::AddOrRetract {
                                            op: OpType::Add,
                                            e: db_id.clone(),
                                            a: AttributePlace::Entid(entmod::EntidOrIdent::Entid(
                                                inner_a,
                                            )),
                                            v: inner_v,
                                        });
                                    }
                                }

                                if dangling {
                                    bail!(DbErrorKind::NotYetImplemented(format!("Cannot explode nested map value that would lead to dangling entity for attribute {}", a)));
                                }

                                in_process.entity_e_into_term_v(db_id)?
                            }
                        };

                        let e = in_process.entity_e_into_term_e(e)?;
                        terms.push(Term::AddOrRetract(op, e, a, v));
                    }
                }
            }
        }
        Ok((terms, in_process.temp_ids, in_process.lookup_refs))
    }

    /// Pipeline stage 2: rewrite `Term` instances with lookup refs into `Term` instances without
    /// lookup refs.
    ///
    /// The `Term` instances produced share interned TempId handles and have no LookupRef references.
    fn resolve_lookup_refs<I>(
        &self,
        lookup_ref_map: &AVMap,
        terms: I,
    ) -> Result<Vec<TermWithTempIds>>
    where
        I: IntoIterator<Item = TermWithTempIdsAndLookupRefs>,
    {
        terms
            .into_iter()
            .map(
                |term: TermWithTempIdsAndLookupRefs| -> Result<TermWithTempIds> {
                    match term {
                        Term::AddOrRetract(op, e, a, v) => {
                            let e = replace_lookup_ref(&lookup_ref_map, e, KnownEntid)?;
                            let v = replace_lookup_ref(&lookup_ref_map, v, TypedValue::Ref)?;
                            Ok(Term::AddOrRetract(op, e, a, v))
                        }
                    }
                },
            )
            .collect::<Result<Vec<_>>>()
    }

    /// Transact the given `entities` against the store.
    ///
    /// This approach is explained in https://github.com/mozilla/mentat/wiki/Transacting.
    // TODO: move this to the transactor layer.
    pub fn transact_entities<I, V: TransactableValue>(&mut self, entities: I) -> Result<TxReport>
    where
        I: IntoIterator<Item = Entity<V>>,
    {
        // Pipeline stage 1: entities -> terms with tempids and lookup refs.
        let (terms_with_temp_ids_and_lookup_refs, tempid_set, lookup_ref_set) =
            self.entities_into_terms_with_temp_ids_and_lookup_refs(entities)?;

        // Pipeline stage 2: resolve lookup refs -> terms with tempids.
        let lookup_ref_avs: Vec<&(i64, TypedValue)> =
            lookup_ref_set.iter().map(|rc| &**rc).collect();
        let lookup_ref_map: AVMap = self.store.resolve_avs(&lookup_ref_avs[..])?;

        let terms_with_temp_ids =
            self.resolve_lookup_refs(&lookup_ref_map, terms_with_temp_ids_and_lookup_refs)?;

        self.transact_simple_terms_with_action(
            terms_with_temp_ids,
            tempid_set,
            TransactorAction::MaterializeAndCommit,
        )
    }

    pub fn transact_simple_terms<I>(
        &mut self,
        terms: I,
        tempid_set: InternSet<TempId>,
    ) -> Result<TxReport>
    where
        I: IntoIterator<Item = TermWithTempIds>,
    {
        self.transact_simple_terms_with_action(
            terms,
            tempid_set,
            TransactorAction::MaterializeAndCommit,
        )
    }

    fn transact_simple_terms_with_action<I>(
        &mut self,
        terms: I,
        tempid_set: InternSet<TempId>,
        action: TransactorAction,
    ) -> Result<TxReport>
    where
        I: IntoIterator<Item = TermWithTempIds>,
    {
        // TODO: push these into an internal transaction report?
        let mut tempids: BTreeMap<TempId, KnownEntid> = BTreeMap::default();

        // Pipeline stage 3: upsert tempids -> terms without tempids or lookup refs.
        // Now we can collect upsert populations.
        let (mut generation, inert_terms) = Generation::from(terms, &self.schema)?;

        // And evolve them forward.
        while generation.can_evolve() {
            debug!("generation {:?}", generation);

            let tempid_avs = generation.temp_id_avs();
            debug!("trying to resolve avs {:?}", tempid_avs);

            // Evolve further.
            let temp_id_map: TempIdMap = self.resolve_temp_id_avs(&tempid_avs[..])?;

            debug!("resolved avs for tempids {:?}", temp_id_map);

            generation = generation.evolve_one_step(&temp_id_map);

            // Errors.  BTree* since we want deterministic results.
            let mut conflicting_upserts: BTreeMap<TempId, BTreeSet<KnownEntid>> =
                BTreeMap::default();

            // Report each tempid that resolves via upsert.
            for (tempid, entid) in temp_id_map {
                // Since `UpsertEV` instances always transition to `UpsertE` instances, it might be
                // that a tempid resolves in two generations, and those resolutions might conflict.
                if let Some(previous) = tempids.insert((*tempid).clone(), entid) {
                    if entid != previous {
                        conflicting_upserts
                            .entry((*tempid).clone())
                            .or_insert_with(|| once(previous).collect::<BTreeSet<_>>())
                            .insert(entid);
                    }
                }
            }

            if !conflicting_upserts.is_empty() {
                bail!(DbErrorKind::SchemaConstraintViolation(
                    errors::SchemaConstraintViolation::ConflictingUpserts {
                        conflicting_upserts
                    }
                ));
            }

            debug!("tempids {:?}", tempids);
        }

        generation.allocate_unresolved_upserts()?;

        debug!("final generation {:?}", generation);

        // Allocate entids for tempids that didn't upsert.  BTreeMap so this is deterministic.
        let unresolved_temp_ids: BTreeMap<TempIdHandle, usize> =
            generation.temp_ids_in_allocations(&self.schema)?;

        debug!("unresolved tempids {:?}", unresolved_temp_ids);

        // TODO: track partitions for temporary IDs.
        let entids = self
            .partition_map
            .allocate_entids(":db.part/user", unresolved_temp_ids.len());

        let temp_id_allocations = unresolved_temp_ids
            .into_iter()
            .map(|(tempid, index)| (tempid, KnownEntid(entids.start + (index as i64))))
            .collect();

        debug!("tempid allocations {:?}", temp_id_allocations);

        let final_populations = generation.into_final_populations(&temp_id_allocations)?;

        // Report each tempid that is allocated.
        for (tempid, &entid) in &temp_id_allocations {
            // Every tempid should be allocated at most once.
            assert!(!tempids.contains_key(&**tempid));
            tempids.insert((**tempid).clone(), entid);
        }

        // Verify that every tempid we interned either resolved or has been allocated.
        assert_eq!(tempids.len(), tempid_set.len());
        for tempid in tempid_set.iter() {
            assert!(tempids.contains_key(&**tempid));
        }

        // Any internal tempid has been allocated by the system and is a private implementation
        // detail; it shouldn't be exposed in the final transaction report.
        let tempids = tempids
            .into_iter()
            .filter_map(|(tempid, e)| tempid.into_external().map(|s| (s, e.0)))
            .collect();

        // A transaction might try to add or retract :db/ident assertions or other metadata mutating
        // assertions , but those assertions might not make it to the store.  If we see a possible
        // metadata mutation, we will figure out if any assertions made it through later.  This is
        // strictly an optimization: it would be correct to _always_ check what made it to the
        // store.
        let mut tx_might_update_metadata = false;

        // Mutable so that we can add the transaction :db/txInstant.
        let mut aev_trie = into_aev_trie(&self.schema, final_populations, inert_terms)?;

        let tx_instant;
        {
            // TODO: Don't use this block to scope borrowing the schema; instead, extract a helper function.

            // Assertions that are :db.cardinality/one and not :db.fulltext.
            let mut non_fts_one: Vec<db::ReducedEntity> = vec![];

            // Assertions that are :db.cardinality/many and not :db.fulltext.
            let mut non_fts_many: Vec<db::ReducedEntity> = vec![];

            // Assertions that are :db.cardinality/one and :db.fulltext.
            let mut fts_one: Vec<db::ReducedEntity> = vec![];

            // Assertions that are :db.cardinality/many and :db.fulltext.
            let mut fts_many: Vec<db::ReducedEntity> = vec![];

            // We need to ensure that callers can't blindly transact entities that haven't been
            // allocated by this store.

            let errors = tx_checking::type_disagreements(&aev_trie);
            if !errors.is_empty() {
                bail!(DbErrorKind::SchemaConstraintViolation(
                    errors::SchemaConstraintViolation::TypeDisagreements {
                        conflicting_datoms: errors
                    }
                ));
            }

            let errors = tx_checking::cardinality_conflicts(&aev_trie);
            if !errors.is_empty() {
                bail!(DbErrorKind::SchemaConstraintViolation(
                    errors::SchemaConstraintViolation::CardinalityConflicts { conflicts: errors }
                ));
            }

            // Pipeline stage 4: final terms (after rewriting) -> DB insertions.
            // Collect into non_fts_*.

            tx_instant = get_or_insert_tx_instant(&mut aev_trie, &self.schema, self.tx_id)?;

            for ((a, attribute), evs) in aev_trie {
                if entids::might_update_metadata(a) {
                    tx_might_update_metadata = true;
                }

                let queue = match (attribute.fulltext, attribute.multival) {
                    (false, true) => &mut non_fts_many,
                    (false, false) => &mut non_fts_one,
                    (true, false) => &mut fts_one,
                    (true, true) => &mut fts_many,
                };

                for (e, ars) in evs {
                    for (added, v) in ars
                        .add
                        .into_iter()
                        .map(|v| (true, v))
                        .chain(ars.retract.into_iter().map(|v| (false, v)))
                    {
                        let op = if added { OpType::Add } else { OpType::Retract };
                        self.watcher.datom(op, e, a, &v);
                        queue.push((e, a, attribute, v, added));
                    }
                }
            }

            if !non_fts_one.is_empty() {
                self.store
                    .insert_non_fts_searches(&non_fts_one[..], db::SearchType::Inexact)?;
            }

            if !non_fts_many.is_empty() {
                self.store
                    .insert_non_fts_searches(&non_fts_many[..], db::SearchType::Exact)?;
            }

            if !fts_one.is_empty() {
                self.store
                    .insert_fts_searches(&fts_one[..], db::SearchType::Inexact)?;
            }

            if !fts_many.is_empty() {
                self.store
                    .insert_fts_searches(&fts_many[..], db::SearchType::Exact)?;
            }

            match action {
                TransactorAction::Materialize => {
                    self.store.materialize_mentat_transaction(self.tx_id)?;
                }
                TransactorAction::MaterializeAndCommit => {
                    self.store.materialize_mentat_transaction(self.tx_id)?;
                    self.store.commit_mentat_transaction(self.tx_id)?;
                }
            }
        }

        self.watcher.done(&self.tx_id, self.schema)?;

        if tx_might_update_metadata {
            // Extract changes to metadata from the store.
            let metadata_assertions = match action {
                TransactorAction::Materialize => self.store.resolved_metadata_assertions()?,
                TransactorAction::MaterializeAndCommit => {
                    db::committed_metadata_assertions(self.store, self.tx_id)?
                }
            };
            let mut new_schema = (*self.schema_for_mutation).clone(); // Clone the underlying Schema for modification.
            let metadata_report = metadata::update_schema_from_entid_quadruples(
                &mut new_schema,
                metadata_assertions,
            )?;
            // We might not have made any changes to the schema, even though it looked like we
            // would.  This should not happen, even during bootstrapping: we mutate an empty
            // `Schema` in this case specifically to run the bootstrapped assertions through the
            // regular transactor code paths, updating the schema and materialized views uniformly.
            // But, belt-and-braces: handle it gracefully.
            if new_schema != *self.schema_for_mutation {
                let old_schema = (*self.schema_for_mutation).clone(); // Clone the original Schema for comparison.
                *self.schema_for_mutation.to_mut() = new_schema; // Store the new Schema.
                db::update_metadata(
                    self.store,
                    &old_schema,
                    &*self.schema_for_mutation,
                    &metadata_report,
                )?;
            }
        }

        Ok(TxReport {
            tx_id: self.tx_id,
            tx_instant,
            tempids,
        })
    }
}

/// Initialize a new Tx object with a new tx id and a tx instant. Kick off the SQLite conn, too.
fn start_tx<'conn, 'a, W>(
    conn: &'conn rusqlite::Connection,
    mut partition_map: PartitionMap,
    schema_for_mutation: &'a Schema,
    schema: &'a Schema,
    watcher: W,
) -> Result<Tx<'conn, 'a, W>>
where
    W: TransactWatcher,
{
    let tx_id = partition_map.allocate_entid(":db.part/tx");
    conn.begin_tx_application()?;

    Ok(Tx::new(
        conn,
        partition_map,
        schema_for_mutation,
        schema,
        watcher,
        tx_id,
    ))
}

fn conclude_tx<W>(
    tx: Tx<W>,
    report: TxReport,
) -> Result<(TxReport, PartitionMap, Option<Schema>, W)>
where
    W: TransactWatcher,
{
    // If the schema has moved on, return it.
    let next_schema = match tx.schema_for_mutation {
        Cow::Borrowed(_) => None,
        Cow::Owned(next_schema) => Some(next_schema),
    };
    Ok((report, tx.partition_map, next_schema, tx.watcher))
}

/// Transact the given `entities` against the given SQLite `conn`, using the given metadata.
/// If you want this work to occur inside a SQLite transaction, establish one on the connection
/// prior to calling this function.
///
/// This approach is explained in https://github.com/mozilla/mentat/wiki/Transacting.
// TODO: move this to the transactor layer.
pub fn transact<'conn, 'a, I, V, W>(
    conn: &'conn rusqlite::Connection,
    partition_map: PartitionMap,
    schema_for_mutation: &'a Schema,
    schema: &'a Schema,
    watcher: W,
    entities: I,
) -> Result<(TxReport, PartitionMap, Option<Schema>, W)>
where
    I: IntoIterator<Item = Entity<V>>,
    V: TransactableValue,
    W: TransactWatcher,
{
    let mut tx = start_tx(conn, partition_map, schema_for_mutation, schema, watcher)?;
    let report = tx.transact_entities(entities)?;
    conclude_tx(tx, report)
}

/// Just like `transact`, but accepts lower-level inputs to allow bypassing the parser interface.
pub fn transact_terms<'conn, 'a, I, W>(
    conn: &'conn rusqlite::Connection,
    partition_map: PartitionMap,
    schema_for_mutation: &'a Schema,
    schema: &'a Schema,
    watcher: W,
    terms: I,
    tempid_set: InternSet<TempId>,
) -> Result<(TxReport, PartitionMap, Option<Schema>, W)>
where
    I: IntoIterator<Item = TermWithTempIds>,
    W: TransactWatcher,
{
    transact_terms_with_action(
        conn,
        partition_map,
        schema_for_mutation,
        schema,
        watcher,
        terms,
        tempid_set,
        TransactorAction::MaterializeAndCommit,
    )
}

#[allow(clippy::too_many_arguments)]
pub(crate) fn transact_terms_with_action<'conn, 'a, I, W>(
    conn: &'conn rusqlite::Connection,
    partition_map: PartitionMap,
    schema_for_mutation: &'a Schema,
    schema: &'a Schema,
    watcher: W,
    terms: I,
    tempid_set: InternSet<TempId>,
    action: TransactorAction,
) -> Result<(TxReport, PartitionMap, Option<Schema>, W)>
where
    I: IntoIterator<Item = TermWithTempIds>,
    W: TransactWatcher,
{
    let mut tx = start_tx(conn, partition_map, schema_for_mutation, schema, watcher)?;
    let report = tx.transact_simple_terms_with_action(terms, tempid_set, action)?;
    conclude_tx(tx, report)
}

fn extend_aev_trie<'schema, I>(
    schema: &'schema Schema,
    terms: I,
    trie: &mut AEVTrie<'schema>,
) -> Result<()>
where
    I: IntoIterator<Item = TermWithoutTempIds>,
{
    for Term::AddOrRetract(op, KnownEntid(e), a, v) in terms.into_iter() {
        let attribute: &Attribute = schema.require_attribute_for_entid(a)?;

        let a_and_r = trie
            .entry((a, attribute))
            .or_insert_with(BTreeMap::default)
            .entry(e)
            .or_insert_with(AddAndRetract::default);

        match op {
            OpType::Add => a_and_r.add.insert(v),
            OpType::Retract => a_and_r.retract.insert(v),
        };
    }

    Ok(())
}

pub(crate) fn into_aev_trie<'schema>(
    schema: &'schema Schema,
    final_populations: FinalPopulations,
    inert_terms: Vec<TermWithTempIds>,
) -> Result<AEVTrie<'schema>> {
    let mut trie = AEVTrie::default();
    extend_aev_trie(schema, final_populations.resolved, &mut trie)?;
    extend_aev_trie(schema, final_populations.allocated, &mut trie)?;
    // Inert terms need to be unwrapped.  It is a coding error if a term can't be unwrapped.
    extend_aev_trie(
        schema,
        inert_terms.into_iter().map(|term| term.unwrap()),
        &mut trie,
    )?;

    Ok(trie)
}

/// Transact [:db/add :db/txInstant tx_instant (transaction-tx)] if the trie doesn't contain it
/// already.  Return the instant from the input or the instant inserted.
fn get_or_insert_tx_instant<'schema>(
    aev_trie: &mut AEVTrie<'schema>,
    schema: &'schema Schema,
    tx_id: Entid,
) -> Result<DateTime<Utc>> {
    let ars = aev_trie
        .entry((
            entids::DB_TX_INSTANT,
            schema.require_attribute_for_entid(entids::DB_TX_INSTANT)?,
        ))
        .or_insert_with(BTreeMap::default)
        .entry(tx_id)
        .or_insert_with(AddAndRetract::default);
    if !ars.retract.is_empty() {
        // Cannot retract :db/txInstant!
    }

    // Otherwise we have a coding error -- we should have cardinality checked this already.
    assert!(ars.add.len() <= 1);

    let first = ars.add.iter().next().cloned();
    match first {
        Some(TypedValue::Instant(instant)) => Ok(instant),
        Some(_) => unreachable!(), // This is a coding error -- we should have typechecked this already.
        None => {
            let instant = now();
            ars.add.insert(instant.into());
            Ok(instant)
        }
    }
}