// Copyright 2020 The Jujutsu Authors
//
// 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
//
// https://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(missing_docs)]

use std::collections::hash_map::Entry;
use std::collections::HashMap;
use std::collections::HashSet;
use std::fmt::Debug;
use std::fmt::Formatter;
use std::fs;
use std::path::Path;
use std::slice;
use std::sync::Arc;

use itertools::Itertools;
use once_cell::sync::OnceCell;
use thiserror::Error;
use tracing::instrument;

use self::dirty_cell::DirtyCell;
use crate::backend::Backend;
use crate::backend::BackendError;
use crate::backend::BackendInitError;
use crate::backend::BackendLoadError;
use crate::backend::BackendResult;
use crate::backend::ChangeId;
use crate::backend::CommitId;
use crate::backend::MergedTreeId;
use crate::commit::Commit;
use crate::commit::CommitByCommitterTimestamp;
use crate::commit_builder::CommitBuilder;
use crate::commit_builder::DetachedCommitBuilder;
use crate::dag_walk;
use crate::default_index::DefaultIndexStore;
use crate::default_index::DefaultMutableIndex;
use crate::default_submodule_store::DefaultSubmoduleStore;
use crate::file_util::IoResultExt as _;
use crate::file_util::PathError;
use crate::index::ChangeIdIndex;
use crate::index::Index;
use crate::index::IndexReadError;
use crate::index::IndexStore;
use crate::index::MutableIndex;
use crate::index::ReadonlyIndex;
use crate::local_backend::LocalBackend;
use crate::merge::MergeBuilder;
use crate::object_id::HexPrefix;
use crate::object_id::ObjectId;
use crate::object_id::PrefixResolution;
use crate::op_heads_store;
use crate::op_heads_store::OpHeadResolutionError;
use crate::op_heads_store::OpHeadsStore;
use crate::op_store;
use crate::op_store::OpStore;
use crate::op_store::OpStoreError;
use crate::op_store::OpStoreResult;
use crate::op_store::OperationId;
use crate::op_store::RefTarget;
use crate::op_store::RemoteRef;
use crate::op_store::RemoteRefState;
use crate::op_store::RootOperationData;
use crate::op_store::WorkspaceId;
use crate::operation::Operation;
use crate::refs::diff_named_ref_targets;
use crate::refs::diff_named_remote_refs;
use crate::refs::merge_ref_targets;
use crate::refs::merge_remote_refs;
use crate::revset;
use crate::revset::RevsetEvaluationError;
use crate::revset::RevsetExpression;
use crate::revset::RevsetIteratorExt;
use crate::rewrite::merge_commit_trees;
use crate::rewrite::CommitRewriter;
use crate::rewrite::DescendantRebaser;
use crate::rewrite::RebaseOptions;
use crate::settings::RepoSettings;
use crate::settings::UserSettings;
use crate::signing::SignInitError;
use crate::signing::Signer;
use crate::simple_op_heads_store::SimpleOpHeadsStore;
use crate::simple_op_store::SimpleOpStore;
use crate::store::Store;
use crate::submodule_store::SubmoduleStore;
use crate::transaction::Transaction;
use crate::view::RenameWorkspaceError;
use crate::view::View;

pub trait Repo {
    /// Base repository that contains all committed data. Returns `self` if this
    /// is a `ReadonlyRepo`,
    fn base_repo(&self) -> &ReadonlyRepo;

    fn store(&self) -> &Arc<Store>;

    fn op_store(&self) -> &Arc<dyn OpStore>;

    fn index(&self) -> &dyn Index;

    fn view(&self) -> &View;

    fn submodule_store(&self) -> &Arc<dyn SubmoduleStore>;

    fn resolve_change_id(&self, change_id: &ChangeId) -> Option<Vec<CommitId>> {
        // Replace this if we added more efficient lookup method.
        let prefix = HexPrefix::from_bytes(change_id.as_bytes());
        match self.resolve_change_id_prefix(&prefix) {
            PrefixResolution::NoMatch => None,
            PrefixResolution::SingleMatch(entries) => Some(entries),
            PrefixResolution::AmbiguousMatch => panic!("complete change_id should be unambiguous"),
        }
    }

    fn resolve_change_id_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<Vec<CommitId>>;

    fn shortest_unique_change_id_prefix_len(&self, target_id_bytes: &ChangeId) -> usize;
}

pub struct ReadonlyRepo {
    loader: RepoLoader,
    operation: Operation,
    index: Box<dyn ReadonlyIndex>,
    change_id_index: OnceCell<Box<dyn ChangeIdIndex>>,
    // TODO: This should eventually become part of the index and not be stored fully in memory.
    view: View,
}

impl Debug for ReadonlyRepo {
    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
        f.debug_struct("ReadonlyRepo")
            .field("store", &self.loader.store)
            .finish_non_exhaustive()
    }
}

#[derive(Error, Debug)]
pub enum RepoInitError {
    #[error(transparent)]
    Backend(#[from] BackendInitError),
    #[error(transparent)]
    Path(#[from] PathError),
}

impl ReadonlyRepo {
    pub fn default_op_store_initializer() -> &'static OpStoreInitializer<'static> {
        &|_settings, store_path, root_data| Box::new(SimpleOpStore::init(store_path, root_data))
    }

    pub fn default_op_heads_store_initializer() -> &'static OpHeadsStoreInitializer<'static> {
        &|_settings, store_path| {
            let store = SimpleOpHeadsStore::init(store_path);
            Box::new(store)
        }
    }

    pub fn default_index_store_initializer() -> &'static IndexStoreInitializer<'static> {
        &|_settings, store_path| Ok(Box::new(DefaultIndexStore::init(store_path)?))
    }

    pub fn default_submodule_store_initializer() -> &'static SubmoduleStoreInitializer<'static> {
        &|_settings, store_path| Box::new(DefaultSubmoduleStore::init(store_path))
    }

    #[allow(clippy::too_many_arguments)]
    pub fn init(
        user_settings: &UserSettings,
        repo_path: &Path,
        backend_initializer: &BackendInitializer,
        signer: Signer,
        op_store_initializer: &OpStoreInitializer,
        op_heads_store_initializer: &OpHeadsStoreInitializer,
        index_store_initializer: &IndexStoreInitializer,
        submodule_store_initializer: &SubmoduleStoreInitializer,
    ) -> Result<Arc<ReadonlyRepo>, RepoInitError> {
        let repo_path = repo_path.canonicalize().context(repo_path)?;

        let store_path = repo_path.join("store");
        fs::create_dir(&store_path).context(&store_path)?;
        let backend = backend_initializer(user_settings, &store_path)?;
        let backend_path = store_path.join("type");
        fs::write(&backend_path, backend.name()).context(&backend_path)?;
        let store = Store::new(backend, signer);
        let repo_settings = user_settings.with_repo(&repo_path).unwrap();

        let op_store_path = repo_path.join("op_store");
        fs::create_dir(&op_store_path).context(&op_store_path)?;
        let root_op_data = RootOperationData {
            root_commit_id: store.root_commit_id().clone(),
        };
        let op_store = op_store_initializer(user_settings, &op_store_path, root_op_data);
        let op_store_type_path = op_store_path.join("type");
        fs::write(&op_store_type_path, op_store.name()).context(&op_store_type_path)?;
        let op_store: Arc<dyn OpStore> = Arc::from(op_store);

        let op_heads_path = repo_path.join("op_heads");
        fs::create_dir(&op_heads_path).context(&op_heads_path)?;
        let op_heads_store = op_heads_store_initializer(user_settings, &op_heads_path);
        let op_heads_type_path = op_heads_path.join("type");
        fs::write(&op_heads_type_path, op_heads_store.name()).context(&op_heads_type_path)?;
        op_heads_store.update_op_heads(&[], op_store.root_operation_id());
        let op_heads_store: Arc<dyn OpHeadsStore> = Arc::from(op_heads_store);

        let index_path = repo_path.join("index");
        fs::create_dir(&index_path).context(&index_path)?;
        let index_store = index_store_initializer(user_settings, &index_path)?;
        let index_type_path = index_path.join("type");
        fs::write(&index_type_path, index_store.name()).context(&index_type_path)?;
        let index_store: Arc<dyn IndexStore> = Arc::from(index_store);

        let submodule_store_path = repo_path.join("submodule_store");
        fs::create_dir(&submodule_store_path).context(&submodule_store_path)?;
        let submodule_store = submodule_store_initializer(user_settings, &submodule_store_path);
        let submodule_store_type_path = submodule_store_path.join("type");
        fs::write(&submodule_store_type_path, submodule_store.name())
            .context(&submodule_store_type_path)?;
        let submodule_store = Arc::from(submodule_store);

        let loader = RepoLoader {
            repo_settings,
            store,
            op_store,
            op_heads_store,
            index_store,
            submodule_store,
        };

        let root_operation = loader.root_operation();
        let root_view = root_operation.view().expect("failed to read root view");
        assert!(!root_view.heads().is_empty());
        let index = loader
            .index_store
            .get_index_at_op(&root_operation, &loader.store)
            // If the root op index couldn't be read, the index backend wouldn't
            // be initialized properly.
            .map_err(|err| BackendInitError(err.into()))?;
        Ok(Arc::new(ReadonlyRepo {
            loader,
            operation: root_operation,
            index,
            change_id_index: OnceCell::new(),
            view: root_view,
        }))
    }

    pub fn loader(&self) -> &RepoLoader {
        &self.loader
    }

    pub fn op_id(&self) -> &OperationId {
        self.operation.id()
    }

    pub fn operation(&self) -> &Operation {
        &self.operation
    }

    pub fn view(&self) -> &View {
        &self.view
    }

    pub fn readonly_index(&self) -> &dyn ReadonlyIndex {
        self.index.as_ref()
    }

    fn change_id_index(&self) -> &dyn ChangeIdIndex {
        self.change_id_index
            .get_or_init(|| {
                self.readonly_index()
                    .change_id_index(&mut self.view().heads().iter())
            })
            .as_ref()
    }

    pub fn op_heads_store(&self) -> &Arc<dyn OpHeadsStore> {
        self.loader.op_heads_store()
    }

    pub fn index_store(&self) -> &Arc<dyn IndexStore> {
        self.loader.index_store()
    }

    pub fn settings(&self) -> &RepoSettings {
        self.loader.settings()
    }

    pub fn start_transaction(
        self: &Arc<ReadonlyRepo>,
        user_settings: &UserSettings,
    ) -> Transaction {
        let mut_repo = MutableRepo::new(self.clone(), self.readonly_index(), &self.view);
        Transaction::new(mut_repo, user_settings)
    }

    pub fn reload_at_head(
        &self,
        user_settings: &UserSettings,
    ) -> Result<Arc<ReadonlyRepo>, RepoLoaderError> {
        self.loader().load_at_head(user_settings)
    }

    #[instrument]
    pub fn reload_at(&self, operation: &Operation) -> Result<Arc<ReadonlyRepo>, RepoLoaderError> {
        self.loader().load_at(operation)
    }
}

impl Repo for ReadonlyRepo {
    fn base_repo(&self) -> &ReadonlyRepo {
        self
    }

    fn store(&self) -> &Arc<Store> {
        self.loader.store()
    }

    fn op_store(&self) -> &Arc<dyn OpStore> {
        self.loader.op_store()
    }

    fn index(&self) -> &dyn Index {
        self.readonly_index().as_index()
    }

    fn view(&self) -> &View {
        &self.view
    }

    fn submodule_store(&self) -> &Arc<dyn SubmoduleStore> {
        self.loader.submodule_store()
    }

    fn resolve_change_id_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<Vec<CommitId>> {
        self.change_id_index().resolve_prefix(prefix)
    }

    fn shortest_unique_change_id_prefix_len(&self, target_id: &ChangeId) -> usize {
        self.change_id_index().shortest_unique_prefix_len(target_id)
    }
}

pub type BackendInitializer<'a> =
    dyn Fn(&UserSettings, &Path) -> Result<Box<dyn Backend>, BackendInitError> + 'a;
pub type OpStoreInitializer<'a> =
    dyn Fn(&UserSettings, &Path, RootOperationData) -> Box<dyn OpStore> + 'a;
pub type OpHeadsStoreInitializer<'a> = dyn Fn(&UserSettings, &Path) -> Box<dyn OpHeadsStore> + 'a;
pub type IndexStoreInitializer<'a> =
    dyn Fn(&UserSettings, &Path) -> Result<Box<dyn IndexStore>, BackendInitError> + 'a;
pub type SubmoduleStoreInitializer<'a> =
    dyn Fn(&UserSettings, &Path) -> Box<dyn SubmoduleStore> + 'a;

type BackendFactory =
    Box<dyn Fn(&UserSettings, &Path) -> Result<Box<dyn Backend>, BackendLoadError>>;
type OpStoreFactory = Box<dyn Fn(&UserSettings, &Path, RootOperationData) -> Box<dyn OpStore>>;
type OpHeadsStoreFactory = Box<dyn Fn(&UserSettings, &Path) -> Box<dyn OpHeadsStore>>;
type IndexStoreFactory =
    Box<dyn Fn(&UserSettings, &Path) -> Result<Box<dyn IndexStore>, BackendLoadError>>;
type SubmoduleStoreFactory = Box<dyn Fn(&UserSettings, &Path) -> Box<dyn SubmoduleStore>>;

pub fn merge_factories_map<F>(base: &mut HashMap<String, F>, ext: HashMap<String, F>) {
    for (name, factory) in ext {
        match base.entry(name) {
            Entry::Vacant(v) => {
                v.insert(factory);
            }
            Entry::Occupied(o) => {
                panic!("Conflicting factory definitions for '{}' factory", o.key())
            }
        }
    }
}

pub struct StoreFactories {
    backend_factories: HashMap<String, BackendFactory>,
    op_store_factories: HashMap<String, OpStoreFactory>,
    op_heads_store_factories: HashMap<String, OpHeadsStoreFactory>,
    index_store_factories: HashMap<String, IndexStoreFactory>,
    submodule_store_factories: HashMap<String, SubmoduleStoreFactory>,
}

impl Default for StoreFactories {
    fn default() -> Self {
        let mut factories = StoreFactories::empty();

        // Backends
        factories.add_backend(
            LocalBackend::name(),
            Box::new(|_settings, store_path| Ok(Box::new(LocalBackend::load(store_path)))),
        );
        #[cfg(feature = "git")]
        factories.add_backend(
            crate::git_backend::GitBackend::name(),
            Box::new(|settings, store_path| {
                Ok(Box::new(crate::git_backend::GitBackend::load(
                    settings, store_path,
                )?))
            }),
        );
        #[cfg(feature = "testing")]
        factories.add_backend(
            crate::secret_backend::SecretBackend::name(),
            Box::new(|settings, store_path| {
                Ok(Box::new(crate::secret_backend::SecretBackend::load(
                    settings, store_path,
                )?))
            }),
        );

        // OpStores
        factories.add_op_store(
            SimpleOpStore::name(),
            Box::new(|_settings, store_path, root_data| {
                Box::new(SimpleOpStore::load(store_path, root_data))
            }),
        );

        // OpHeadsStores
        factories.add_op_heads_store(
            SimpleOpHeadsStore::name(),
            Box::new(|_settings, store_path| Box::new(SimpleOpHeadsStore::load(store_path))),
        );

        // Index
        factories.add_index_store(
            DefaultIndexStore::name(),
            Box::new(|_settings, store_path| Ok(Box::new(DefaultIndexStore::load(store_path)))),
        );

        // SubmoduleStores
        factories.add_submodule_store(
            DefaultSubmoduleStore::name(),
            Box::new(|_settings, store_path| Box::new(DefaultSubmoduleStore::load(store_path))),
        );

        factories
    }
}

#[derive(Debug, Error)]
pub enum StoreLoadError {
    #[error("Unsupported {store} backend type '{store_type}'")]
    UnsupportedType {
        store: &'static str,
        store_type: String,
    },
    #[error("Failed to read {store} backend type")]
    ReadError {
        store: &'static str,
        source: PathError,
    },
    #[error(transparent)]
    Backend(#[from] BackendLoadError),
    #[error(transparent)]
    Signing(#[from] SignInitError),
}

impl StoreFactories {
    pub fn empty() -> Self {
        StoreFactories {
            backend_factories: HashMap::new(),
            op_store_factories: HashMap::new(),
            op_heads_store_factories: HashMap::new(),
            index_store_factories: HashMap::new(),
            submodule_store_factories: HashMap::new(),
        }
    }

    pub fn merge(&mut self, ext: StoreFactories) {
        let StoreFactories {
            backend_factories,
            op_store_factories,
            op_heads_store_factories,
            index_store_factories,
            submodule_store_factories,
        } = ext;

        merge_factories_map(&mut self.backend_factories, backend_factories);
        merge_factories_map(&mut self.op_store_factories, op_store_factories);
        merge_factories_map(&mut self.op_heads_store_factories, op_heads_store_factories);
        merge_factories_map(&mut self.index_store_factories, index_store_factories);
        merge_factories_map(
            &mut self.submodule_store_factories,
            submodule_store_factories,
        );
    }

    pub fn add_backend(&mut self, name: &str, factory: BackendFactory) {
        self.backend_factories.insert(name.to_string(), factory);
    }

    pub fn load_backend(
        &self,
        settings: &UserSettings,
        store_path: &Path,
    ) -> Result<Box<dyn Backend>, StoreLoadError> {
        let backend_type = read_store_type("commit", store_path.join("type"))?;
        let backend_factory = self.backend_factories.get(&backend_type).ok_or_else(|| {
            StoreLoadError::UnsupportedType {
                store: "commit",
                store_type: backend_type.to_string(),
            }
        })?;
        Ok(backend_factory(settings, store_path)?)
    }

    pub fn add_op_store(&mut self, name: &str, factory: OpStoreFactory) {
        self.op_store_factories.insert(name.to_string(), factory);
    }

    pub fn load_op_store(
        &self,
        settings: &UserSettings,
        store_path: &Path,
        root_data: RootOperationData,
    ) -> Result<Box<dyn OpStore>, StoreLoadError> {
        let op_store_type = read_store_type("operation", store_path.join("type"))?;
        let op_store_factory = self.op_store_factories.get(&op_store_type).ok_or_else(|| {
            StoreLoadError::UnsupportedType {
                store: "operation",
                store_type: op_store_type.to_string(),
            }
        })?;
        Ok(op_store_factory(settings, store_path, root_data))
    }

    pub fn add_op_heads_store(&mut self, name: &str, factory: OpHeadsStoreFactory) {
        self.op_heads_store_factories
            .insert(name.to_string(), factory);
    }

    pub fn load_op_heads_store(
        &self,
        settings: &UserSettings,
        store_path: &Path,
    ) -> Result<Box<dyn OpHeadsStore>, StoreLoadError> {
        let op_heads_store_type = read_store_type("operation heads", store_path.join("type"))?;
        let op_heads_store_factory = self
            .op_heads_store_factories
            .get(&op_heads_store_type)
            .ok_or_else(|| StoreLoadError::UnsupportedType {
                store: "operation heads",
                store_type: op_heads_store_type.to_string(),
            })?;
        Ok(op_heads_store_factory(settings, store_path))
    }

    pub fn add_index_store(&mut self, name: &str, factory: IndexStoreFactory) {
        self.index_store_factories.insert(name.to_string(), factory);
    }

    pub fn load_index_store(
        &self,
        settings: &UserSettings,
        store_path: &Path,
    ) -> Result<Box<dyn IndexStore>, StoreLoadError> {
        let index_store_type = read_store_type("index", store_path.join("type"))?;
        let index_store_factory = self
            .index_store_factories
            .get(&index_store_type)
            .ok_or_else(|| StoreLoadError::UnsupportedType {
                store: "index",
                store_type: index_store_type.to_string(),
            })?;
        Ok(index_store_factory(settings, store_path)?)
    }

    pub fn add_submodule_store(&mut self, name: &str, factory: SubmoduleStoreFactory) {
        self.submodule_store_factories
            .insert(name.to_string(), factory);
    }

    pub fn load_submodule_store(
        &self,
        settings: &UserSettings,
        store_path: &Path,
    ) -> Result<Box<dyn SubmoduleStore>, StoreLoadError> {
        let submodule_store_type = read_store_type("submodule_store", store_path.join("type"))?;
        let submodule_store_factory = self
            .submodule_store_factories
            .get(&submodule_store_type)
            .ok_or_else(|| StoreLoadError::UnsupportedType {
                store: "submodule_store",
                store_type: submodule_store_type.to_string(),
            })?;

        Ok(submodule_store_factory(settings, store_path))
    }
}

pub fn read_store_type(
    store: &'static str,
    path: impl AsRef<Path>,
) -> Result<String, StoreLoadError> {
    let path = path.as_ref();
    fs::read_to_string(path)
        .context(path)
        .map_err(|source| StoreLoadError::ReadError { store, source })
}

#[derive(Debug, Error)]
pub enum RepoLoaderError {
    #[error(transparent)]
    Backend(#[from] BackendError),
    #[error(transparent)]
    IndexRead(#[from] IndexReadError),
    #[error(transparent)]
    OpHeadResolution(#[from] OpHeadResolutionError),
    #[error(transparent)]
    OpStore(#[from] OpStoreError),
}

/// Helps create `ReadonlyRepoo` instances of a repo at the head operation or at
/// a given operation.
#[derive(Clone)]
pub struct RepoLoader {
    repo_settings: RepoSettings,
    store: Arc<Store>,
    op_store: Arc<dyn OpStore>,
    op_heads_store: Arc<dyn OpHeadsStore>,
    index_store: Arc<dyn IndexStore>,
    submodule_store: Arc<dyn SubmoduleStore>,
}

impl RepoLoader {
    pub fn new(
        repo_settings: RepoSettings,
        store: Arc<Store>,
        op_store: Arc<dyn OpStore>,
        op_heads_store: Arc<dyn OpHeadsStore>,
        index_store: Arc<dyn IndexStore>,
        submodule_store: Arc<dyn SubmoduleStore>,
    ) -> Self {
        Self {
            repo_settings,
            store,
            op_store,
            op_heads_store,
            index_store,
            submodule_store,
        }
    }

    /// Creates a `RepoLoader` for the repo at `repo_path` by reading the
    /// various `.jj/repo/<backend>/type` files and loading the right
    /// backends from `store_factories`.
    pub fn init_from_file_system(
        user_settings: &UserSettings,
        repo_path: &Path,
        store_factories: &StoreFactories,
    ) -> Result<Self, StoreLoadError> {
        let store = Store::new(
            store_factories.load_backend(user_settings, &repo_path.join("store"))?,
            Signer::from_settings(user_settings)?,
        );
        let repo_settings = user_settings.with_repo(repo_path).unwrap();
        let root_op_data = RootOperationData {
            root_commit_id: store.root_commit_id().clone(),
        };
        let op_store = Arc::from(store_factories.load_op_store(
            user_settings,
            &repo_path.join("op_store"),
            root_op_data,
        )?);
        let op_heads_store = Arc::from(
            store_factories.load_op_heads_store(user_settings, &repo_path.join("op_heads"))?,
        );
        let index_store =
            Arc::from(store_factories.load_index_store(user_settings, &repo_path.join("index"))?);
        let submodule_store = Arc::from(
            store_factories
                .load_submodule_store(user_settings, &repo_path.join("submodule_store"))?,
        );
        Ok(Self {
            repo_settings,
            store,
            op_store,
            op_heads_store,
            index_store,
            submodule_store,
        })
    }

    pub fn settings(&self) -> &RepoSettings {
        &self.repo_settings
    }

    pub fn store(&self) -> &Arc<Store> {
        &self.store
    }

    pub fn index_store(&self) -> &Arc<dyn IndexStore> {
        &self.index_store
    }

    pub fn op_store(&self) -> &Arc<dyn OpStore> {
        &self.op_store
    }

    pub fn op_heads_store(&self) -> &Arc<dyn OpHeadsStore> {
        &self.op_heads_store
    }

    pub fn submodule_store(&self) -> &Arc<dyn SubmoduleStore> {
        &self.submodule_store
    }

    pub fn load_at_head(
        &self,
        user_settings: &UserSettings,
    ) -> Result<Arc<ReadonlyRepo>, RepoLoaderError> {
        let op = op_heads_store::resolve_op_heads(
            self.op_heads_store.as_ref(),
            &self.op_store,
            |op_heads| self._resolve_op_heads(op_heads, user_settings),
        )?;
        let view = op.view()?;
        self._finish_load(op, view)
    }

    #[instrument(skip(self))]
    pub fn load_at(&self, op: &Operation) -> Result<Arc<ReadonlyRepo>, RepoLoaderError> {
        let view = op.view()?;
        self._finish_load(op.clone(), view)
    }

    pub fn create_from(
        &self,
        operation: Operation,
        view: View,
        index: Box<dyn ReadonlyIndex>,
    ) -> Arc<ReadonlyRepo> {
        let repo = ReadonlyRepo {
            loader: self.clone(),
            operation,
            index,
            change_id_index: OnceCell::new(),
            view,
        };
        Arc::new(repo)
    }

    // If we add a higher-level abstraction of OpStore, root_operation() and
    // load_operation() will be moved there.

    /// Returns the root operation.
    pub fn root_operation(&self) -> Operation {
        self.load_operation(self.op_store.root_operation_id())
            .expect("failed to read root operation")
    }

    /// Loads the specified operation from the operation store.
    pub fn load_operation(&self, id: &OperationId) -> OpStoreResult<Operation> {
        let data = self.op_store.read_operation(id)?;
        Ok(Operation::new(self.op_store.clone(), id.clone(), data))
    }

    /// Merges the given `operations` into a single operation. Returns the root
    /// operation if the `operations` is empty.
    pub fn merge_operations(
        &self,
        settings: &UserSettings,
        operations: Vec<Operation>,
        tx_description: Option<&str>,
    ) -> Result<Operation, RepoLoaderError> {
        let num_operations = operations.len();
        let mut operations = operations.into_iter();
        let Some(base_op) = operations.next() else {
            return Ok(self.root_operation());
        };
        let final_op = if num_operations > 1 {
            let base_repo = self.load_at(&base_op)?;
            let mut tx = base_repo.start_transaction(settings);
            for other_op in operations {
                tx.merge_operation(other_op)?;
                tx.repo_mut().rebase_descendants(settings)?;
            }
            let tx_description = tx_description.map_or_else(
                || format!("merge {num_operations} operations"),
                |tx_description| tx_description.to_string(),
            );
            let merged_repo = tx.write(tx_description).leave_unpublished();
            merged_repo.operation().clone()
        } else {
            base_op
        };

        Ok(final_op)
    }

    fn _resolve_op_heads(
        &self,
        op_heads: Vec<Operation>,
        user_settings: &UserSettings,
    ) -> Result<Operation, RepoLoaderError> {
        assert!(!op_heads.is_empty());
        self.merge_operations(
            user_settings,
            op_heads,
            Some("reconcile divergent operations"),
        )
    }

    fn _finish_load(
        &self,
        operation: Operation,
        view: View,
    ) -> Result<Arc<ReadonlyRepo>, RepoLoaderError> {
        let index = self.index_store.get_index_at_op(&operation, &self.store)?;
        let repo = ReadonlyRepo {
            loader: self.clone(),
            operation,
            index,
            change_id_index: OnceCell::new(),
            view,
        };
        Ok(Arc::new(repo))
    }
}

#[derive(Clone, Debug, PartialEq, Eq)]
enum Rewrite {
    /// The old commit was rewritten as this new commit. Children should be
    /// rebased onto the new commit.
    Rewritten(CommitId),
    /// The old commit was rewritten as multiple other commits. Children should
    /// not be rebased.
    Divergent(Vec<CommitId>),
    /// The old commit was abandoned. Children should be rebased onto the given
    /// commits (typically the parents of the old commit).
    Abandoned(Vec<CommitId>),
}

impl Rewrite {
    fn new_parent_ids(&self) -> &[CommitId] {
        match self {
            Rewrite::Rewritten(new_parent_id) => std::slice::from_ref(new_parent_id),
            Rewrite::Divergent(new_parent_ids) => new_parent_ids.as_slice(),
            Rewrite::Abandoned(new_parent_ids) => new_parent_ids.as_slice(),
        }
    }
}

pub struct MutableRepo {
    base_repo: Arc<ReadonlyRepo>,
    index: Box<dyn MutableIndex>,
    view: DirtyCell<View>,
    // The commit identified by the key has been replaced by all the ones in the value.
    // * Bookmarks pointing to the old commit should be updated to the new commit, resulting in a
    //   conflict if there multiple new commits.
    // * Children of the old commit should be rebased onto the new commits. However, if the type is
    //   `Divergent`, they should be left in place.
    // * Working copies pointing to the old commit should be updated to the first of the new
    //   commits. However, if the type is `Abandoned`, a new working-copy commit should be created
    //   on top of all of the new commits instead.
    parent_mapping: HashMap<CommitId, Rewrite>,
}

impl MutableRepo {
    pub fn new(
        base_repo: Arc<ReadonlyRepo>,
        index: &dyn ReadonlyIndex,
        view: &View,
    ) -> MutableRepo {
        let mut_view = view.clone();
        let mut_index = index.start_modification();
        MutableRepo {
            base_repo,
            index: mut_index,
            view: DirtyCell::with_clean(mut_view),
            parent_mapping: Default::default(),
        }
    }

    pub fn base_repo(&self) -> &Arc<ReadonlyRepo> {
        &self.base_repo
    }

    fn view_mut(&mut self) -> &mut View {
        self.view.get_mut()
    }

    pub fn mutable_index(&self) -> &dyn MutableIndex {
        self.index.as_ref()
    }

    pub fn has_changes(&self) -> bool {
        !(self.parent_mapping.is_empty() && self.view() == &self.base_repo.view)
    }

    pub(crate) fn consume(self) -> (Box<dyn MutableIndex>, View) {
        self.view.ensure_clean(|v| self.enforce_view_invariants(v));
        (self.index, self.view.into_inner())
    }

    /// Returns a [`CommitBuilder`] to write new commit to the repo.
    pub fn new_commit(
        &mut self,
        settings: &UserSettings,
        parents: Vec<CommitId>,
        tree_id: MergedTreeId,
    ) -> CommitBuilder {
        DetachedCommitBuilder::for_new_commit(self, settings, parents, tree_id).attach(self)
    }

    /// Returns a [`CommitBuilder`] to rewrite an existing commit in the repo.
    pub fn rewrite_commit(
        &mut self,
        settings: &UserSettings,
        predecessor: &Commit,
    ) -> CommitBuilder {
        DetachedCommitBuilder::for_rewrite_from(self, settings, predecessor).attach(self)
        // CommitBuilder::write will record the rewrite in
        // `self.rewritten_commits`
    }

    /// Record a commit as having been rewritten to another commit in this
    /// transaction.
    ///
    /// This record is used by `rebase_descendants` to know which commits have
    /// children that need to be rebased, and where to rebase them to. See the
    /// docstring for `record_rewritten_commit` for details.
    pub fn set_rewritten_commit(&mut self, old_id: CommitId, new_id: CommitId) {
        assert_ne!(old_id, *self.store().root_commit_id());
        self.parent_mapping
            .insert(old_id, Rewrite::Rewritten(new_id));
    }

    /// Record a commit as being rewritten into multiple other commits in this
    /// transaction.
    ///
    /// A later call to `rebase_descendants()` will update bookmarks pointing to
    /// `old_id` be conflicted and pointing to all pf `new_ids`. Working copies
    /// pointing to `old_id` will be updated to point to the first commit in
    /// `new_ids``. Descendants of `old_id` will be left alone.
    pub fn set_divergent_rewrite(
        &mut self,
        old_id: CommitId,
        new_ids: impl IntoIterator<Item = CommitId>,
    ) {
        assert_ne!(old_id, *self.store().root_commit_id());
        self.parent_mapping.insert(
            old_id.clone(),
            Rewrite::Divergent(new_ids.into_iter().collect()),
        );
    }

    /// Record a commit as having been abandoned in this transaction.
    ///
    /// This record is used by `rebase_descendants` to know which commits have
    /// children that need to be rebased, and where to rebase the children (as
    /// well as bookmarks) to.
    ///
    /// The `rebase_descendants` logic will rebase the descendants of `old_id`
    /// to become the descendants of parent(s) of `old_id`. Any bookmarks at
    /// `old_id` would be moved to the parent(s) of `old_id` as well.
    // TODO: Propagate errors from commit lookup or take a Commit as argument.
    pub fn record_abandoned_commit(&mut self, old_id: CommitId) {
        assert_ne!(old_id, *self.store().root_commit_id());
        // Descendants should be rebased onto the commit's parents
        let old_commit = self.store().get_commit(&old_id).unwrap();
        self.record_abandoned_commit_with_parents(old_id, old_commit.parent_ids().to_vec());
    }

    /// Record a commit as having been abandoned in this transaction.
    ///
    /// A later `rebase_descendants()` will rebase children of `old_id` onto
    /// `new_parent_ids`. A working copy pointing to `old_id` will point to a
    /// new commit on top of `new_parent_ids`.
    pub fn record_abandoned_commit_with_parents(
        &mut self,
        old_id: CommitId,
        new_parent_ids: impl IntoIterator<Item = CommitId>,
    ) {
        assert_ne!(old_id, *self.store().root_commit_id());
        self.parent_mapping.insert(
            old_id,
            Rewrite::Abandoned(new_parent_ids.into_iter().collect()),
        );
    }

    pub fn has_rewrites(&self) -> bool {
        !self.parent_mapping.is_empty()
    }

    /// Calculates new parents for a commit that's currently based on the given
    /// parents. It does that by considering how previous commits have been
    /// rewritten and abandoned.
    ///
    /// If `parent_mapping` contains cycles, this function may either panic or
    /// drop parents that caused cycles.
    pub fn new_parents(&self, old_ids: &[CommitId]) -> Vec<CommitId> {
        self.rewritten_ids_with(old_ids, |rewrite| !matches!(rewrite, Rewrite::Divergent(_)))
    }

    fn rewritten_ids_with(
        &self,
        old_ids: &[CommitId],
        mut predicate: impl FnMut(&Rewrite) -> bool,
    ) -> Vec<CommitId> {
        assert!(!old_ids.is_empty());
        let mut new_ids = Vec::with_capacity(old_ids.len());
        let mut to_visit = old_ids.iter().rev().collect_vec();
        let mut visited = HashSet::new();
        while let Some(id) = to_visit.pop() {
            if !visited.insert(id) {
                continue;
            }
            match self.parent_mapping.get(id).filter(|&v| predicate(v)) {
                None => {
                    new_ids.push(id.clone());
                }
                Some(rewrite) => {
                    let replacements = rewrite.new_parent_ids();
                    assert!(
                        // Each commit must have a parent, so a parent can
                        // not just be mapped to nothing. This assertion
                        // could be removed if this function is used for
                        // mapping something other than a commit's parents.
                        !replacements.is_empty(),
                        "Found empty value for key {id:?} in the parent mapping",
                    );
                    to_visit.extend(replacements.iter().rev());
                }
            }
        }
        assert!(
            !new_ids.is_empty(),
            "new ids become empty because of cycle in the parent mapping"
        );
        debug_assert!(new_ids.iter().all_unique());
        new_ids
    }

    /// Fully resolves transitive replacements in `parent_mapping`.
    ///
    /// If `parent_mapping` contains cycles, this function will panic.
    fn resolve_rewrite_mapping_with(
        &self,
        mut predicate: impl FnMut(&Rewrite) -> bool,
    ) -> HashMap<CommitId, Vec<CommitId>> {
        let sorted_ids = dag_walk::topo_order_forward(
            self.parent_mapping.keys(),
            |&id| id,
            |&id| match self.parent_mapping.get(id).filter(|&v| predicate(v)) {
                None => &[],
                Some(rewrite) => rewrite.new_parent_ids(),
            },
        );
        let mut new_mapping: HashMap<CommitId, Vec<CommitId>> = HashMap::new();
        for old_id in sorted_ids {
            let Some(rewrite) = self.parent_mapping.get(old_id).filter(|&v| predicate(v)) else {
                continue;
            };
            let lookup = |id| new_mapping.get(id).map_or(slice::from_ref(id), |ids| ids);
            let new_ids = match rewrite.new_parent_ids() {
                [id] => lookup(id).to_vec(), // unique() not needed
                ids => ids.iter().flat_map(lookup).unique().cloned().collect(),
            };
            debug_assert_eq!(
                new_ids,
                self.rewritten_ids_with(slice::from_ref(old_id), &mut predicate)
            );
            new_mapping.insert(old_id.clone(), new_ids);
        }
        new_mapping
    }

    /// Updates bookmarks, working copies, and anonymous heads after rewriting
    /// and/or abandoning commits.
    pub fn update_rewritten_references(&mut self, settings: &UserSettings) -> BackendResult<()> {
        self.update_all_references(settings)?;
        self.update_heads();
        Ok(())
    }

    fn update_all_references(&mut self, settings: &UserSettings) -> BackendResult<()> {
        let rewrite_mapping = self.resolve_rewrite_mapping_with(|_| true);
        self.update_local_bookmarks(&rewrite_mapping);
        self.update_wc_commits(settings, &rewrite_mapping)?;
        Ok(())
    }

    fn update_local_bookmarks(&mut self, rewrite_mapping: &HashMap<CommitId, Vec<CommitId>>) {
        let changed_branches = self
            .view()
            .local_bookmarks()
            .flat_map(|(name, target)| {
                target.added_ids().filter_map(|id| {
                    let change = rewrite_mapping.get_key_value(id)?;
                    Some((name.to_owned(), change))
                })
            })
            .collect_vec();
        for (bookmark_name, (old_commit_id, new_commit_ids)) in changed_branches {
            let old_target = RefTarget::normal(old_commit_id.clone());
            let new_target = RefTarget::from_merge(
                MergeBuilder::from_iter(
                    itertools::intersperse(new_commit_ids, old_commit_id)
                        .map(|id| Some(id.clone())),
                )
                .build(),
            );

            self.merge_local_bookmark(&bookmark_name, &old_target, &new_target);
        }
    }

    fn update_wc_commits(
        &mut self,
        settings: &UserSettings,
        rewrite_mapping: &HashMap<CommitId, Vec<CommitId>>,
    ) -> BackendResult<()> {
        let changed_wc_commits = self
            .view()
            .wc_commit_ids()
            .iter()
            .filter_map(|(workspace_id, commit_id)| {
                let change = rewrite_mapping.get_key_value(commit_id)?;
                Some((workspace_id.to_owned(), change))
            })
            .collect_vec();
        let mut recreated_wc_commits: HashMap<&CommitId, Commit> = HashMap::new();
        for (workspace_id, (old_commit_id, new_commit_ids)) in changed_wc_commits {
            let abandoned_old_commit = matches!(
                self.parent_mapping.get(old_commit_id),
                Some(Rewrite::Abandoned(_))
            );
            let new_wc_commit = if !abandoned_old_commit {
                // We arbitrarily pick a new working-copy commit among the candidates.
                self.store().get_commit(&new_commit_ids[0])?
            } else if let Some(commit) = recreated_wc_commits.get(old_commit_id) {
                commit.clone()
            } else {
                let new_commits: Vec<_> = new_commit_ids
                    .iter()
                    .map(|id| self.store().get_commit(id))
                    .try_collect()?;
                let merged_parents_tree = merge_commit_trees(self, &new_commits)?;
                let commit = self
                    .new_commit(
                        settings,
                        new_commit_ids.clone(),
                        merged_parents_tree.id().clone(),
                    )
                    .write()?;
                recreated_wc_commits.insert(old_commit_id, commit.clone());
                commit
            };
            self.edit(workspace_id, &new_wc_commit).unwrap();
        }
        Ok(())
    }

    fn update_heads(&mut self) {
        let old_commits_expression =
            RevsetExpression::commits(self.parent_mapping.keys().cloned().collect());
        let heads_to_add_expression = old_commits_expression
            .parents()
            .minus(&old_commits_expression);
        let heads_to_add = heads_to_add_expression
            .evaluate_programmatic(self)
            .unwrap()
            .iter();

        let mut view = self.view().store_view().clone();
        for commit_id in self.parent_mapping.keys() {
            view.head_ids.remove(commit_id);
        }
        view.head_ids.extend(heads_to_add);
        self.set_view(view);
    }

    /// Find descendants of `root`, unless they've already been rewritten
    /// (according to `parent_mapping`), and then return them in
    /// an order they should be rebased in. The result is in reverse order
    /// so the next value can be removed from the end.
    fn find_descendants_to_rebase(&self, roots: Vec<CommitId>) -> BackendResult<Vec<Commit>> {
        let store = self.store();
        let to_visit_expression =
            RevsetExpression::commits(roots)
                .descendants()
                .minus(&RevsetExpression::commits(
                    self.parent_mapping.keys().cloned().collect(),
                ));
        let to_visit_revset = to_visit_expression
            .evaluate_programmatic(self)
            .map_err(|err| match err {
                RevsetEvaluationError::StoreError(err) => err,
                RevsetEvaluationError::Other(_) => panic!("Unexpected revset error: {err}"),
            })?;
        let to_visit: Vec<_> = to_visit_revset.iter().commits(store).try_collect()?;
        drop(to_visit_revset);
        let to_visit_set: HashSet<CommitId> =
            to_visit.iter().map(|commit| commit.id().clone()).collect();
        let mut visited = HashSet::new();
        // Calculate an order where we rebase parents first, but if the parents were
        // rewritten, make sure we rebase the rewritten parent first.
        dag_walk::topo_order_reverse_ok(
            to_visit.into_iter().map(Ok),
            |commit| commit.id().clone(),
            |commit| -> Vec<BackendResult<Commit>> {
                visited.insert(commit.id().clone());
                let mut dependents = vec![];
                for parent in commit.parents() {
                    let Ok(parent) = parent else {
                        dependents.push(parent);
                        continue;
                    };
                    if let Some(rewrite) = self.parent_mapping.get(parent.id()) {
                        for target in rewrite.new_parent_ids() {
                            if to_visit_set.contains(target) && !visited.contains(target) {
                                dependents.push(store.get_commit(target));
                            }
                        }
                    }
                    if to_visit_set.contains(parent.id()) {
                        dependents.push(Ok(parent));
                    }
                }
                dependents
            },
        )
    }

    /// Rewrite descendants of the given roots.
    ///
    /// The callback will be called for each commit with the new parents
    /// prepopulated. The callback may change the parents and write the new
    /// commit, or it may abandon the commit, or it may leave the old commit
    /// unchanged.
    ///
    /// The set of commits to visit is determined at the start. If the callback
    /// adds new descendants, then the callback will not be called for those.
    /// Similarly, if the callback rewrites unrelated commits, then the callback
    /// will not be called for descendants of those commits.
    pub fn transform_descendants(
        &mut self,
        settings: &UserSettings,
        roots: Vec<CommitId>,
        mut callback: impl FnMut(CommitRewriter) -> BackendResult<()>,
    ) -> BackendResult<()> {
        let mut to_visit = self.find_descendants_to_rebase(roots)?;
        while let Some(old_commit) = to_visit.pop() {
            let new_parent_ids = self.new_parents(old_commit.parent_ids());
            let rewriter = CommitRewriter::new(self, old_commit, new_parent_ids);
            callback(rewriter)?;
        }
        self.update_rewritten_references(settings)?;
        // Since we didn't necessarily visit all descendants of rewritten commits (e.g.
        // if they were rewritten in the callback), there can still be commits left to
        // rebase, so we don't clear `parent_mapping` here.
        // TODO: Should we make this stricter? We could check that there were no
        // rewrites before this function was called, and we can check that only
        // commits in the `to_visit` set were added by the callback. Then we
        // could clear `parent_mapping` here and not have to scan it again at
        // the end of the transaction when we call `rebase_descendants()`.

        Ok(())
    }

    /// After the rebaser returned by this function is dropped,
    /// self.parent_mapping needs to be cleared.
    fn rebase_descendants_return_rebaser<'settings, 'repo>(
        &'repo mut self,
        settings: &'settings UserSettings,
        options: RebaseOptions,
    ) -> BackendResult<Option<DescendantRebaser<'settings, 'repo>>> {
        if !self.has_rewrites() {
            // Optimization
            return Ok(None);
        }

        let to_visit =
            self.find_descendants_to_rebase(self.parent_mapping.keys().cloned().collect())?;
        let mut rebaser = DescendantRebaser::new(settings, self, to_visit);
        *rebaser.mut_options() = options;
        rebaser.rebase_all()?;
        Ok(Some(rebaser))
    }

    // TODO(ilyagr): Either document that this also moves bookmarks (rename the
    // function and the related functions?) or change things so that this only
    // rebases descendants.
    pub fn rebase_descendants_with_options(
        &mut self,
        settings: &UserSettings,
        options: RebaseOptions,
    ) -> BackendResult<usize> {
        let result = self
            .rebase_descendants_return_rebaser(settings, options)?
            .map_or(0, |rebaser| rebaser.into_map().len());
        self.parent_mapping.clear();
        Ok(result)
    }

    /// This is similar to `rebase_descendants_return_map`, but the return value
    /// needs more explaining.
    ///
    /// If the `options.empty` is the default, this function will only
    /// rebase commits, and the return value is what you'd expect it to be.
    ///
    /// Otherwise, this function may rebase some commits and abandon others. The
    /// behavior is such that only commits with a single parent will ever be
    /// abandoned. In the returned map, an abandoned commit will look as a
    /// key-value pair where the key is the abandoned commit and the value is
    /// **its parent**. One can tell this case apart since the change ids of the
    /// key and the value will not match. The parent will inherit the
    /// descendants and the bookmarks of the abandoned commit.
    // TODO: Rewrite this using `transform_descendants()`
    pub fn rebase_descendants_with_options_return_map(
        &mut self,
        settings: &UserSettings,
        options: RebaseOptions,
    ) -> BackendResult<HashMap<CommitId, CommitId>> {
        let result = Ok(self
            // We do not set RebaseOptions here, since this function does not currently return
            // enough information to describe the results of a rebase if some commits got
            // abandoned
            .rebase_descendants_return_rebaser(settings, options)?
            .map_or(HashMap::new(), |rebaser| rebaser.into_map()));
        self.parent_mapping.clear();
        result
    }

    /// Rebase descendants of the rewritten commits.
    ///
    /// The descendants of the commits registered in `self.parent_mappings` will
    /// be recursively rebased onto the new version of their parents.
    /// Returns the number of rebased descendants.
    pub fn rebase_descendants(&mut self, settings: &UserSettings) -> BackendResult<usize> {
        let roots = self.parent_mapping.keys().cloned().collect_vec();
        let mut num_rebased = 0;
        self.transform_descendants(settings, roots, |rewriter| {
            if rewriter.parents_changed() {
                let builder = rewriter.rebase(settings)?;
                builder.write()?;
                num_rebased += 1;
            }
            Ok(())
        })?;
        self.parent_mapping.clear();
        Ok(num_rebased)
    }

    /// Reparent descendants of the rewritten commits.
    ///
    /// The descendants of the commits registered in `self.parent_mappings` will
    /// be recursively reparented onto the new version of their parents.
    /// The content of those descendants will remain untouched.
    /// Returns the number of reparented descendants.
    pub fn reparent_descendants(&mut self, settings: &UserSettings) -> BackendResult<usize> {
        let roots = self.parent_mapping.keys().cloned().collect_vec();
        let mut num_reparented = 0;
        self.transform_descendants(settings, roots, |rewriter| {
            if rewriter.parents_changed() {
                let builder = rewriter.reparent(settings)?;
                builder.write()?;
                num_reparented += 1;
            }
            Ok(())
        })?;
        self.parent_mapping.clear();
        Ok(num_reparented)
    }

    pub fn rebase_descendants_return_map(
        &mut self,
        settings: &UserSettings,
    ) -> BackendResult<HashMap<CommitId, CommitId>> {
        self.rebase_descendants_with_options_return_map(settings, Default::default())
    }

    pub fn set_wc_commit(
        &mut self,
        workspace_id: WorkspaceId,
        commit_id: CommitId,
    ) -> Result<(), RewriteRootCommit> {
        if &commit_id == self.store().root_commit_id() {
            return Err(RewriteRootCommit);
        }
        self.view_mut().set_wc_commit(workspace_id, commit_id);
        Ok(())
    }

    pub fn remove_wc_commit(&mut self, workspace_id: &WorkspaceId) -> Result<(), EditCommitError> {
        self.maybe_abandon_wc_commit(workspace_id)?;
        self.view_mut().remove_wc_commit(workspace_id);
        Ok(())
    }

    pub fn rename_workspace(
        &mut self,
        old_workspace_id: &WorkspaceId,
        new_workspace_id: WorkspaceId,
    ) -> Result<(), RenameWorkspaceError> {
        self.view_mut()
            .rename_workspace(old_workspace_id, new_workspace_id)
    }

    pub fn check_out(
        &mut self,
        workspace_id: WorkspaceId,
        settings: &UserSettings,
        commit: &Commit,
    ) -> Result<Commit, CheckOutCommitError> {
        let wc_commit = self
            .new_commit(
                settings,
                vec![commit.id().clone()],
                commit.tree_id().clone(),
            )
            .write()?;
        self.edit(workspace_id, &wc_commit)?;
        Ok(wc_commit)
    }

    pub fn edit(
        &mut self,
        workspace_id: WorkspaceId,
        commit: &Commit,
    ) -> Result<(), EditCommitError> {
        self.maybe_abandon_wc_commit(&workspace_id)?;
        self.add_head(commit)?;
        self.set_wc_commit(workspace_id, commit.id().clone())
            .map_err(|RewriteRootCommit| EditCommitError::RewriteRootCommit)
    }

    fn maybe_abandon_wc_commit(
        &mut self,
        workspace_id: &WorkspaceId,
    ) -> Result<(), EditCommitError> {
        let is_commit_referenced = |view: &View, commit_id: &CommitId| -> bool {
            view.wc_commit_ids()
                .iter()
                .filter(|&(ws_id, _)| ws_id != workspace_id)
                .map(|(_, wc_id)| wc_id)
                .chain(
                    view.local_bookmarks()
                        .flat_map(|(_, target)| target.added_ids()),
                )
                .any(|id| id == commit_id)
        };

        let maybe_wc_commit_id = self
            .view
            .with_ref(|v| v.get_wc_commit_id(workspace_id).cloned());
        if let Some(wc_commit_id) = maybe_wc_commit_id {
            let wc_commit = self
                .store()
                .get_commit(&wc_commit_id)
                .map_err(EditCommitError::WorkingCopyCommitNotFound)?;
            if wc_commit.is_discardable(self)?
                && self
                    .view
                    .with_ref(|v| !is_commit_referenced(v, wc_commit.id()))
                && self.view().heads().contains(wc_commit.id())
            {
                // Abandon the working-copy commit we're leaving if it's
                // discardable, not pointed by local bookmark or other working
                // copies, and a head commit.
                self.record_abandoned_commit(wc_commit_id);
            }
        }

        Ok(())
    }

    fn enforce_view_invariants(&self, view: &mut View) {
        let view = view.store_view_mut();
        let root_commit_id = self.store().root_commit_id();
        if view.head_ids.is_empty() {
            view.head_ids.insert(root_commit_id.clone());
        } else if view.head_ids.len() > 1 {
            // An empty head_ids set is padded with the root_commit_id, but the
            // root id is unwanted during the heads resolution.
            view.head_ids.remove(root_commit_id);
            view.head_ids = self
                .index()
                .heads(&mut view.head_ids.iter())
                .into_iter()
                .collect();
        }
        assert!(!view.head_ids.is_empty());
    }

    /// Ensures that the given `head` and ancestor commits are reachable from
    /// the visible heads.
    pub fn add_head(&mut self, head: &Commit) -> BackendResult<()> {
        self.add_heads(slice::from_ref(head))
    }

    /// Ensures that the given `heads` and ancestor commits are reachable from
    /// the visible heads.
    ///
    /// The `heads` may contain redundant commits such as already visible ones
    /// and ancestors of the other heads. The `heads` and ancestor commits
    /// should exist in the store.
    pub fn add_heads(&mut self, heads: &[Commit]) -> BackendResult<()> {
        let current_heads = self.view.get_mut().heads();
        // Use incremental update for common case of adding a single commit on top a
        // current head. TODO: Also use incremental update when adding a single
        // commit on top a non-head.
        match heads {
            [] => {}
            [head]
                if head
                    .parent_ids()
                    .iter()
                    .all(|parent_id| current_heads.contains(parent_id)) =>
            {
                self.index.add_commit(head);
                self.view.get_mut().add_head(head.id());
                for parent_id in head.parent_ids() {
                    self.view.get_mut().remove_head(parent_id);
                }
            }
            _ => {
                let missing_commits = dag_walk::topo_order_reverse_ord_ok(
                    heads
                        .iter()
                        .cloned()
                        .map(CommitByCommitterTimestamp)
                        .map(Ok),
                    |CommitByCommitterTimestamp(commit)| commit.id().clone(),
                    |CommitByCommitterTimestamp(commit)| {
                        commit
                            .parent_ids()
                            .iter()
                            .filter(|id| !self.index().has_id(id))
                            .map(|id| self.store().get_commit(id))
                            .map_ok(CommitByCommitterTimestamp)
                            .collect_vec()
                    },
                )?;
                for CommitByCommitterTimestamp(missing_commit) in missing_commits.iter().rev() {
                    self.index.add_commit(missing_commit);
                }
                for head in heads {
                    self.view.get_mut().add_head(head.id());
                }
                self.view.mark_dirty();
            }
        }
        Ok(())
    }

    pub fn remove_head(&mut self, head: &CommitId) {
        self.view_mut().remove_head(head);
        self.view.mark_dirty();
    }

    pub fn get_local_bookmark(&self, name: &str) -> RefTarget {
        self.view.with_ref(|v| v.get_local_bookmark(name).clone())
    }

    pub fn set_local_bookmark_target(&mut self, name: &str, target: RefTarget) {
        self.view_mut().set_local_bookmark_target(name, target);
    }

    pub fn merge_local_bookmark(
        &mut self,
        name: &str,
        base_target: &RefTarget,
        other_target: &RefTarget,
    ) {
        let view = self.view.get_mut();
        let index = self.index.as_index();
        let self_target = view.get_local_bookmark(name);
        let new_target = merge_ref_targets(index, self_target, base_target, other_target);
        view.set_local_bookmark_target(name, new_target);
    }

    pub fn get_remote_bookmark(&self, name: &str, remote_name: &str) -> RemoteRef {
        self.view
            .with_ref(|v| v.get_remote_bookmark(name, remote_name).clone())
    }

    pub fn set_remote_bookmark(&mut self, name: &str, remote_name: &str, remote_ref: RemoteRef) {
        self.view_mut()
            .set_remote_bookmark(name, remote_name, remote_ref);
    }

    fn merge_remote_bookmark(
        &mut self,
        name: &str,
        remote_name: &str,
        base_ref: &RemoteRef,
        other_ref: &RemoteRef,
    ) {
        let view = self.view.get_mut();
        let index = self.index.as_index();
        let self_ref = view.get_remote_bookmark(name, remote_name);
        let new_ref = merge_remote_refs(index, self_ref, base_ref, other_ref);
        view.set_remote_bookmark(name, remote_name, new_ref);
    }

    /// Merges the specified remote bookmark in to local bookmark, and starts
    /// tracking it.
    pub fn track_remote_bookmark(&mut self, name: &str, remote_name: &str) {
        let mut remote_ref = self.get_remote_bookmark(name, remote_name);
        let base_target = remote_ref.tracking_target();
        self.merge_local_bookmark(name, base_target, &remote_ref.target);
        remote_ref.state = RemoteRefState::Tracking;
        self.set_remote_bookmark(name, remote_name, remote_ref);
    }

    /// Stops tracking the specified remote bookmark.
    pub fn untrack_remote_bookmark(&mut self, name: &str, remote_name: &str) {
        let mut remote_ref = self.get_remote_bookmark(name, remote_name);
        remote_ref.state = RemoteRefState::New;
        self.set_remote_bookmark(name, remote_name, remote_ref);
    }

    pub fn remove_remote(&mut self, remote_name: &str) {
        self.view_mut().remove_remote(remote_name);
    }

    pub fn rename_remote(&mut self, old: &str, new: &str) {
        self.view_mut().rename_remote(old, new);
    }

    pub fn get_tag(&self, name: &str) -> RefTarget {
        self.view.with_ref(|v| v.get_tag(name).clone())
    }

    pub fn set_tag_target(&mut self, name: &str, target: RefTarget) {
        self.view_mut().set_tag_target(name, target);
    }

    pub fn merge_tag(&mut self, name: &str, base_target: &RefTarget, other_target: &RefTarget) {
        let view = self.view.get_mut();
        let index = self.index.as_index();
        let self_target = view.get_tag(name);
        let new_target = merge_ref_targets(index, self_target, base_target, other_target);
        view.set_tag_target(name, new_target);
    }

    pub fn get_git_ref(&self, name: &str) -> RefTarget {
        self.view.with_ref(|v| v.get_git_ref(name).clone())
    }

    pub fn set_git_ref_target(&mut self, name: &str, target: RefTarget) {
        self.view_mut().set_git_ref_target(name, target);
    }

    fn merge_git_ref(&mut self, name: &str, base_target: &RefTarget, other_target: &RefTarget) {
        let view = self.view.get_mut();
        let index = self.index.as_index();
        let self_target = view.get_git_ref(name);
        let new_target = merge_ref_targets(index, self_target, base_target, other_target);
        view.set_git_ref_target(name, new_target);
    }

    pub fn git_head(&self) -> RefTarget {
        self.view.with_ref(|v| v.git_head().clone())
    }

    pub fn set_git_head_target(&mut self, target: RefTarget) {
        self.view_mut().set_git_head_target(target);
    }

    pub fn set_view(&mut self, data: op_store::View) {
        self.view_mut().set_view(data);
        self.view.mark_dirty();
    }

    pub fn merge(&mut self, base_repo: &ReadonlyRepo, other_repo: &ReadonlyRepo) {
        // First, merge the index, so we can take advantage of a valid index when
        // merging the view. Merging in base_repo's index isn't typically
        // necessary, but it can be if base_repo is ahead of either self or other_repo
        // (e.g. because we're undoing an operation that hasn't been published).
        self.index.merge_in(base_repo.readonly_index());
        self.index.merge_in(other_repo.readonly_index());

        self.view.ensure_clean(|v| self.enforce_view_invariants(v));
        self.merge_view(&base_repo.view, &other_repo.view);
        self.view.mark_dirty();
    }

    pub fn merge_index(&mut self, other_repo: &ReadonlyRepo) {
        self.index.merge_in(other_repo.readonly_index());
    }

    fn merge_view(&mut self, base: &View, other: &View) {
        // Merge working-copy commits. If there's a conflict, we keep the self side.
        for (workspace_id, base_wc_commit) in base.wc_commit_ids() {
            let self_wc_commit = self.view().get_wc_commit_id(workspace_id);
            let other_wc_commit = other.get_wc_commit_id(workspace_id);
            if other_wc_commit == Some(base_wc_commit) || other_wc_commit == self_wc_commit {
                // The other side didn't change or both sides changed in the
                // same way.
            } else if let Some(other_wc_commit) = other_wc_commit {
                if self_wc_commit == Some(base_wc_commit) {
                    self.view_mut()
                        .set_wc_commit(workspace_id.clone(), other_wc_commit.clone());
                }
            } else {
                // The other side removed the workspace. We want to remove it even if the self
                // side changed the working-copy commit.
                self.view_mut().remove_wc_commit(workspace_id);
            }
        }
        for (workspace_id, other_wc_commit) in other.wc_commit_ids() {
            if self.view().get_wc_commit_id(workspace_id).is_none()
                && base.get_wc_commit_id(workspace_id).is_none()
            {
                // The other side added the workspace.
                self.view_mut()
                    .set_wc_commit(workspace_id.clone(), other_wc_commit.clone());
            }
        }
        let base_heads = base.heads().iter().cloned().collect_vec();
        let own_heads = self.view().heads().iter().cloned().collect_vec();
        let other_heads = other.heads().iter().cloned().collect_vec();

        // HACK: Don't walk long ranges of commits to find rewrites when using other
        // custom implementations. The only custom index implementation we're currently
        // aware of is Google's. That repo has too high commit rate for it to be
        // feasible to walk all added and removed commits.
        // TODO: Fix this somehow. Maybe a method on `Index` to find rewritten commits
        // given `base_heads`, `own_heads` and `other_heads`?
        if self.index.as_any().is::<DefaultMutableIndex>() {
            self.record_rewrites(&base_heads, &own_heads);
            self.record_rewrites(&base_heads, &other_heads);
            // No need to remove heads removed by `other` because we already
            // marked them abandoned or rewritten.
        } else {
            for removed_head in base.heads().difference(other.heads()) {
                self.view_mut().remove_head(removed_head);
            }
        }
        for added_head in other.heads().difference(base.heads()) {
            self.view_mut().add_head(added_head);
        }

        let changed_local_bookmarks =
            diff_named_ref_targets(base.local_bookmarks(), other.local_bookmarks());
        for (name, (base_target, other_target)) in changed_local_bookmarks {
            self.merge_local_bookmark(name, base_target, other_target);
        }

        let changed_tags = diff_named_ref_targets(base.tags(), other.tags());
        for (name, (base_target, other_target)) in changed_tags {
            self.merge_tag(name, base_target, other_target);
        }

        let changed_git_refs = diff_named_ref_targets(base.git_refs(), other.git_refs());
        for (name, (base_target, other_target)) in changed_git_refs {
            self.merge_git_ref(name, base_target, other_target);
        }

        let changed_remote_bookmarks =
            diff_named_remote_refs(base.all_remote_bookmarks(), other.all_remote_bookmarks());
        for ((name, remote_name), (base_ref, other_ref)) in changed_remote_bookmarks {
            self.merge_remote_bookmark(name, remote_name, base_ref, other_ref);
        }

        let new_git_head_target = merge_ref_targets(
            self.index(),
            self.view().git_head(),
            base.git_head(),
            other.git_head(),
        );
        self.set_git_head_target(new_git_head_target);
    }

    /// Finds and records commits that were rewritten or abandoned between
    /// `old_heads` and `new_heads`.
    fn record_rewrites(&mut self, old_heads: &[CommitId], new_heads: &[CommitId]) {
        let mut removed_changes: HashMap<ChangeId, Vec<CommitId>> = HashMap::new();
        for (commit_id, change_id) in revset::walk_revs(self, old_heads, new_heads)
            .unwrap()
            .commit_change_ids()
        {
            removed_changes
                .entry(change_id)
                .or_default()
                .push(commit_id);
        }
        if removed_changes.is_empty() {
            return;
        }

        let mut rewritten_changes = HashSet::new();
        let mut rewritten_commits: HashMap<CommitId, Vec<CommitId>> = HashMap::new();
        for (commit_id, change_id) in revset::walk_revs(self, new_heads, old_heads)
            .unwrap()
            .commit_change_ids()
        {
            if let Some(old_commits) = removed_changes.get(&change_id) {
                for old_commit in old_commits {
                    rewritten_commits
                        .entry(old_commit.clone())
                        .or_default()
                        .push(commit_id.clone());
                }
            }
            rewritten_changes.insert(change_id);
        }
        for (old_commit, new_commits) in rewritten_commits {
            if new_commits.len() == 1 {
                self.set_rewritten_commit(
                    old_commit.clone(),
                    new_commits.into_iter().next().unwrap(),
                );
            } else {
                self.set_divergent_rewrite(old_commit.clone(), new_commits);
            }
        }

        for (change_id, removed_commit_ids) in &removed_changes {
            if !rewritten_changes.contains(change_id) {
                for removed_commit_id in removed_commit_ids {
                    self.record_abandoned_commit(removed_commit_id.clone());
                }
            }
        }
    }
}

impl Repo for MutableRepo {
    fn base_repo(&self) -> &ReadonlyRepo {
        &self.base_repo
    }

    fn store(&self) -> &Arc<Store> {
        self.base_repo.store()
    }

    fn op_store(&self) -> &Arc<dyn OpStore> {
        self.base_repo.op_store()
    }

    fn index(&self) -> &dyn Index {
        self.index.as_index()
    }

    fn view(&self) -> &View {
        self.view
            .get_or_ensure_clean(|v| self.enforce_view_invariants(v))
    }

    fn submodule_store(&self) -> &Arc<dyn SubmoduleStore> {
        self.base_repo.submodule_store()
    }

    fn resolve_change_id_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<Vec<CommitId>> {
        let change_id_index = self.index.change_id_index(&mut self.view().heads().iter());
        change_id_index.resolve_prefix(prefix)
    }

    fn shortest_unique_change_id_prefix_len(&self, target_id: &ChangeId) -> usize {
        let change_id_index = self.index.change_id_index(&mut self.view().heads().iter());
        change_id_index.shortest_unique_prefix_len(target_id)
    }
}

/// Error from attempts to check out the root commit for editing
#[derive(Debug, Error)]
#[error("Cannot rewrite the root commit")]
pub struct RewriteRootCommit;

/// Error from attempts to edit a commit
#[derive(Debug, Error)]
pub enum EditCommitError {
    #[error("Current working-copy commit not found")]
    WorkingCopyCommitNotFound(#[source] BackendError),
    #[error("Cannot rewrite the root commit")]
    RewriteRootCommit,
    #[error(transparent)]
    BackendError(#[from] BackendError),
}

/// Error from attempts to check out a commit
#[derive(Debug, Error)]
pub enum CheckOutCommitError {
    #[error("Failed to create new working-copy commit")]
    CreateCommit(#[from] BackendError),
    #[error("Failed to edit commit")]
    EditCommit(#[from] EditCommitError),
}

mod dirty_cell {
    use std::cell::OnceCell;
    use std::cell::RefCell;

    /// Cell that lazily updates the value after `mark_dirty()`.
    ///
    /// A clean value can be immutably borrowed within the `self` lifetime.
    #[derive(Clone, Debug)]
    pub struct DirtyCell<T> {
        // Either clean or dirty value is set. The value is boxed to reduce stack space
        // and memcopy overhead.
        clean: OnceCell<Box<T>>,
        dirty: RefCell<Option<Box<T>>>,
    }

    impl<T> DirtyCell<T> {
        pub fn with_clean(value: T) -> Self {
            DirtyCell {
                clean: OnceCell::from(Box::new(value)),
                dirty: RefCell::new(None),
            }
        }

        pub fn get_or_ensure_clean(&self, f: impl FnOnce(&mut T)) -> &T {
            self.clean.get_or_init(|| {
                // Panics if ensure_clean() is invoked from with_ref() callback for example.
                let mut value = self.dirty.borrow_mut().take().unwrap();
                f(&mut value);
                value
            })
        }

        pub fn ensure_clean(&self, f: impl FnOnce(&mut T)) {
            self.get_or_ensure_clean(f);
        }

        pub fn into_inner(self) -> T {
            *self
                .clean
                .into_inner()
                .or_else(|| self.dirty.into_inner())
                .unwrap()
        }

        pub fn with_ref<R>(&self, f: impl FnOnce(&T) -> R) -> R {
            if let Some(value) = self.clean.get() {
                f(value)
            } else {
                f(self.dirty.borrow().as_ref().unwrap())
            }
        }

        pub fn get_mut(&mut self) -> &mut T {
            self.clean
                .get_mut()
                .or_else(|| self.dirty.get_mut().as_mut())
                .unwrap()
        }

        pub fn mark_dirty(&mut self) {
            if let Some(value) = self.clean.take() {
                *self.dirty.get_mut() = Some(value);
            }
        }
    }
}