// 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.

//! Defines the commit backend trait and related types. This is the lowest-level
//! trait for reading and writing commits, trees, files, etc.

use std::any::Any;
use std::borrow::Borrow;
use std::fmt::Debug;
use std::fmt::Write as _;
use std::iter::zip;
use std::pin::Pin;
use std::slice;
use std::time::SystemTime;

use async_trait::async_trait;
use chrono::TimeZone as _;
use futures::AsyncRead;
use futures::stream::BoxStream;
use smallvec::SmallVec;
use thiserror::Error;

use crate::conflict_labels::ConflictLabels;
use crate::content_hash::ContentHash;
use crate::hex_util;
use crate::index::Index;
use crate::merge::Merge;
use crate::object_id::ObjectId as _;
use crate::object_id::id_type;
use crate::repo_path::RepoPath;
use crate::repo_path::RepoPathBuf;
use crate::repo_path::RepoPathComponent;
use crate::repo_path::RepoPathComponentBuf;
use crate::signing::SignResult;

id_type!(
    /// Identifier for a [`Commit`] based on its content. When a commit is
    /// rewritten, its `CommitId` changes.
    pub CommitId { hex() }
);
id_type!(
    /// Stable identifier for a [`Commit`]. Unlike the `CommitId`, the `ChangeId`
    /// follows the commit and is not updated when the commit is rewritten.
    pub ChangeId { reverse_hex() }
);
id_type!(
    /// Identifier for a tree object.
    pub TreeId { hex() }
);
id_type!(
    /// Identifier for a file content.
    pub FileId { hex() }
);
id_type!(
    /// Identifier for a symlink.
    pub SymlinkId { hex() }
);
id_type!(
    /// Identifier for a copy history.
    pub CopyId { hex() }
);

impl ChangeId {
    /// Parses the given "reverse" hex string into a `ChangeId`.
    pub fn try_from_reverse_hex(hex: impl AsRef<[u8]>) -> Option<Self> {
        hex_util::decode_reverse_hex(hex).map(Self)
    }

    /// Returns the hex string representation of this ID, which uses `z-k`
    /// "digits" instead of `0-9a-f`.
    pub fn reverse_hex(&self) -> String {
        hex_util::encode_reverse_hex(&self.0)
    }
}

impl CopyId {
    /// Returns a placeholder copy id to be used when we don't have a real copy
    /// id yet.
    // TODO: Delete this
    pub fn placeholder() -> Self {
        Self::new(vec![])
    }
}

/// Error that may occur when converting a `Timestamp` to a `Datetime``.
#[derive(Debug, Error)]
#[error("Out-of-range date")]
pub struct TimestampOutOfRange;

/// The number of milliseconds since the Unix epoch.
#[derive(ContentHash, Hash, Debug, PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
pub struct MillisSinceEpoch(pub i64);

/// A timestamp with millisecond precision and a time zone offset.
#[derive(ContentHash, Hash, Debug, PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
pub struct Timestamp {
    /// The number of milliseconds since the Unix epoch.
    pub timestamp: MillisSinceEpoch,
    /// Timezone offset in minutes
    pub tz_offset: i32,
}

impl Timestamp {
    /// Returns the current local time as a `Timestamp`.
    pub fn now() -> Self {
        Self::from_datetime(chrono::offset::Local::now())
    }

    /// Creates a `Timestamp` from the given `DateTime`.
    pub fn from_datetime<Tz: chrono::TimeZone<Offset = chrono::offset::FixedOffset>>(
        datetime: chrono::DateTime<Tz>,
    ) -> Self {
        Self {
            timestamp: MillisSinceEpoch(datetime.timestamp_millis()),
            tz_offset: datetime.offset().local_minus_utc() / 60,
        }
    }

    /// Converts this `Timestamp` to a `DateTime`.
    pub fn to_datetime(
        &self,
    ) -> Result<chrono::DateTime<chrono::FixedOffset>, TimestampOutOfRange> {
        let utc = match chrono::Utc.timestamp_opt(
            self.timestamp.0.div_euclid(1000),
            (self.timestamp.0.rem_euclid(1000)) as u32 * 1000000,
        ) {
            chrono::LocalResult::None => {
                return Err(TimestampOutOfRange);
            }
            chrono::LocalResult::Single(x) => x,
            chrono::LocalResult::Ambiguous(y, _z) => y,
        };

        Ok(utc.with_timezone(
            &chrono::FixedOffset::east_opt(self.tz_offset * 60)
                .unwrap_or_else(|| chrono::FixedOffset::east_opt(0).unwrap()),
        ))
    }
}

impl serde::Serialize for Timestamp {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        // TODO: test is_human_readable() to use raw format?
        let t = self.to_datetime().map_err(serde::ser::Error::custom)?;
        t.serialize(serializer)
    }
}

/// Represents a person/entity and a timestamp for when they authored or
/// committed a commit.
#[derive(ContentHash, Hash, Debug, PartialEq, Eq, Clone, serde::Serialize)]
pub struct Signature {
    /// The name of the person/entity.
    pub name: String,
    /// The email address of the person/entity.
    pub email: String,
    /// The timestamp for when the person/entity authored or committed the
    /// commit.
    pub timestamp: Timestamp,
}

/// Represents a cryptographically signed [`Commit`] signature.
#[derive(ContentHash, Debug, PartialEq, Eq, Clone)]
pub struct SecureSig {
    /// The raw data that was signed to produce this signature.
    pub data: Vec<u8>,
    /// The signature itself.
    pub sig: Vec<u8>,
}

/// Function called to sign a commit. The input is the raw data to sign, and the
/// output is the signature.
pub type SigningFn<'a> = dyn FnMut(&[u8]) -> SignResult<Vec<u8>> + Send + 'a;

/// Represents a commit object, which contains a reference to the contents a
/// that point in time, along with metadata about the commit.
#[derive(ContentHash, Debug, PartialEq, Eq, Clone, serde::Serialize)]
pub struct Commit {
    /// The parent commits of this commit. Commits typically have one parents,
    /// but they can have any number of parents. Only the root commit has no
    /// parents.
    pub parents: Vec<CommitId>,
    /// The predecessor commits of this commit, i.e. commits that were rewritten
    /// to create this commit.
    //
    // TODO: delete commit.predecessors when we can assume that most commits are
    // tracked by op.commit_predecessors. (in jj 0.42 or so?)
    #[serde(skip)] // deprecated
    pub predecessors: Vec<CommitId>,
    /// The tree at the root directory in this commit.
    #[serde(skip)] // TODO: should be exposed?
    pub root_tree: Merge<TreeId>,
    /// If resolved, must be empty string. Otherwise, must have same number of
    /// terms as `root_tree`.
    #[serde(skip)]
    pub conflict_labels: Merge<String>,
    /// The change ID of this commit. This is a stable identifier that follows
    /// the commit when it's rewritten.
    pub change_id: ChangeId,
    /// The description (commit message).
    pub description: String,
    /// The person/entity that authored this commit.
    pub author: Signature,
    /// The person/entity that committed this commit.
    pub committer: Signature,
    /// A cryptographic signature of this commit.
    #[serde(skip)] // raw data wouldn't be useful
    pub secure_sig: Option<SecureSig>,
}

/// An individual copy event, from file A -> B.
#[derive(Debug, PartialEq, Eq, Clone)]
pub struct CopyRecord {
    /// The destination of the copy, B.
    pub target: RepoPathBuf,
    /// The CommitId where the copy took place.
    pub target_commit: CommitId,
    /// The source path a target was copied from.
    ///
    /// It is not required that the source path is different than the target
    /// path. A custom backend may choose to represent 'rollbacks' as copies
    /// from a file unto itself, from a specific prior commit.
    pub source: RepoPathBuf,
    /// The file id of the source file.
    pub source_file: FileId,
    /// The source commit the target was copied from. Backends may use this
    /// field to implement 'integration' logic, where a source may be
    /// periodically merged into a target, similar to a branch, but the
    /// branching occurs at the file level rather than the repository level. It
    /// also follows naturally that any copy source targeted to a specific
    /// commit should avoid copy propagation on rebasing, which is desirable
    /// for 'fork' style copies.
    ///
    /// It is required that the commit id is an ancestor of the commit with
    /// which this copy source is associated.
    pub source_commit: CommitId,
}

/// Describes the copy history of a file. The copy object is unchanged when a
/// file is modified.
#[derive(ContentHash, Debug, PartialEq, Eq, Clone, PartialOrd, Ord)]
pub struct CopyHistory {
    /// The file's current path.
    pub current_path: RepoPathBuf,
    /// IDs of the files that became the current incarnation of this file.
    ///
    /// A newly created file has no parents. A regular copy or rename has one
    /// parent. A merge of multiple files has multiple parents.
    pub parents: Vec<CopyId>,
    /// An optional piece of data to give the Copy object a different ID. May be
    /// randomly generated. This allows a commit to say that a file was replaced
    /// by a new incarnation of it, indicating a logically distinct file
    /// taking the place of the previous file at the path.
    pub salt: Vec<u8>,
}

/// A `CopyHistory` along with its `CopyId`.
#[derive(Debug, Eq, PartialEq)]
pub struct RelatedCopy {
    /// The copy id.
    pub id: CopyId,
    /// The copy history.
    pub history: CopyHistory,
}

/// Error that may occur during backend initialization.
#[derive(Debug, Error)]
#[error(transparent)]
pub struct BackendInitError(pub Box<dyn std::error::Error + Send + Sync>);

/// Error that may occur during backend loading.
#[derive(Debug, Error)]
#[error(transparent)]
pub struct BackendLoadError(pub Box<dyn std::error::Error + Send + Sync>);

/// Commit-backend error that may occur after the backend is loaded.
#[derive(Debug, Error)]
pub enum BackendError {
    /// The caller attempted to read an object by specifying an ID with an
    /// invalid hash length for this backend.
    #[error(
        "Invalid hash length for object of type {object_type} (expected {expected} bytes, got \
         {actual} bytes): {hash}"
    )]
    InvalidHashLength {
        /// The expected length of the hash in bytes for this backend.
        expected: usize,
        /// The actual length of the hash in bytes that was provided.
        actual: usize,
        /// The type of the object that we attempted to read, e.g. "commit" or
        /// "tree".
        object_type: String,
        /// The hex hash that had an invalid length.
        hash: String,
    },
    /// The caller attempted to read an object that internally stored as invalid
    /// UTF-8, such as a symlink target with invalid UTF-8 stored in the Git
    /// backend.
    #[error("Invalid UTF-8 for object {hash} of type {object_type}")]
    InvalidUtf8 {
        /// The type of the object that we attempted to read, e.g. "commit" or
        /// "tree".
        object_type: String,
        /// The hex hash of the object that had invalid UTF-8.
        hash: String,
        /// The source error.
        source: std::str::Utf8Error,
    },
    /// The caller attempted to read an object that doesn't exist.
    #[error("Object {hash} of type {object_type} not found")]
    ObjectNotFound {
        /// The type of the object that we attempted to read, e.g. "commit" or
        /// "tree".
        object_type: String,
        /// The hex hash of the object that was not found.
        hash: String,
        /// The source error.
        source: Box<dyn std::error::Error + Send + Sync>,
    },
    /// Failed to read an object due to an I/O error or other unexpected error.
    #[error("Error when reading object {hash} of type {object_type}")]
    ReadObject {
        /// The type of the object that we attempted to read, e.g. "commit" or
        /// "tree".
        object_type: String,
        /// The hex hash of the object that we failed to read.
        hash: String,
        /// The source error.
        source: Box<dyn std::error::Error + Send + Sync>,
    },
    /// The caller attempted to read an object but doesn't have permission to
    /// read it.
    #[error("Access denied to read object {hash} of type {object_type}")]
    ReadAccessDenied {
        /// The type of the object that we attempted to read, e.g. "commit" or
        /// "tree".
        object_type: String,
        /// The hex hash of the object that the caller doesn't have permission
        /// to read.
        hash: String,
        /// The source error.
        source: Box<dyn std::error::Error + Send + Sync>,
    },
    /// Failed to read a file's content due to an I/O error or other unexpected
    /// error.
    #[error(
        "Error when reading file content for file {path} with id {id}",
        path = path.as_internal_file_string()
    )]
    ReadFile {
        /// The path of the file we failed to read.
        path: RepoPathBuf,
        /// The ID of the file we failed to read.
        id: FileId,
        /// The source error.
        source: Box<dyn std::error::Error + Send + Sync>,
    },
    /// Failed to write an object due to an I/O error or other unexpected error.
    #[error("Could not write object of type {object_type}")]
    WriteObject {
        /// The type of the object that we attempted to write, e.g. "commit" or
        /// "tree".
        object_type: &'static str,
        /// The source error.
        source: Box<dyn std::error::Error + Send + Sync>,
    },
    /// Some other error that doesn't fit into the above categories.
    #[error(transparent)]
    Other(Box<dyn std::error::Error + Send + Sync>),
    /// A valid operation was attempted, but it failed because it isn't
    /// supported by the particular backend.
    #[error("{0}")]
    Unsupported(String),
}

/// A specialized [`Result`] type for commit backend errors.
pub type BackendResult<T> = Result<T, BackendError>;

/// Identifies the content at a given path in a tree.
#[derive(ContentHash, Debug, PartialEq, Eq, Clone, Hash)]
pub enum TreeValue {
    // TODO: When there's a CopyId here, the copy object's path must match
    // the path identified by the tree.
    /// This path is a regular file, possibly executable.
    File {
        /// The file's content ID.
        id: FileId,
        /// Whether the file is executable.
        executable: bool,
        /// The copy id.
        copy_id: CopyId,
    },
    /// This path is a symbolic link.
    Symlink(SymlinkId),
    /// This path is a directory.
    Tree(TreeId),
    /// This path is a Git submodule.
    GitSubmodule(CommitId),
}

impl TreeValue {
    /// The copy id if this value represents a file.
    pub fn copy_id(&self) -> Option<&CopyId> {
        match self {
            Self::File { copy_id, .. } => Some(copy_id),
            _ => None,
        }
    }
}

/// Borrowed `MergedTreeValue`.
pub type MergedTreeVal<'a> = Merge<Option<&'a TreeValue>>;

/// The value at a given path in a commit.
///
/// It depends on the context whether it can be absent
/// (`Merge::is_absent()`). For example, when getting the value at a
/// specific path, it may be, but when iterating over entries in a
/// tree, it shouldn't be.
pub type MergedTreeValue = Merge<Option<TreeValue>>;

/// Extension methods for tree-value merges such as [`MergedTreeValue`] and
/// [`MergedTreeVal`].
pub trait MergedTreeValueExt {
    /// Whether this merge should be recursed into when doing directory walks.
    fn is_tree(&self) -> bool;

    /// Whether this merge is present and not a tree
    fn is_file_like(&self) -> bool;

    /// If this merge contains only files or absent entries, returns a merge of
    /// the `FileId`s. The executable bits and copy IDs will be ignored. Use
    /// `Merge::with_new_file_ids()` to produce a new merge with the original
    /// executable bits preserved.
    fn to_file_merge(&self) -> Option<Merge<Option<FileId>>>;

    /// If this merge contains only files or absent entries, returns a merge of
    /// the files' executable bits.
    fn to_executable_merge(&self) -> Option<Merge<Option<bool>>>;

    /// If this merge contains only files or absent entries, returns a merge of
    /// the files' copy IDs.
    fn to_copy_id_merge(&self) -> Option<Merge<Option<CopyId>>>;

    /// Creates a new merge with the file ids from the given merge. In other
    /// words, the executable bits and copy IDs from `self` will be preserved.
    ///
    /// The given `file_ids` should have the same shape as `self`. Only the
    /// `FileId` values may differ.
    fn with_new_file_ids(&self, file_ids: &Merge<Option<FileId>>) -> Merge<Option<TreeValue>>;

    /// Give a summary description of the conflict's "removes" and "adds"
    fn describe(&self, labels: &ConflictLabels) -> String;
}

impl<T> MergedTreeValueExt for Merge<Option<T>>
where
    T: Borrow<TreeValue>,
{
    fn is_tree(&self) -> bool {
        self.is_present()
            && self.iter().all(|value| {
                matches!(
                    borrow_tree_value(value.as_ref()),
                    Some(TreeValue::Tree(_)) | None
                )
            })
    }

    fn is_file_like(&self) -> bool {
        self.is_present() && !self.is_tree()
    }

    fn to_file_merge(&self) -> Option<Merge<Option<FileId>>> {
        let file_ids = self
            .try_map(|term| match borrow_tree_value(term.as_ref()) {
                None => Ok(None),
                Some(TreeValue::File {
                    id,
                    executable: _,
                    copy_id: _,
                }) => Ok(Some(id.clone())),
                _ => Err(()),
            })
            .ok()?;

        Some(file_ids)
    }

    fn to_executable_merge(&self) -> Option<Merge<Option<bool>>> {
        self.try_map(|term| match borrow_tree_value(term.as_ref()) {
            None => Ok(None),
            Some(TreeValue::File {
                id: _,
                executable,
                copy_id: _,
            }) => Ok(Some(*executable)),
            _ => Err(()),
        })
        .ok()
    }

    fn to_copy_id_merge(&self) -> Option<Merge<Option<CopyId>>> {
        self.try_map(|term| match borrow_tree_value(term.as_ref()) {
            None => Ok(None),
            Some(TreeValue::File {
                id: _,
                executable: _,
                copy_id,
            }) => Ok(Some(copy_id.clone())),
            _ => Err(()),
        })
        .ok()
    }

    fn with_new_file_ids(&self, file_ids: &Merge<Option<FileId>>) -> Merge<Option<TreeValue>> {
        assert_eq!(self.num_sides(), file_ids.num_sides());
        let values: SmallVec<_> = zip(self, file_ids.iter().cloned())
            .map(
                |(tree_value, file_id)| match (borrow_tree_value(tree_value.as_ref()), file_id) {
                    (
                        Some(TreeValue::File {
                            id: _,
                            executable,
                            copy_id,
                        }),
                        Some(id),
                    ) => Some(TreeValue::File {
                        id,
                        executable: *executable,
                        copy_id: copy_id.clone(),
                    }),
                    (None, None) => None,
                    // New files are populated to preserve the materialized conflict. The file won't
                    // be checked out to the disk. So the metadata is not important, and we will
                    // just use the default values.
                    (None, Some(id)) => Some(TreeValue::File {
                        id,
                        executable: false,
                        copy_id: CopyId::placeholder(),
                    }),
                    (old, new) => panic!("incompatible update: {old:?} to {new:?}"),
                },
            )
            .collect();
        Merge::from_vec(values)
    }

    fn describe(&self, labels: &ConflictLabels) -> String {
        let mut buf = String::new();
        writeln!(buf, "Conflict:").unwrap();
        for (term, label) in self
            .removes()
            .enumerate()
            .filter_map(|(i, term)| term.as_ref().map(|term| (term, labels.get_remove(i))))
        {
            write!(buf, "  Removing {}", describe_conflict_term(term.borrow())).unwrap();
            if let Some(label) = label {
                write!(buf, " ({label})").unwrap();
            }
            buf.push('\n');
        }
        for (term, label) in self
            .adds()
            .enumerate()
            .filter_map(|(i, term)| term.as_ref().map(|term| (term, labels.get_add(i))))
        {
            write!(buf, "  Adding {}", describe_conflict_term(term.borrow())).unwrap();
            if let Some(label) = label {
                write!(buf, " ({label})").unwrap();
            }
            buf.push('\n');
        }
        buf
    }
}

pub(crate) fn borrow_tree_value<T: Borrow<TreeValue> + ?Sized>(
    term: Option<&T>,
) -> Option<&TreeValue> {
    term.map(|value| value.borrow())
}

fn describe_conflict_term(value: &TreeValue) -> String {
    match value {
        TreeValue::File {
            id,
            executable: false,
            copy_id: _,
        } => {
            // TODO: include the copy here once we start using it
            format!("file with id {id}")
        }
        TreeValue::File {
            id,
            executable: true,
            copy_id: _,
        } => {
            // TODO: include the copy here once we start using it
            format!("executable file with id {id}")
        }
        TreeValue::Symlink(id) => {
            format!("symlink with id {id}")
        }
        TreeValue::Tree(id) => {
            format!("tree with id {id}")
        }
        TreeValue::GitSubmodule(id) => {
            format!("Git submodule with id {id}")
        }
    }
}

/// An entry in a `Tree` consisting of a basename and a `TreeValue`.
#[derive(Debug, PartialEq, Eq, Clone)]
pub struct TreeEntry<'a> {
    name: &'a RepoPathComponent,
    value: &'a TreeValue,
}

impl<'a> TreeEntry<'a> {
    /// Creates a new `TreeEntry` with the given name and value.
    pub fn new(name: &'a RepoPathComponent, value: &'a TreeValue) -> Self {
        Self { name, value }
    }

    /// Returns the basename at this path.
    pub fn name(&self) -> &'a RepoPathComponent {
        self.name
    }

    /// Returns the tree value at this path.
    pub fn value(&self) -> &'a TreeValue {
        self.value
    }
}

/// Iterator over the direct entries in a `Tree`.
pub struct TreeEntriesNonRecursiveIterator<'a> {
    iter: slice::Iter<'a, (RepoPathComponentBuf, TreeValue)>,
}

impl<'a> Iterator for TreeEntriesNonRecursiveIterator<'a> {
    type Item = TreeEntry<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter
            .next()
            .map(|(name, value)| TreeEntry { name, value })
    }
}

/// A tree object, which represents a directory. It contains the direct entries
/// of the directory. Subdirectories are represented by the `TreeValue::Tree`
/// variant. The `Tree` object associated with the root directory thus
/// represents the entire repository at a given point in time.
///
/// The entries must be sorted (by `RepoPathComponentBuf`'s ordering) and must
/// not contain duplicate names.
#[derive(ContentHash, Default, PartialEq, Eq, Debug, Clone)]
pub struct Tree {
    entries: Vec<(RepoPathComponentBuf, TreeValue)>,
}

impl Tree {
    /// Creates a new `Tree` from the given entries. The entries must be sorted
    /// by name and must not contain duplicate names.
    pub fn from_sorted_entries(entries: Vec<(RepoPathComponentBuf, TreeValue)>) -> Self {
        debug_assert!(entries.is_sorted_by(|(a, _), (b, _)| a < b));
        Self { entries }
    }

    /// Checks if this tree has no entries.
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }

    /// Returns an iterator over the names of the entries in this tree.
    pub fn names(&self) -> impl Iterator<Item = &RepoPathComponent> {
        self.entries.iter().map(|(name, _)| name.as_ref())
    }

    /// Returns an iterator over the entries in this tree.
    pub fn entries(&self) -> TreeEntriesNonRecursiveIterator<'_> {
        TreeEntriesNonRecursiveIterator {
            iter: self.entries.iter(),
        }
    }

    /// Returns the entry at the given basename, if it exists.
    pub fn entry(&self, name: &RepoPathComponent) -> Option<TreeEntry<'_>> {
        let index = self
            .entries
            .binary_search_by_key(&name, |(name, _)| name)
            .ok()?;
        let (name, value) = &self.entries[index];
        Some(TreeEntry { name, value })
    }

    /// Returns the value at the given basename, if it exists.
    pub fn value(&self, name: &RepoPathComponent) -> Option<&TreeValue> {
        self.entry(name).map(|entry| entry.value)
    }
}

/// Creates a root commit object.
pub fn make_root_commit(root_change_id: ChangeId, empty_tree_id: TreeId) -> Commit {
    let timestamp = Timestamp {
        timestamp: MillisSinceEpoch(0),
        tz_offset: 0,
    };
    let signature = Signature {
        name: String::new(),
        email: String::new(),
        timestamp,
    };
    Commit {
        parents: vec![],
        predecessors: vec![],
        root_tree: Merge::resolved(empty_tree_id),
        conflict_labels: Merge::resolved(String::new()),
        change_id: root_change_id,
        description: String::new(),
        author: signature.clone(),
        committer: signature,
        secure_sig: None,
    }
}

/// Defines the interface for commit backends.
#[async_trait]
pub trait Backend: Any + Send + Sync + Debug {
    /// A unique name that identifies this backend. Written to
    /// `.jj/repo/store/type` when the repo is created.
    fn name(&self) -> &str;

    /// The length of commit IDs in bytes.
    fn commit_id_length(&self) -> usize;

    /// The length of change IDs in bytes.
    fn change_id_length(&self) -> usize;

    /// The root commit's ID.
    ///
    /// The root commit is a possibly virtual commit that is an ancestor of all
    /// commits in the repository. It is the only commit that has no parents.
    fn root_commit_id(&self) -> &CommitId;

    /// The root commit's change ID.
    fn root_change_id(&self) -> &ChangeId;

    /// The empty tree's ID. All empty trees must have the same ID regardless of
    /// the path.
    fn empty_tree_id(&self) -> &TreeId;

    /// An estimate of how many concurrent requests this backend handles well. A
    /// local backend like the Git backend (at until it supports partial clones)
    /// may want to set this to 1. A cloud-backed backend may want to set it to
    /// 100 or so.
    /// It is guaranteed to return at least 1.
    ///
    /// It is not guaranteed that at most this number of concurrent requests are
    /// sent. It is the backend's responsibility to make sure it doesn't put
    /// too much load on its storage, e.g. by queueing requests if necessary.
    fn concurrency(&self) -> usize;

    /// Returns a reader for reading the contents of a file from the backend.
    async fn read_file(
        &self,
        path: &RepoPath,
        id: &FileId,
    ) -> BackendResult<Pin<Box<dyn AsyncRead + Send>>>;

    /// Writes the contents of the writer to the backend. Returns the ID of the
    /// written file.
    async fn write_file(
        &self,
        path: &RepoPath,
        contents: &mut (dyn AsyncRead + Send + Unpin),
    ) -> BackendResult<FileId>;

    /// Reads the target of a symlink from the backend. Returns the target path.
    /// It is not a `RepoPath` because it doesn't necessarily point within the
    /// repository.
    async fn read_symlink(&self, path: &RepoPath, id: &SymlinkId) -> BackendResult<String>;

    /// Writes a symlink with the given target to the backend and returns its
    /// ID.
    async fn write_symlink(&self, path: &RepoPath, target: &str) -> BackendResult<SymlinkId>;

    /// Read the specified `CopyHistory` object.
    ///
    /// Backends that don't support copy tracking may return
    /// `BackendError::Unsupported`.
    async fn read_copy(&self, id: &CopyId) -> BackendResult<CopyHistory>;

    /// Write the `CopyHistory` object and return its ID.
    ///
    /// Backends that don't support copy tracking may return
    /// `BackendError::Unsupported`.
    async fn write_copy(&self, copy: &CopyHistory) -> BackendResult<CopyId>;

    /// Find all copy histories that are related to the specified one. This is
    /// defined as those that are ancestors of the given specified one, plus
    /// all descendants of those ancestors. Children must be returned before
    /// parents, and the order should be deterministic.
    ///
    /// It is valid (but wasteful) to include other copy histories, such as
    /// siblings, or even completely unrelated copy histories.
    ///
    /// Backends that don't support copy tracking may return
    /// `BackendError::Unsupported`.
    async fn get_related_copies(&self, copy_id: &CopyId) -> BackendResult<Vec<RelatedCopy>>;

    /// Reads the tree at the given path with the given ID.
    async fn read_tree(&self, path: &RepoPath, id: &TreeId) -> BackendResult<Tree>;

    /// Writes the given tree at the given path to the backend and returns its
    /// ID.
    async fn write_tree(&self, path: &RepoPath, contents: &Tree) -> BackendResult<TreeId>;

    /// Reads the commit with the given ID.
    async fn read_commit(&self, id: &CommitId) -> BackendResult<Commit>;

    /// Writes a commit and returns its ID and the commit itself. The commit
    /// should contain the data that was actually written, which may differ
    /// from the data passed in. For example, the backend may change the
    /// committer name to an authenticated user's name, or the backend's
    /// timestamps may have less precision than the millisecond precision in
    /// `Commit`.
    ///
    /// The `sign_with` parameter could contain a function to cryptographically
    /// sign some binary representation of the commit.
    /// If the backend supports it, it could call it and store the result in
    /// an implementation specific fashion, and both `read_commit` and the
    /// return of `write_commit` should read it back as the `secure_sig`
    /// field.
    async fn write_commit(
        &self,
        contents: Commit,
        sign_with: Option<&mut SigningFn>,
    ) -> BackendResult<(CommitId, Commit)>;

    /// Get copy records for the dag range `root..head`. If `paths` is None
    /// include all paths, otherwise restrict to only `paths`.
    ///
    /// The exact order these are returned is unspecified, but it is guaranteed
    /// to be reverse-topological. That is, for any two copy records with
    /// different commit ids A and B, if A is an ancestor of B, A is streamed
    /// after B.
    ///
    /// Streaming by design to better support large backends which may have very
    /// large single-file histories. This also allows more iterative algorithms
    /// like blame/annotate to short-circuit after a point without wasting
    /// unnecessary resources.
    fn get_copy_records(
        &self,
        paths: Option<&[RepoPathBuf]>,
        root: &CommitId,
        head: &CommitId,
    ) -> BackendResult<BoxStream<'_, BackendResult<CopyRecord>>>;

    /// Perform garbage collection.
    ///
    /// All commits found in the `index` won't be removed. In addition to that,
    /// objects created after `keep_newer` will be preserved. This mitigates a
    /// risk of deleting new commits created concurrently by another process.
    fn gc(&self, index: &dyn Index, keep_newer: SystemTime) -> BackendResult<()>;
}

impl dyn Backend {
    /// Returns reference of the implementation type.
    pub fn downcast_ref<T: Backend>(&self) -> Option<&T> {
        (self as &dyn Any).downcast_ref()
    }
}

#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
mod tests {
    use super::*;

    #[test]
    fn test_display_object_id() {
        let commit_id = CommitId::from_hex("deadbeef0123");
        assert_eq!(format!("{commit_id}"), "deadbeef0123");
        assert_eq!(format!("{commit_id:.6}"), "deadbe");

        let change_id = ChangeId::from_hex("deadbeef0123");
        assert_eq!(format!("{change_id}"), "mlpmollkzyxw");
        assert_eq!(format!("{change_id:.6}"), "mlpmol");
    }
}