aft/

backup.rs

use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Arc, Mutex, RwLock};

use rusqlite::Connection;

use crate::db::backups::BackupRow;
use crate::error::AftError;
use sha2::{Digest, Sha256};

const MAX_UNDO_DEPTH: usize = 20;

/// Current on-disk backup metadata schema version.
///
/// Bump this when the `meta.json` shape changes. Readers check the field and
/// refuse or migrate older versions instead of misinterpreting them.
const SCHEMA_VERSION: u32 = 4;

/// A single backup entry for a file.
#[derive(Debug, Clone)]
pub struct BackupEntry {
    pub backup_id: String,
    /// UTF-8 view of the captured regular-file bytes, kept for API/tests that
    /// inspect text backups. Restore uses `content_bytes` so binary files round-trip.
    pub content: String,
    pub content_bytes: Vec<u8>,
    pub timestamp: u64,
    pub order: u128,
    pub description: String,
    pub op_id: Option<String>,
    pub kind: BackupEntryKind,
    pub mode: Option<u32>,
    pub link_target: Option<PathBuf>,
    pub created_dirs: Vec<PathBuf>,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum BackupEntryKind {
    Content,
    Symlink,
    Tombstone,
}

#[derive(Debug, Clone)]
struct BackupEntryHead {
    order: u128,
    op_id: Option<String>,
}

impl BackupEntryHead {
    fn from_entry(entry: &BackupEntry) -> Self {
        Self {
            order: entry.order,
            op_id: entry.op_id.clone(),
        }
    }

    fn from_row(row: &BackupRow) -> Self {
        Self {
            order: row.order,
            op_id: row.op_id.clone(),
        }
    }
}

impl BackupEntry {
    fn to_backup_row(
        &self,
        harness: &str,
        session_id: &str,
        project_key: &str,
        file_path: &str,
        path_hash: &str,
        backup_path: Option<&str>,
    ) -> BackupRow {
        BackupRow {
            backup_id: self.backup_id.clone(),
            harness: harness.to_string(),
            session_id: session_id.to_string(),
            project_key: project_key.to_string(),
            op_id: self.op_id.clone(),
            order: self.order,
            file_path: file_path.to_string(),
            path_hash: path_hash.to_string(),
            backup_path: backup_path.map(str::to_string),
            kind: match self.kind {
                BackupEntryKind::Content => "content".to_string(),
                BackupEntryKind::Symlink => "symlink".to_string(),
                BackupEntryKind::Tombstone => "tombstone".to_string(),
            },
            description: self.description.clone(),
            created_at: i64::try_from(self.timestamp).unwrap_or(i64::MAX),
            is_tombstone: matches!(self.kind, BackupEntryKind::Tombstone),
        }
    }
}

impl TryFrom<BackupRow> for BackupEntry {
    type Error = std::io::Error;

    fn try_from(row: BackupRow) -> Result<Self, Self::Error> {
        let kind = if row.is_tombstone || row.kind == "tombstone" {
            BackupEntryKind::Tombstone
        } else if row.kind == "symlink" {
            BackupEntryKind::Symlink
        } else {
            BackupEntryKind::Content
        };
        let backup_path = row.backup_path.clone();
        let disk_metadata = backup_path
            .as_deref()
            .and_then(|path| read_entry_disk_metadata(Path::new(path), &row.backup_id));
        let content_bytes = match kind {
            BackupEntryKind::Content | BackupEntryKind::Symlink => {
                let backup_path = backup_path.ok_or_else(|| {
                    std::io::Error::new(
                        std::io::ErrorKind::NotFound,
                        format!("backup DB row {} has no backup_path", row.backup_id),
                    )
                })?;
                std::fs::read(backup_path)?
            }
            BackupEntryKind::Tombstone => Vec::new(),
        };
        let link_target = if kind == BackupEntryKind::Symlink {
            disk_metadata
                .as_ref()
                .and_then(|metadata| metadata.link_target.clone())
                .or_else(|| {
                    Some(PathBuf::from(
                        String::from_utf8_lossy(&content_bytes).into_owned(),
                    ))
                })
        } else {
            None
        };
        let content = match kind {
            BackupEntryKind::Content => String::from_utf8_lossy(&content_bytes).into_owned(),
            BackupEntryKind::Symlink => link_target
                .as_ref()
                .map(|target| target.display().to_string())
                .unwrap_or_default(),
            BackupEntryKind::Tombstone => String::new(),
        };

        Ok(BackupEntry {
            backup_id: row.backup_id,
            content,
            content_bytes,
            timestamp: u64::try_from(row.created_at).unwrap_or_default(),
            order: row.order,
            description: row.description,
            op_id: row.op_id,
            kind,
            mode: disk_metadata.as_ref().and_then(|metadata| metadata.mode),
            link_target,
            created_dirs: disk_metadata
                .map(|metadata| metadata.created_dirs)
                .unwrap_or_default(),
        })
    }
}

#[derive(Debug, Clone)]
pub struct RestoredOperation {
    pub op_id: String,
    pub restored: Vec<RestoredFile>,
    pub warnings: Vec<String>,
}

#[derive(Debug, Clone)]
pub struct RestoredFile {
    pub path: PathBuf,
    pub backup_id: String,
}

/// Per-(session, file) undo store with optional disk persistence.
///
/// Introduced alongside project-shared bridges (issue #14): one bridge can now
/// serve many OpenCode sessions in the same project, so undo history must be
/// partitioned by session to keep session A's edits invisible to session B.
///
/// The 20-entry cap is enforced **per (session, file)** deliberately — a global
/// per-file LRU would re-couple sessions and let one busy session evict
/// another's history.
///
/// Disk layout (schema v2):
///   `<storage_dir>/backups/<session_hash>/session.json` — session metadata
///   `<storage_dir>/backups/<session_hash>/<path_hash>/meta.json` — file path + count + session
///   `<storage_dir>/backups/<session_hash>/<path_hash>/0.bak` … `19.bak` — snapshots
///
/// Legacy layouts from before sessionization (flat `<path_hash>/` directly under
/// `backups/`) are migrated on first `set_storage_dir` call into the default
/// session namespace.
#[derive(Debug)]
pub struct BackupStore {
    /// session -> path -> entry stack
    entries: HashMap<String, HashMap<PathBuf, Vec<BackupEntry>>>,
    /// session -> path -> disk metadata
    disk_index: HashMap<String, HashMap<PathBuf, DiskMeta>>,
    /// session -> metadata
    session_meta: HashMap<String, SessionMeta>,
    counter: AtomicU64,
    storage_dir: Option<PathBuf>,
    storage_harness: Option<String>,
    db_pool: RwLock<Option<Arc<Mutex<Connection>>>>,
    db_harness: RwLock<Option<String>>,
    db_project_key: RwLock<Option<String>>,
}

#[derive(Debug, Clone)]
struct DiskMeta {
    dir: PathBuf,
    count: usize,
}

#[derive(Debug, Clone, Default)]
struct SessionMeta {
    /// Unix timestamp of last read/write activity in this session namespace.
    /// Maintained in-memory now, reserved for future inactivity-TTL cleanup.
    last_accessed: u64,
}

impl BackupStore {
    pub fn new() -> Self {
        BackupStore {
            entries: HashMap::new(),
            disk_index: HashMap::new(),
            session_meta: HashMap::new(),
            counter: AtomicU64::new(0),
            storage_dir: None,
            storage_harness: None,
            db_pool: RwLock::new(None),
            db_harness: RwLock::new(None),
            db_project_key: RwLock::new(None),
        }
    }

    pub fn set_db_pool(&self, conn: Arc<Mutex<Connection>>) {
        if let Ok(mut slot) = self.db_pool.write() {
            *slot = Some(conn);
        }
    }

    pub fn clear_db_pool(&self) {
        if let Ok(mut slot) = self.db_pool.write() {
            *slot = None;
        }
    }

    pub fn set_db_harness(&self, harness: crate::harness::Harness) {
        if let Ok(mut slot) = self.db_harness.write() {
            *slot = Some(harness.as_str().to_string());
        }
    }

    pub fn set_db_project_key(&self, project_key: String) {
        if let Ok(mut slot) = self.db_project_key.write() {
            *slot = Some(project_key);
        }
    }

    /// Set storage directory for disk persistence (called during configure).
    ///
    /// Loads the disk index for all session namespaces, removes stale session
    /// directories, and migrates any legacy pre-session (flat) layout into the
    /// default namespace.
    pub fn set_storage_dir(&mut self, dir: PathBuf, ttl_hours: u32) {
        self.set_storage_dir_inner(dir, None, ttl_hours);
    }

    pub fn set_storage_dir_for_harness(
        &mut self,
        dir: PathBuf,
        harness: crate::harness::Harness,
        ttl_hours: u32,
    ) {
        self.set_storage_dir_inner(dir, Some(harness.as_str().to_string()), ttl_hours);
    }

    fn set_storage_dir_inner(&mut self, dir: PathBuf, harness: Option<String>, ttl_hours: u32) {
        self.storage_dir = Some(dir);
        self.storage_harness = harness;
        self.entries.clear();
        self.disk_index.clear();
        self.session_meta.clear();
        self.repair_root_backups_if_needed();
        self.gc_stale_sessions(ttl_hours);
        self.migrate_legacy_layout_if_needed();
        self.load_disk_index();
    }

    /// Snapshot the current contents of `path` under the given session namespace.
    pub fn snapshot(
        &mut self,
        session: &str,
        path: &Path,
        description: &str,
    ) -> Result<String, AftError> {
        self.snapshot_with_op(session, path, description, None)
    }

    /// Snapshot the current contents of `path` under the given session namespace,
    /// optionally tagging it with an operation id shared by all files touched by
    /// one mutating tool call.
    pub fn snapshot_with_op(
        &mut self,
        session: &str,
        path: &Path,
        description: &str,
        op_id: Option<&str>,
    ) -> Result<String, AftError> {
        let key = canonicalize_key(path);
        // Hydrate any prior on-disk history before appending, so a snapshot
        // taken on a fresh store (post-restart) extends the existing stack and
        // advances the id counter instead of overwriting history with a single
        // entry and reusing backup-0.
        self.ensure_stack_hydrated(session, &key);
        let (id, order) = self.next_id_and_order();
        let entry = backup_entry_from_path(path, id.clone(), order, description, op_id)?;

        let session_entries = self.entries.entry(session.to_string()).or_default();
        let stack = session_entries.entry(key.clone()).or_default();
        if stack.len() >= MAX_UNDO_DEPTH {
            stack.remove(0);
        }
        stack.push(entry);

        // Persist to disk
        let stack_clone = stack.clone();
        self.write_snapshot_to_disk(session, &key, &stack_clone);
        self.touch_session(session);

        Ok(id)
    }

    /// Record that `path` was created by the operation and should be removed
    /// if that operation is undone. No file content is captured.
    pub fn snapshot_op_tombstone(
        &mut self,
        session: &str,
        op_id: &str,
        path: &Path,
        description: &str,
    ) -> Result<String, AftError> {
        let key = canonicalize_key(path);
        self.ensure_stack_hydrated(session, &key);
        let created_dirs = path.parent().map(missing_parent_dirs).unwrap_or_default();
        let (id, order) = self.next_id_and_order();
        let entry = BackupEntry {
            backup_id: id.clone(),
            content: String::new(),
            content_bytes: Vec::new(),
            timestamp: current_timestamp(),
            order,
            description: description.to_string(),
            op_id: Some(op_id.to_string()),
            kind: BackupEntryKind::Tombstone,
            mode: None,
            link_target: None,
            created_dirs,
        };

        let session_entries = self.entries.entry(session.to_string()).or_default();
        let stack = session_entries.entry(key.clone()).or_default();
        if stack.len() >= MAX_UNDO_DEPTH {
            stack.remove(0);
        }
        stack.push(entry);

        let stack_clone = stack.clone();
        self.write_snapshot_to_disk(session, &key, &stack_clone);
        self.touch_session(session);

        Ok(id)
    }

    /// Restore every top-of-stack backup entry belonging to the most recent
    /// operation in this session.
    pub fn restore_last_operation(&mut self, session: &str) -> Result<RestoredOperation, AftError> {
        match self.load_latest_operation_from_db(session) {
            Some(Ok(true)) => {}
            Some(Ok(false)) => {
                crate::slog_info!(
                    "backup latest operation DB miss for session {}; falling back to disk",
                    session
                );
                self.load_all_disk_backups(session);
            }
            Some(Err(error)) => {
                crate::slog_warn!(
                    "backup latest operation DB lookup failed for session {}; falling back to disk: {}",
                    session,
                    error
                );
                self.load_all_disk_backups(session);
            }
            None => {
                crate::slog_info!(
                    "backup latest operation DB unavailable for session {}; falling back to disk",
                    session
                );
                self.load_all_disk_backups(session);
            }
        }

        let mut latest: Option<(u128, String)> = None;
        if let Some(files) = self.entries.get(session) {
            for stack in files.values() {
                if let Some(entry) = stack.last() {
                    if let Some(op_id) = &entry.op_id {
                        let order = entry.order;
                        if latest
                            .as_ref()
                            .map_or(true, |(latest_order, _)| order > *latest_order)
                        {
                            latest = Some((order, op_id.clone()));
                        }
                    }
                }
            }
        }

        let Some((_, op_id)) = latest else {
            return Err(AftError::NoUndoHistory {
                path: "operation".to_string(),
            });
        };

        let mut keys_to_restore: Vec<PathBuf> = self
            .entries
            .get(session)
            .map(|files| {
                files
                    .iter()
                    .filter_map(|(key, stack)| {
                        stack.last().and_then(|entry| {
                            (entry.op_id.as_deref() == Some(op_id.as_str())).then(|| key.clone())
                        })
                    })
                    .collect()
            })
            .unwrap_or_default();
        keys_to_restore.sort();

        if keys_to_restore.is_empty() {
            return Err(AftError::NoUndoHistory {
                path: "operation".to_string(),
            });
        }

        let mut content_targets = Vec::new();
        let mut tombstone_targets = Vec::new();
        for key in &keys_to_restore {
            let entry = self
                .entries
                .get(session)
                .and_then(|files| files.get(key))
                .and_then(|stack| stack.last())
                .cloned()
                .ok_or_else(|| AftError::NoUndoHistory {
                    path: key.display().to_string(),
                })?;
            match entry.kind {
                BackupEntryKind::Content | BackupEntryKind::Symlink => {
                    let existing_state = capture_path_state(key)?;
                    let warning = self.check_external_modification(session, key, key);
                    content_targets.push((key.clone(), entry, warning, existing_state));
                }
                BackupEntryKind::Tombstone => {
                    let existing_state = capture_path_state(key)?;
                    tombstone_targets.push((key.clone(), entry, existing_state));
                }
            }
        }

        let mut created_dirs = Vec::new();
        for (key, _, _, _) in &content_targets {
            if let Some(parent) = key.parent() {
                if !parent.as_os_str().is_empty() {
                    let missing_dirs = missing_parent_dirs(parent);
                    if let Err(e) = std::fs::create_dir_all(parent) {
                        let mut dirs_to_remove = created_dirs;
                        dirs_to_remove.extend(missing_dirs);
                        let rollback_ok = rollback_created_dirs(&dirs_to_remove);
                        return Err(AftError::IoError {
                            path: parent.display().to_string(),
                            message: format!(
                                "{}; restore_last_operation aborted; partial_rollback: {}; rollback_succeeded: {}",
                                e,
                                !rollback_ok,
                                rollback_ok
                            ),
                        });
                    }
                    created_dirs.extend(missing_dirs);
                }
            }
        }

        let mut written = Vec::new();
        for (key, entry, _, existing_state) in &content_targets {
            if let Err(e) = restore_entry_to_path(key, entry) {
                let files_rollback_ok =
                    rollback_transactional_restore(&written, Some((key, existing_state)));
                let dirs_rollback_ok = rollback_created_dirs(&created_dirs);
                let rollback_ok = files_rollback_ok && dirs_rollback_ok;
                return Err(AftError::IoError {
                    path: key.display().to_string(),
                    message: format!(
                        "{}; restore_last_operation aborted; partial_rollback: {}; rollback_succeeded: {}",
                        e,
                        !rollback_ok,
                        rollback_ok
                    ),
                });
            }
            written.push((key.clone(), existing_state.clone()));
        }

        let mut deleted_tombstones = Vec::new();
        for (key, _, existing_state) in &tombstone_targets {
            match remove_tombstone_path(key) {
                Ok(()) => deleted_tombstones.push((key.clone(), existing_state.clone())),
                Err(e) => {
                    let files_rollback_ok = rollback_transactional_restore(&written, None);
                    let tombstone_rollback_ok = rollback_deleted_tombstones(&deleted_tombstones);
                    let dirs_rollback_ok = rollback_created_dirs(&created_dirs);
                    let rollback_ok =
                        files_rollback_ok && tombstone_rollback_ok && dirs_rollback_ok;
                    return Err(AftError::IoError {
                        path: key.display().to_string(),
                        message: format!(
                            "{}; restore_last_operation aborted; partial_rollback: {}; rollback_succeeded: {}",
                            e,
                            !rollback_ok,
                            rollback_ok
                        ),
                    });
                }
            }
        }
        let tombstone_created_dirs = tombstone_targets
            .iter()
            .flat_map(|(_, entry, _)| entry.created_dirs.iter().cloned())
            .collect::<Vec<_>>();
        remove_created_dirs_best_effort(&tombstone_created_dirs);

        let mut restored = Vec::new();
        let mut warnings = Vec::new();
        for (key, entry, warning, _) in content_targets {
            self.commit_restored_backup(session, &key);
            if let Some(warning) = warning {
                warnings.push(format!("{}: {}", key.display(), warning));
            }
            restored.push(RestoredFile {
                path: key,
                backup_id: entry.backup_id,
            });
        }
        for (key, _, _) in tombstone_targets {
            self.commit_restored_backup(session, &key);
        }
        self.touch_session(session);

        Ok(RestoredOperation {
            op_id,
            restored,
            warnings,
        })
    }

    /// Pop the most recent backup for `(session, path)` and restore the file.
    /// Returns `(entry, optional_warning)`.
    pub fn restore_latest(
        &mut self,
        session: &str,
        path: &Path,
    ) -> Result<(BackupEntry, Option<String>), AftError> {
        let key = canonicalize_key(path);

        match self.load_from_db_if_present(session, &key) {
            Some(Ok(true)) => {
                let warning = self.check_external_modification(session, &key, path);
                let result = self
                    .do_restore(session, &key, path)
                    .map(|(entry, _)| (entry, warning));
                if result.is_ok() {
                    self.touch_session(session);
                }
                return result;
            }
            Some(Ok(false)) => {
                crate::slog_info!(
                    "backup DB miss for session {} path {}; falling back to disk",
                    session,
                    key.display()
                );
            }
            Some(Err(error)) => {
                crate::slog_warn!(
                    "backup DB lookup failed for session {} path {}; falling back to disk: {}",
                    session,
                    key.display(),
                    error
                );
            }
            None => {
                crate::slog_info!(
                    "backup DB unavailable for session {} path {}; falling back to disk",
                    session,
                    key.display()
                );
            }
        }

        // Try memory first
        let in_memory = self
            .entries
            .get(session)
            .and_then(|s| s.get(&key))
            .map_or(false, |s| !s.is_empty());
        if in_memory {
            let warning = self.check_external_modification(session, &key, path);
            let result = self
                .do_restore(session, &key, path)
                .map(|(entry, _)| (entry, warning));
            if result.is_ok() {
                self.touch_session(session);
            }
            return result;
        }

        // Try disk fallback
        if self.load_from_disk_if_needed(session, &key) {
            // Check for external modification
            let warning = self.check_external_modification(session, &key, path);
            let (entry, _) = self.do_restore(session, &key, path)?;
            self.touch_session(session);
            return Ok((entry, warning));
        }

        Err(AftError::NoUndoHistory {
            path: path.display().to_string(),
        })
    }

    /// Return the backup history for `(session, path)` (oldest first).
    pub fn history(&self, session: &str, path: &Path) -> Vec<BackupEntry> {
        let key = canonicalize_key(path);
        match self.read_stack_from_db(session, &key) {
            Some(Ok(stack)) if !stack.is_empty() => return stack,
            Some(Ok(_)) => {
                crate::slog_info!(
                    "backup history DB miss for session {} path {}; falling back to disk",
                    session,
                    key.display()
                );
            }
            Some(Err(error)) => {
                crate::slog_warn!(
                    "backup history DB lookup failed for session {} path {}; falling back to disk: {}",
                    session,
                    key.display(),
                    error
                );
            }
            None => {
                crate::slog_info!(
                    "backup history DB unavailable for session {} path {}; falling back to disk",
                    session,
                    key.display()
                );
            }
        }

        self.entries
            .get(session)
            .and_then(|s| s.get(&key))
            .cloned()
            .or_else(|| self.read_stack_from_disk(session, &key))
            .unwrap_or_default()
    }

    /// Return the number of on-disk backup entries for `(session, file)`.
    pub fn disk_history_count(&self, session: &str, path: &Path) -> usize {
        let key = canonicalize_key(path);
        self.disk_index
            .get(session)
            .and_then(|s| s.get(&key))
            .map(|m| m.count)
            .unwrap_or(0)
    }

    /// Return all files that have at least one backup entry in this session
    /// (memory + disk). Other sessions' files are not visible.
    pub fn tracked_files(&self, session: &str) -> Vec<PathBuf> {
        let mut files: std::collections::HashSet<PathBuf> = self
            .entries
            .get(session)
            .map(|s| s.keys().cloned().collect())
            .unwrap_or_default();
        if let Some(disk) = self.disk_index.get(session) {
            for key in disk.keys() {
                files.insert(key.clone());
            }
        }
        files.into_iter().collect()
    }

    /// Preview the file path that `restore_latest` would write for `(session, path)`.
    ///
    /// This is intentionally read-only: it inspects DB/disk/in-memory backup metadata
    /// without popping the undo stack or writing restored file contents.
    pub fn preview_latest_path(&self, session: &str, path: &Path) -> Result<PathBuf, AftError> {
        let key = canonicalize_key(path);
        if self.latest_head_for_key(session, &key).is_some() {
            Ok(key)
        } else {
            Err(AftError::NoUndoHistory {
                path: path.display().to_string(),
            })
        }
    }

    /// Preview the paths that `restore_last_operation` would touch for `session`.
    ///
    /// This mirrors the operation selection logic used by restore, but only reads
    /// backup metadata. It includes tombstone targets because undoing a create
    /// operation deletes those paths and therefore still requires write permission.
    pub fn preview_last_operation_paths(&self, session: &str) -> Result<Vec<PathBuf>, AftError> {
        let mut heads_by_path: HashMap<PathBuf, BackupEntryHead> = self
            .entries
            .get(session)
            .map(|files| {
                files
                    .iter()
                    .filter_map(|(key, stack)| {
                        stack
                            .last()
                            .map(|entry| (key.clone(), BackupEntryHead::from_entry(entry)))
                    })
                    .collect()
            })
            .unwrap_or_default();

        match self.read_latest_operation_heads_from_db(session) {
            Some(Ok(db_heads)) if !db_heads.is_empty() => {
                for (key, head) in db_heads {
                    heads_by_path.insert(key, head);
                }
            }
            Some(Ok(_)) => {
                crate::slog_info!(
                    "backup latest operation preview DB miss for session {}; falling back to disk",
                    session
                );
                self.merge_disk_stack_heads(session, &mut heads_by_path);
            }
            Some(Err(error)) => {
                crate::slog_warn!(
                    "backup latest operation preview DB lookup failed for session {}; falling back to disk: {}",
                    session,
                    error
                );
                self.merge_disk_stack_heads(session, &mut heads_by_path);
            }
            None => {
                crate::slog_info!(
                    "backup latest operation preview DB unavailable for session {}; falling back to disk",
                    session
                );
                self.merge_disk_stack_heads(session, &mut heads_by_path);
            }
        }

        let mut latest: Option<(u128, String)> = None;
        for head in heads_by_path.values() {
            if let Some(op_id) = &head.op_id {
                if latest
                    .as_ref()
                    .map_or(true, |(latest_order, _)| head.order > *latest_order)
                {
                    latest = Some((head.order, op_id.clone()));
                }
            }
        }

        let Some((_, op_id)) = latest else {
            return Err(AftError::NoUndoHistory {
                path: "operation".to_string(),
            });
        };

        let mut paths: Vec<PathBuf> = heads_by_path
            .into_iter()
            .filter_map(|(key, head)| {
                (head.op_id.as_deref() == Some(op_id.as_str())).then_some(key)
            })
            .collect();
        paths.sort();

        if paths.is_empty() {
            Err(AftError::NoUndoHistory {
                path: "operation".to_string(),
            })
        } else {
            Ok(paths)
        }
    }

    /// Return all session namespaces that currently have any backup state
    /// (memory or disk). Exposed for `/aft-status` aggregate reporting.
    pub fn sessions_with_backups(&self) -> Vec<String> {
        let mut sessions: std::collections::HashSet<String> =
            self.entries.keys().cloned().collect();
        for s in self.disk_index.keys() {
            sessions.insert(s.clone());
        }
        sessions.into_iter().collect()
    }

    /// Total on-disk bytes across all sessions (best-effort, reads metadata only).
    /// Used by `/aft-status` to surface storage footprint.
    pub fn total_disk_bytes(&self) -> u64 {
        let mut total = 0u64;
        for session_dirs in self.disk_index.values() {
            for meta in session_dirs.values() {
                if let Ok(read_dir) = std::fs::read_dir(&meta.dir) {
                    for entry in read_dir.flatten() {
                        if let Ok(m) = entry.metadata() {
                            if m.is_file() {
                                total += m.len();
                            }
                        }
                    }
                }
            }
        }
        total
    }

    fn next_id_and_order(&self) -> (String, u128) {
        let n = self.counter.fetch_add(1, Ordering::Relaxed);
        let order = ((current_timestamp_nanos() as u128) << 32) | u128::from(n);
        (format!("backup-{}", n), order)
    }

    fn db_pool_and_harness(&self) -> Option<(Arc<Mutex<Connection>>, String)> {
        let pool = self.db_pool.read().ok().and_then(|slot| slot.clone())?;
        let harness = self.db_harness.read().ok().and_then(|slot| slot.clone())?;
        Some((pool, harness))
    }

    fn latest_head_for_key(&self, session: &str, key: &Path) -> Option<BackupEntryHead> {
        match self.read_stack_heads_from_db(session, key) {
            Some(Ok(stack)) if !stack.is_empty() => return stack.last().cloned(),
            Some(Ok(_)) => {
                crate::slog_info!(
                    "backup preview DB miss for session {} path {}; falling back to disk",
                    session,
                    key.display()
                );
            }
            Some(Err(error)) => {
                crate::slog_warn!(
                    "backup preview DB lookup failed for session {} path {}; falling back to disk: {}",
                    session,
                    key.display(),
                    error
                );
            }
            None => {
                crate::slog_info!(
                    "backup preview DB unavailable for session {} path {}; falling back to disk",
                    session,
                    key.display()
                );
            }
        }

        self.entries
            .get(session)
            .and_then(|files| files.get(key))
            .and_then(|stack| stack.last())
            .map(BackupEntryHead::from_entry)
            .or_else(|| {
                self.read_stack_heads_from_disk(session, key)
                    .and_then(|stack| stack.last().cloned())
            })
    }

    fn merge_disk_stack_heads(
        &self,
        session: &str,
        heads_by_path: &mut HashMap<PathBuf, BackupEntryHead>,
    ) {
        let disk_keys: Vec<PathBuf> = self
            .disk_index
            .get(session)
            .map(|files| files.keys().cloned().collect())
            .unwrap_or_default();
        for key in disk_keys {
            if let Some(head) = self
                .read_stack_heads_from_disk(session, &key)
                .and_then(|stack| stack.last().cloned())
            {
                heads_by_path.insert(key, head);
            }
        }
    }

    fn read_stack_heads_from_db(
        &self,
        session: &str,
        key: &Path,
    ) -> Option<Result<Vec<BackupEntryHead>, String>> {
        let (pool, harness) = self.db_pool_and_harness()?;
        let conn = match pool.lock() {
            Ok(conn) => conn,
            Err(_) => return Some(Err("db mutex poisoned".to_string())),
        };
        let path_hash = Self::path_hash(key);
        Some(
            crate::db::backups::list_backups(&conn, &harness, session, &path_hash)
                .map_err(|error| error.to_string())
                .map(|rows| {
                    rows.iter()
                        .map(BackupEntryHead::from_row)
                        .collect::<Vec<_>>()
                }),
        )
    }

    fn read_latest_operation_heads_from_db(
        &self,
        session: &str,
    ) -> Option<Result<HashMap<PathBuf, BackupEntryHead>, String>> {
        let (pool, harness) = self.db_pool_and_harness()?;
        let conn = match pool.lock() {
            Ok(conn) => conn,
            Err(_) => return Some(Err("db mutex poisoned".to_string())),
        };
        let latest = match crate::db::backups::get_latest_operation_backup(&conn, &harness, session)
        {
            Ok(Some(row)) => row,
            Ok(None) => return Some(Ok(HashMap::new())),
            Err(error) => return Some(Err(error.to_string())),
        };
        let Some(op_id) = latest.op_id else {
            return Some(Ok(HashMap::new()));
        };
        let rows = match crate::db::backups::list_backups_by_op(&conn, &harness, session, &op_id) {
            Ok(rows) => rows,
            Err(error) => return Some(Err(error.to_string())),
        };
        if rows.is_empty() {
            return Some(Ok(HashMap::new()));
        }
        let path_hashes: std::collections::HashSet<String> =
            rows.into_iter().map(|row| row.path_hash).collect();
        drop(conn);

        let mut heads = HashMap::new();
        for path_hash in path_hashes {
            let conn = match pool.lock() {
                Ok(conn) => conn,
                Err(_) => return Some(Err("db mutex poisoned".to_string())),
            };
            let rows = match crate::db::backups::list_backups(&conn, &harness, session, &path_hash)
            {
                Ok(rows) => rows,
                Err(error) => return Some(Err(error.to_string())),
            };
            drop(conn);

            let Some(file_path) = rows.first().map(|row| row.file_path.clone()) else {
                continue;
            };
            let Some(head) = rows.last().map(BackupEntryHead::from_row) else {
                continue;
            };
            heads.insert(PathBuf::from(file_path), head);
        }

        Some(Ok(heads))
    }

    fn read_stack_from_db(
        &self,
        session: &str,
        key: &Path,
    ) -> Option<Result<Vec<BackupEntry>, String>> {
        let (pool, harness) = self.db_pool_and_harness()?;
        let conn = match pool.lock() {
            Ok(conn) => conn,
            Err(_) => return Some(Err("db mutex poisoned".to_string())),
        };
        let path_hash = Self::path_hash(key);
        Some(
            crate::db::backups::list_backups(&conn, &harness, session, &path_hash)
                .map_err(|error| error.to_string())
                .and_then(|rows| {
                    rows.into_iter()
                        .map(BackupEntry::try_from)
                        .collect::<Result<Vec<_>, _>>()
                        .map_err(|error| error.to_string())
                }),
        )
    }

    fn load_from_db_if_present(
        &mut self,
        session: &str,
        key: &Path,
    ) -> Option<Result<bool, String>> {
        match self.read_stack_from_db(session, key) {
            Some(Ok(stack)) if !stack.is_empty() => {
                self.update_counter_from_entries(&stack);
                self.entries
                    .entry(session.to_string())
                    .or_default()
                    .insert(key.to_path_buf(), stack);
                Some(Ok(true))
            }
            Some(Ok(_)) => Some(Ok(false)),
            Some(Err(error)) => Some(Err(error)),
            None => None,
        }
    }

    fn load_latest_operation_from_db(&mut self, session: &str) -> Option<Result<bool, String>> {
        let (pool, harness) = self.db_pool_and_harness()?;
        let conn = match pool.lock() {
            Ok(conn) => conn,
            Err(_) => return Some(Err("db mutex poisoned".to_string())),
        };
        let latest = match crate::db::backups::get_latest_operation_backup(&conn, &harness, session)
        {
            Ok(Some(row)) => row,
            Ok(None) => return Some(Ok(false)),
            Err(error) => return Some(Err(error.to_string())),
        };
        let Some(op_id) = latest.op_id else {
            return Some(Ok(false));
        };
        let rows = match crate::db::backups::list_backups_by_op(&conn, &harness, session, &op_id) {
            Ok(rows) => rows,
            Err(error) => return Some(Err(error.to_string())),
        };
        if rows.is_empty() {
            return Some(Ok(false));
        }
        let path_hashes: std::collections::HashSet<String> =
            rows.into_iter().map(|row| row.path_hash).collect();
        drop(conn);

        let mut loaded_any = false;
        for path_hash in path_hashes {
            let conn = match pool.lock() {
                Ok(conn) => conn,
                Err(_) => return Some(Err("db mutex poisoned".to_string())),
            };
            let loaded =
                match crate::db::backups::list_backups(&conn, &harness, session, &path_hash) {
                    Ok(rows) => {
                        let file_path = rows.first().map(|row| row.file_path.clone());
                        rows.into_iter()
                            .map(BackupEntry::try_from)
                            .collect::<Result<Vec<_>, _>>()
                            .map(|stack| (file_path, stack))
                            .map_err(|error| error.to_string())
                    }
                    Err(error) => Err(error.to_string()),
                };
            drop(conn);
            let (file_path, stack) = match loaded {
                Ok((file_path, stack)) if !stack.is_empty() => (file_path, stack),
                Ok(_) => continue,
                Err(error) => return Some(Err(error)),
            };
            let Some(file_path) = file_path else {
                return Some(Err(format!(
                    "backup DB rows for path hash {path_hash} have no file path"
                )));
            };
            let key = PathBuf::from(file_path);
            self.update_counter_from_entries(&stack);
            self.entries
                .entry(session.to_string())
                .or_default()
                .insert(key, stack);
            loaded_any = true;
        }

        Some(Ok(loaded_any))
    }

    fn update_counter_from_entries(&self, entries: &[BackupEntry]) {
        if let Some(next_counter) = entries
            .iter()
            .filter_map(|entry| backup_sequence(&entry.backup_id))
            .max()
            .and_then(|max| max.checked_add(1))
        {
            let _ = self
                .counter
                .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
                    (current < next_counter).then_some(next_counter)
                });
        }
    }

    pub fn discard_operation_entries(&mut self, session: &str, op_id: &str) {
        let keys: Vec<PathBuf> = self
            .entries
            .get(session)
            .map(|files| files.keys().cloned().collect())
            .unwrap_or_default();

        for key in keys {
            let mut remove_key = false;
            let mut remaining_stack = None;
            if let Some(session_entries) = self.entries.get_mut(session) {
                if let Some(stack) = session_entries.get_mut(&key) {
                    while stack
                        .last()
                        .is_some_and(|entry| entry.op_id.as_deref() == Some(op_id))
                    {
                        stack.pop();
                    }
                    if stack.is_empty() {
                        remove_key = true;
                    } else {
                        remaining_stack = Some(stack.clone());
                    }
                }
                if remove_key {
                    session_entries.remove(&key);
                }
            }

            if remove_key {
                self.remove_disk_backups(session, &key);
            } else if let Some(stack) = remaining_stack {
                self.write_snapshot_to_disk(session, &key, &stack);
            }
        }

        if self
            .entries
            .get(session)
            .is_some_and(|session_entries| session_entries.is_empty())
        {
            self.entries.remove(session);
        }
    }

    fn touch_session(&mut self, session: &str) {
        let now = current_timestamp();
        self.session_meta
            .entry(session.to_string())
            .or_default()
            .last_accessed = now;
        self.write_session_marker(session, now);
    }

    // ---- Internal helpers ----

    fn do_restore(
        &mut self,
        session: &str,
        key: &Path,
        path: &Path,
    ) -> Result<(BackupEntry, Option<String>), AftError> {
        let session_entries =
            self.entries
                .get_mut(session)
                .ok_or_else(|| AftError::NoUndoHistory {
                    path: path.display().to_string(),
                })?;
        let stack = session_entries
            .get_mut(key)
            .ok_or_else(|| AftError::NoUndoHistory {
                path: path.display().to_string(),
            })?;

        let entry = stack
            .last()
            .cloned()
            .ok_or_else(|| AftError::NoUndoHistory {
                path: path.display().to_string(),
            })?;

        match entry.kind {
            BackupEntryKind::Content | BackupEntryKind::Symlink => {
                restore_entry_to_path(path, &entry).map_err(|e| AftError::IoError {
                    path: path.display().to_string(),
                    message: e.to_string(),
                })?;
            }
            BackupEntryKind::Tombstone => {
                remove_tombstone_path(path).map_err(|e| AftError::IoError {
                    path: path.display().to_string(),
                    message: e.to_string(),
                })?;
                remove_created_dirs_best_effort(&entry.created_dirs);
            }
        }

        stack.pop();
        if stack.is_empty() {
            session_entries.remove(key);
            // Also prune the session map when its last file is gone.
            if session_entries.is_empty() {
                self.entries.remove(session);
            }
            self.remove_disk_backups(session, key);
        } else {
            let stack_clone = self
                .entries
                .get(session)
                .and_then(|s| s.get(key))
                .cloned()
                .unwrap_or_default();
            self.write_snapshot_to_disk(session, key, &stack_clone);
        }

        Ok((entry, None))
    }

    fn commit_restored_backup(&mut self, session: &str, key: &Path) {
        let mut remove_key = false;
        let mut remove_session = false;
        let mut remaining_stack = None;

        if let Some(session_entries) = self.entries.get_mut(session) {
            if let Some(stack) = session_entries.get_mut(key) {
                stack.pop();
                if stack.is_empty() {
                    remove_key = true;
                } else {
                    remaining_stack = Some(stack.clone());
                }
            }

            if remove_key {
                session_entries.remove(key);
                remove_session = session_entries.is_empty();
            }
        }

        if remove_session {
            self.entries.remove(session);
        }

        if remove_key {
            self.remove_disk_backups(session, key);
        } else if let Some(stack) = remaining_stack {
            self.write_snapshot_to_disk(session, key, &stack);
        }
    }

    fn check_external_modification(
        &self,
        session: &str,
        key: &Path,
        path: &Path,
    ) -> Option<String> {
        let stack = self.entries.get(session).and_then(|s| s.get(key))?;
        let latest = stack.last()?;
        let modified = match latest.kind {
            BackupEntryKind::Content => std::fs::read(path)
                .map(|current| current != latest.content_bytes)
                .unwrap_or(true),
            BackupEntryKind::Symlink => std::fs::read_link(path)
                .map(|target| latest.link_target.as_ref() != Some(&target))
                .unwrap_or(true),
            BackupEntryKind::Tombstone => false,
        };
        modified.then(|| "file was modified externally since last backup".to_string())
    }

    // ---- Disk persistence ----

    fn backups_dir(&self) -> Option<PathBuf> {
        self.storage_dir
            .as_ref()
            .map(|dir| match &self.storage_harness {
                Some(harness) => dir.join(harness).join("backups"),
                None => dir.join("backups"),
            })
    }

    fn session_dir(&self, session: &str) -> Option<PathBuf> {
        self.backups_dir()
            .map(|d| d.join(Self::session_hash(session)))
    }

    fn session_hash(session: &str) -> String {
        hash_session(session)
    }

    fn path_hash(key: &Path) -> String {
        // v0.16.0 intentionally switched from DefaultHasher to SHA-256 for
        // stable on-disk names. Existing DefaultHasher backup directories are
        // not migrated: backups are short-lived/session-scoped, so one-time
        // loss of pre-upgrade undo history is acceptable.
        stable_hash_16(key.to_string_lossy().as_bytes())
    }

    fn write_session_marker(&self, session: &str, last_accessed: u64) {
        let Some(session_dir) = self.session_dir(session) else {
            return;
        };
        if let Err(e) = std::fs::create_dir_all(&session_dir) {
            crate::slog_warn!("failed to create session dir: {}", e);
            return;
        }
        let marker = session_dir.join("session.json");
        let json = serde_json::json!({
            "schema_version": SCHEMA_VERSION,
            "session_id": session,
            "last_accessed": last_accessed,
        });
        if let Ok(s) = serde_json::to_string_pretty(&json) {
            let tmp = session_dir.join("session.json.tmp");
            if std::fs::write(&tmp, s).is_ok() {
                let _ = std::fs::rename(&tmp, marker);
            }
        }
    }

    fn repair_root_backups_if_needed(&self) {
        let (Some(storage_dir), Some(harness)) = (&self.storage_dir, &self.storage_harness) else {
            return;
        };
        let root_backups = storage_dir.join("backups");
        if !dir_has_entries(&root_backups) {
            return;
        }
        let harness_backups = storage_dir.join(harness).join("backups");
        if dir_has_entries(&harness_backups) {
            return;
        }
        if let Some(parent) = harness_backups.parent() {
            if let Err(error) = std::fs::create_dir_all(parent) {
                crate::slog_warn!(
                    "failed to create harness backup dir {}: {}",
                    parent.display(),
                    error
                );
                return;
            }
        }
        if harness_backups.exists() {
            let _ = std::fs::remove_dir(&harness_backups);
        }
        match std::fs::rename(&root_backups, &harness_backups) {
            Ok(()) => {
                crate::slog_info!(
                    "moved legacy root backups into harness namespace: {}",
                    harness_backups.display()
                );
            }
            Err(error) => {
                crate::slog_warn!(
                    "failed to move legacy root backups into {}: {}; trying child merge",
                    harness_backups.display(),
                    error
                );
                if std::fs::create_dir_all(&harness_backups).is_err() {
                    return;
                }
                if let Ok(entries) = std::fs::read_dir(&root_backups) {
                    for entry in entries.flatten() {
                        let source = entry.path();
                        let target = harness_backups.join(entry.file_name());
                        if !target.exists() {
                            let _ = std::fs::rename(source, target);
                        }
                    }
                }
                let _ = std::fs::remove_dir(&root_backups);
            }
        }
    }

    fn gc_stale_sessions(&mut self, ttl_hours: u32) {
        let backups_dir = match self.backups_dir() {
            Some(d) if d.exists() => d,
            _ => return,
        };
        let ttl_secs = u64::from(if ttl_hours == 0 { 72 } else { ttl_hours }) * 60 * 60;
        let cutoff = current_timestamp().saturating_sub(ttl_secs);
        let entries = match std::fs::read_dir(&backups_dir) {
            Ok(entries) => entries,
            Err(_) => return,
        };

        for entry in entries.flatten() {
            let session_dir = entry.path();
            if !session_dir.is_dir() || session_dir.join("meta.json").exists() {
                continue;
            }
            let Some(last_accessed) = Self::read_session_last_accessed(&session_dir) else {
                continue;
            };
            if last_accessed >= cutoff {
                continue;
            }
            if let Err(e) = std::fs::remove_dir_all(&session_dir) {
                crate::slog_warn!(
                    "failed to remove stale backup session {}: {}",
                    session_dir.display(),
                    e
                );
            } else {
                crate::slog_warn!(
                    "removed stale backup session {} (last_accessed={})",
                    session_dir.display(),
                    last_accessed
                );
            }
        }
    }

    /// One-time migration: move pre-session flat layout into the default
    /// session namespace. Called from `set_storage_dir` so existing backups
    /// survive the upgrade.
    ///
    /// Detection: any directory directly under `backups/` that contains a
    /// `meta.json` (as opposed to a `session.json` marker or subdirectories)
    /// is treated as a legacy entry.
    fn migrate_legacy_layout_if_needed(&mut self) {
        let backups_dir = match self.backups_dir() {
            Some(d) if d.exists() => d,
            _ => return,
        };
        let default_session_dir =
            backups_dir.join(Self::session_hash(crate::protocol::DEFAULT_SESSION_ID));

        let entries = match std::fs::read_dir(&backups_dir) {
            Ok(e) => e,
            Err(_) => return,
        };
        let mut migrated = 0usize;
        for entry in entries.flatten() {
            let entry_path = entry.path();
            // Skip non-directories and already-sessionized layouts.
            if !entry_path.is_dir() {
                continue;
            }
            if entry_path == default_session_dir {
                continue;
            }
            let meta_path = entry_path.join("meta.json");
            if !meta_path.exists() {
                continue; // Already a session-hash dir (contains per-path subdirs), skip
            }
            // This is a legacy flat-layout path-hash directory. Move it under
            // the default session namespace.
            if let Err(e) = std::fs::create_dir_all(&default_session_dir) {
                crate::slog_warn!("failed to create default session dir: {}", e);
                return;
            }
            let leaf = match entry_path.file_name() {
                Some(n) => n,
                None => continue,
            };
            let target = default_session_dir.join(leaf);
            if target.exists() {
                // Already migrated on a prior run that was interrupted —
                // leave both and let the regular load pick up the target.
                continue;
            }
            match std::fs::rename(&entry_path, &target) {
                Ok(()) => {
                    // Bump meta.json to include session_id + schema_version.
                    Self::upgrade_meta_file(
                        &target.join("meta.json"),
                        crate::protocol::DEFAULT_SESSION_ID,
                    );
                    migrated += 1;
                }
                Err(e) => {
                    crate::slog_warn!(
                        "failed to migrate legacy backup {}: {}",
                        entry_path.display(),
                        e
                    );
                }
            }
        }
        if migrated > 0 {
            crate::slog_info!(
                "migrated {} legacy backup entries into default session namespace",
                migrated
            );
            // Write a session.json marker so future scans don't re-migrate.
            let marker = default_session_dir.join("session.json");
            let json = serde_json::json!({
                "schema_version": SCHEMA_VERSION,
                "session_id": crate::protocol::DEFAULT_SESSION_ID,
                "last_accessed": current_timestamp(),
            });
            if let Ok(s) = serde_json::to_string_pretty(&json) {
                let _ = std::fs::write(&marker, s);
            }
        }
    }

    fn upgrade_meta_file(meta_path: &Path, session_id: &str) {
        let content = match std::fs::read_to_string(meta_path) {
            Ok(c) => c,
            Err(_) => return,
        };
        let mut parsed: serde_json::Value = match serde_json::from_str(&content) {
            Ok(v) => v,
            Err(_) => return,
        };
        if let Some(obj) = parsed.as_object_mut() {
            let count = obj.get("count").and_then(|v| v.as_u64()).unwrap_or(0);
            obj.insert(
                "schema_version".to_string(),
                serde_json::json!(SCHEMA_VERSION),
            );
            obj.insert("session_id".to_string(), serde_json::json!(session_id));
            obj.entry("entries").or_insert_with(|| {
                serde_json::Value::Array(
                    (0..count)
                        .map(|i| {
                            serde_json::json!({
                                "backup_id": format!("disk-{}", i),
                                "timestamp": 0,
                                "description": "restored from disk",
                                "op_id": null,
                            })
                        })
                        .collect(),
                )
            });
        }
        if let Ok(s) = serde_json::to_string_pretty(&parsed) {
            let tmp = meta_path.with_extension("json.tmp");
            if std::fs::write(&tmp, &s).is_ok() {
                let _ = std::fs::rename(&tmp, meta_path);
            }
        }
    }

    fn load_disk_index(&mut self) {
        let backups_dir = match self.backups_dir() {
            Some(d) if d.exists() => d,
            _ => return,
        };
        let session_dirs = match std::fs::read_dir(&backups_dir) {
            Ok(e) => e,
            Err(_) => return,
        };
        let mut total_entries = 0usize;
        let mut skipped_legacy = 0usize;
        for session_entry in session_dirs.flatten() {
            let session_dir = session_entry.path();
            if !session_dir.is_dir() {
                continue;
            }
            // Recover the session_id from session.json if present, otherwise skip
            // (can't invert the hash to recover the original).
            let session_id = match Self::read_session_marker(&session_dir) {
                Some(session_id) => session_id,
                None => {
                    crate::slog_warn!(
                        "skipping backup session dir without readable session marker: {}",
                        session_dir.display()
                    );
                    continue;
                }
            };

            let path_dirs = match std::fs::read_dir(&session_dir) {
                Ok(e) => e,
                Err(_) => continue,
            };
            let per_session = self.disk_index.entry(session_id.clone()).or_default();
            for path_entry in path_dirs.flatten() {
                let path_dir = path_entry.path();
                if !path_dir.is_dir() {
                    continue;
                }
                let meta_path = path_dir.join("meta.json");
                if let Ok(content) = std::fs::read_to_string(&meta_path) {
                    if let Ok(meta) = serde_json::from_str::<serde_json::Value>(&content) {
                        if let (Some(path_str), Some(count)) = (
                            meta.get("path").and_then(|v| v.as_str()),
                            meta.get("count").and_then(|v| v.as_u64()),
                        ) {
                            let key = PathBuf::from(path_str);
                            if !is_loadable_backup_path(&key, &path_dir) {
                                // Legacy/relocated backup dirs whose folder name came
                                // from an older path-hash scheme can never be loaded by
                                // the current hasher. They are harmless dead husks
                                // (active undo is DB-backed), so skip quietly and
                                // summarize once at debug instead of warning per entry.
                                skipped_legacy += 1;
                                crate::slog_debug!(
                                    "skipping backup entry with invalid path metadata: {}",
                                    meta_path.display()
                                );
                                continue;
                            }
                            per_session.insert(
                                key,
                                DiskMeta {
                                    dir: path_dir.clone(),
                                    count: count as usize,
                                },
                            );
                            total_entries += 1;
                        }
                    }
                }
            }
            if per_session.is_empty() {
                self.disk_index.remove(&session_id);
            }
        }
        if skipped_legacy > 0 {
            crate::slog_debug!(
                "skipped {} legacy backup entries with mismatched path-hash directories",
                skipped_legacy
            );
        }
        if total_entries > 0 {
            crate::slog_info!(
                "loaded {} backup entries across {} session(s) from disk",
                total_entries,
                self.disk_index.len()
            );
        }
    }

    fn read_session_marker(session_dir: &Path) -> Option<String> {
        let marker = session_dir.join("session.json");
        let content = std::fs::read_to_string(&marker).ok()?;
        let parsed: serde_json::Value = serde_json::from_str(&content).ok()?;
        parsed
            .get("session_id")
            .and_then(|v| v.as_str())
            .map(|s| s.to_string())
    }

    fn read_session_last_accessed(session_dir: &Path) -> Option<u64> {
        let marker = session_dir.join("session.json");
        let content = std::fs::read_to_string(&marker).ok()?;
        let parsed: serde_json::Value = serde_json::from_str(&content).ok()?;
        parsed.get("last_accessed").and_then(|v| v.as_u64())
    }

    fn load_from_disk_if_needed(&mut self, session: &str, key: &Path) -> bool {
        let Some(entries) = self.read_stack_from_disk(session, key) else {
            return false;
        };

        self.update_counter_from_entries(&entries);

        self.entries
            .entry(session.to_string())
            .or_default()
            .insert(key.to_path_buf(), entries);
        true
    }

    /// Ensure the in-memory undo stack for `(session, key)` reflects any prior
    /// on-disk history before a new snapshot is appended.
    ///
    /// Without this, a fresh `BackupStore` (e.g. after a bridge/process
    /// restart, which clears `self.entries` and resets `counter` to 0) would
    /// append a new snapshot onto an EMPTY in-memory stack and then
    /// `write_snapshot_to_disk` would overwrite the file's `meta.json`/`.bak`
    /// set with that single entry — silently discarding all undo history
    /// captured before the restart, and reusing `backup-0` because the counter
    /// was never advanced past the persisted entries. Hydrating here preserves
    /// the prior stack AND advances the counter via
    /// `update_counter_from_entries`. Only loads when nothing is in memory yet,
    /// so it never clobbers a stack already mutated in this run and adds at
    /// most one disk read per file per session.
    fn ensure_stack_hydrated(&mut self, session: &str, key: &Path) {
        let already_in_memory = self
            .entries
            .get(session)
            .and_then(|files| files.get(key))
            .is_some_and(|stack| !stack.is_empty());
        if !already_in_memory {
            self.load_from_disk_if_needed(session, key);
        }
    }

    fn load_all_disk_backups(&mut self, session: &str) {
        let disk_keys: Vec<PathBuf> = self
            .disk_index
            .get(session)
            .map(|files| files.keys().cloned().collect())
            .unwrap_or_default();
        for key in disk_keys {
            self.load_from_disk_if_needed(session, &key);
        }
    }

    fn read_stack_heads_from_disk(
        &self,
        session: &str,
        key: &Path,
    ) -> Option<Vec<BackupEntryHead>> {
        let disk_meta = match self
            .disk_index
            .get(session)
            .and_then(|s| s.get(key))
            .cloned()
        {
            Some(m) if m.count > 0 => m,
            _ => return None,
        };

        let entry_meta = std::fs::read_to_string(disk_meta.dir.join("meta.json"))
            .ok()
            .and_then(|content| serde_json::from_str::<serde_json::Value>(&content).ok())
            .and_then(|meta| meta.get("entries").and_then(|v| v.as_array()).cloned())
            .unwrap_or_default();

        let mut heads = Vec::new();
        for i in 0..disk_meta.count {
            let meta = entry_meta.get(i);
            let backup_id = meta
                .and_then(|m| m.get("backup_id"))
                .and_then(|v| v.as_str())
                .map(str::to_string)
                .unwrap_or_else(|| format!("disk-{}", i));
            let timestamp = meta
                .and_then(|m| m.get("timestamp"))
                .and_then(|v| v.as_u64())
                .unwrap_or(0);
            let order = meta
                .and_then(|m| m.get("order"))
                .and_then(parse_order_value)
                .unwrap_or_else(|| legacy_entry_order(timestamp, &backup_id));
            heads.push(BackupEntryHead {
                order,
                op_id: meta
                    .and_then(|m| m.get("op_id"))
                    .and_then(|v| v.as_str())
                    .map(str::to_string),
            });
        }

        if heads.is_empty() {
            return None;
        }
        Some(heads)
    }

    fn read_stack_from_disk(&self, session: &str, key: &Path) -> Option<Vec<BackupEntry>> {
        let disk_meta = match self
            .disk_index
            .get(session)
            .and_then(|s| s.get(key))
            .cloned()
        {
            Some(m) if m.count > 0 => m,
            _ => return None,
        };

        let mut entries = Vec::new();
        let entry_meta = std::fs::read_to_string(disk_meta.dir.join("meta.json"))
            .ok()
            .and_then(|content| serde_json::from_str::<serde_json::Value>(&content).ok())
            .and_then(|meta| meta.get("entries").and_then(|v| v.as_array()).cloned())
            .unwrap_or_default();

        for i in 0..disk_meta.count {
            let meta = entry_meta.get(i);
            let kind = match meta.and_then(|m| m.get("kind")).and_then(|v| v.as_str()) {
                Some("tombstone") => BackupEntryKind::Tombstone,
                Some("symlink") => BackupEntryKind::Symlink,
                _ => BackupEntryKind::Content,
            };
            let content_bytes = match kind {
                BackupEntryKind::Content | BackupEntryKind::Symlink => {
                    let bak_path = disk_meta.dir.join(format!("{}.bak", i));
                    match std::fs::read(&bak_path) {
                        Ok(content) => content,
                        Err(_) => continue,
                    }
                }
                BackupEntryKind::Tombstone => Vec::new(),
            };
            let link_target = if kind == BackupEntryKind::Symlink {
                meta.and_then(|m| m.get("link_target"))
                    .and_then(|v| v.as_str())
                    .map(PathBuf::from)
                    .or_else(|| {
                        Some(PathBuf::from(
                            String::from_utf8_lossy(&content_bytes).into_owned(),
                        ))
                    })
            } else {
                None
            };
            let content = match kind {
                BackupEntryKind::Content => String::from_utf8_lossy(&content_bytes).into_owned(),
                BackupEntryKind::Symlink => link_target
                    .as_ref()
                    .map(|target| target.display().to_string())
                    .unwrap_or_default(),
                BackupEntryKind::Tombstone => String::new(),
            };
            let backup_id = meta
                .and_then(|m| m.get("backup_id"))
                .and_then(|v| v.as_str())
                .map(str::to_string)
                .unwrap_or_else(|| format!("disk-{}", i));
            let timestamp = meta
                .and_then(|m| m.get("timestamp"))
                .and_then(|v| v.as_u64())
                .unwrap_or(0);
            let order = meta
                .and_then(|m| m.get("order"))
                .and_then(parse_order_value)
                .unwrap_or_else(|| legacy_entry_order(timestamp, &backup_id));
            entries.push(BackupEntry {
                backup_id,
                content,
                content_bytes,
                timestamp,
                order,
                description: meta
                    .and_then(|m| m.get("description"))
                    .and_then(|v| v.as_str())
                    .unwrap_or("restored from disk")
                    .to_string(),
                op_id: meta
                    .and_then(|m| m.get("op_id"))
                    .and_then(|v| v.as_str())
                    .map(str::to_string),
                kind,
                mode: meta
                    .and_then(|m| m.get("mode"))
                    .and_then(|v| v.as_u64())
                    .and_then(|mode| u32::try_from(mode).ok()),
                link_target,
                created_dirs: meta
                    .and_then(|m| m.get("created_dirs"))
                    .and_then(|v| v.as_array())
                    .map(|dirs| {
                        dirs.iter()
                            .filter_map(|dir| dir.as_str())
                            .map(PathBuf::from)
                            .collect()
                    })
                    .unwrap_or_default(),
            });
        }

        if entries.is_empty() {
            return None;
        }
        Some(entries)
    }

    fn write_snapshot_to_disk(&mut self, session: &str, key: &Path, stack: &[BackupEntry]) {
        let session_dir = match self.session_dir(session) {
            Some(d) => d,
            None => return,
        };

        // Ensure session dir + marker exist.
        if let Err(e) = std::fs::create_dir_all(&session_dir) {
            crate::slog_warn!("failed to create session dir: {}", e);
            return;
        }
        let marker = session_dir.join("session.json");
        if !marker.exists() {
            let json = serde_json::json!({
                "schema_version": SCHEMA_VERSION,
                "session_id": session,
                "last_accessed": current_timestamp(),
            });
            if let Ok(s) = serde_json::to_string_pretty(&json) {
                let _ = std::fs::write(&marker, s);
            }
        }

        let hash = Self::path_hash(key);
        let dir = session_dir.join(&hash);
        if let Err(e) = std::fs::create_dir_all(&dir) {
            crate::slog_warn!("failed to create backup dir: {}", e);
            return;
        }

        for (i, entry) in stack.iter().enumerate() {
            let bak_path = dir.join(format!("{}.bak", i));
            let tmp_path = dir.join(format!("{}.bak.tmp", i));
            match entry.kind {
                BackupEntryKind::Content => {
                    if std::fs::write(&tmp_path, &entry.content_bytes).is_ok() {
                        let _ = std::fs::rename(&tmp_path, &bak_path);
                    }
                }
                BackupEntryKind::Symlink => {
                    let target = entry
                        .link_target
                        .as_ref()
                        .map(|target| target.as_os_str().to_string_lossy().as_bytes().to_vec())
                        .unwrap_or_default();
                    if std::fs::write(&tmp_path, target).is_ok() {
                        let _ = std::fs::rename(&tmp_path, &bak_path);
                    }
                }
                BackupEntryKind::Tombstone => {
                    let _ = std::fs::remove_file(&bak_path);
                    let _ = std::fs::remove_file(&tmp_path);
                }
            }
        }

        // Clean up extra .bak files if stack shrank.
        for i in stack.len()..MAX_UNDO_DEPTH {
            let old = dir.join(format!("{}.bak", i));
            if old.exists() {
                let _ = std::fs::remove_file(&old);
            }
        }

        let entries: Vec<serde_json::Value> = stack
            .iter()
            .map(|entry| {
                serde_json::json!({
                    "backup_id": entry.backup_id,
                    "timestamp": entry.timestamp,
                    "order": entry.order.to_string(),
                    "description": entry.description,
                    "op_id": entry.op_id,
                    "kind": match entry.kind {
                        BackupEntryKind::Content => "content",
                        BackupEntryKind::Symlink => "symlink",
                        BackupEntryKind::Tombstone => "tombstone",
                    },
                    "mode": entry.mode,
                    "link_target": entry.link_target.as_ref().map(|target| target.display().to_string()),
                    "created_dirs": entry
                        .created_dirs
                        .iter()
                        .map(|dir| dir.display().to_string())
                        .collect::<Vec<_>>(),
                })
            })
            .collect();
        let meta = serde_json::json!({
            "schema_version": SCHEMA_VERSION,
            "session_id": session,
            "path": key.display().to_string(),
            "count": stack.len(),
            "entries": entries,
        });
        let meta_path = dir.join("meta.json");
        let meta_tmp = dir.join("meta.json.tmp");
        if let Ok(content) = serde_json::to_string_pretty(&meta) {
            if std::fs::write(&meta_tmp, &content).is_ok() {
                let _ = std::fs::rename(&meta_tmp, &meta_path);
            }
        }

        // Keep the in-memory disk_index in sync so tracked_files() and
        // disk_history_count() immediately reflect what we just wrote.
        self.disk_index
            .entry(session.to_string())
            .or_default()
            .insert(
                key.to_path_buf(),
                DiskMeta {
                    dir: dir.clone(),
                    count: stack.len(),
                },
            );
        self.dual_write_stack_to_db(session, key, &dir, stack);
    }

    fn dual_write_stack_to_db(&self, session: &str, key: &Path, dir: &Path, stack: &[BackupEntry]) {
        let pool = self.db_pool.read().ok().and_then(|slot| slot.clone());
        let Some(pool) = pool else {
            return;
        };
        let harness = self.db_harness.read().ok().and_then(|slot| slot.clone());
        let Some(harness) = harness else {
            crate::slog_warn!(
                "dual-write backup to DB skipped for {}: harness not configured",
                key.display()
            );
            return;
        };
        let project_key = self
            .db_project_key
            .read()
            .ok()
            .and_then(|slot| slot.clone());
        let Some(project_key) = project_key else {
            crate::slog_warn!(
                "dual-write backup to DB skipped for {}: project key not configured",
                key.display()
            );
            return;
        };

        let conn = match pool.lock() {
            Ok(conn) => conn,
            Err(_) => {
                crate::slog_warn!(
                    "dual-write backup to DB failed for {}: db mutex poisoned",
                    key.display()
                );
                return;
            }
        };
        let path_hash = Self::path_hash(key);
        let file_path = key.display().to_string();
        if let Err(error) =
            crate::db::backups::delete_backups_for_path(&conn, &harness, session, &path_hash)
        {
            crate::slog_warn!(
                "delete old backup DB rows failed for {}: {}",
                key.display(),
                error
            );
            return;
        }
        for (index, entry) in stack.iter().enumerate() {
            let backup_path = match entry.kind {
                BackupEntryKind::Content | BackupEntryKind::Symlink => {
                    Some(dir.join(format!("{}.bak", index)).display().to_string())
                }
                BackupEntryKind::Tombstone => Some(dir.join("meta.json").display().to_string()),
            };
            let row = entry.to_backup_row(
                &harness,
                session,
                &project_key,
                &file_path,
                &path_hash,
                backup_path.as_deref(),
            );
            if let Err(error) = crate::db::backups::upsert_backup(&conn, &row) {
                crate::slog_warn!(
                    "dual-write backup to DB failed for {}: {}",
                    entry.backup_id,
                    error
                );
            }
        }
    }

    fn remove_disk_backups(&mut self, session: &str, key: &Path) {
        self.remove_db_backups(session, key);
        let removed = self.disk_index.get_mut(session).and_then(|s| s.remove(key));
        if let Some(meta) = removed {
            let _ = std::fs::remove_dir_all(&meta.dir);
        } else if let Some(session_dir) = self.session_dir(session) {
            let hash = Self::path_hash(key);
            let dir = session_dir.join(&hash);
            if dir.exists() {
                let _ = std::fs::remove_dir_all(&dir);
            }
        }

        // If this session has no more disk entries, drop the map slot (session
        // dir itself is kept so the marker survives future sessions).
        let empty = self
            .disk_index
            .get(session)
            .map(|s| s.is_empty())
            .unwrap_or(false);
        if empty {
            self.disk_index.remove(session);
        }
    }

    fn remove_db_backups(&self, session: &str, key: &Path) {
        let Some((pool, harness)) = self.db_pool_and_harness() else {
            return;
        };
        let conn = match pool.lock() {
            Ok(conn) => conn,
            Err(_) => {
                crate::slog_warn!(
                    "delete backup DB rows failed for {}: db mutex poisoned",
                    key.display()
                );
                return;
            }
        };
        let path_hash = Self::path_hash(key);
        if let Err(error) =
            crate::db::backups::delete_backups_for_path(&conn, &harness, session, &path_hash)
        {
            crate::slog_warn!(
                "delete backup DB rows failed for {}: {}",
                key.display(),
                error
            );
        }
    }
}

pub fn hash_session(session: &str) -> String {
    stable_hash_16(session.as_bytes())
}

pub fn new_op_id() -> String {
    let mut bytes = [0u8; 4];
    if getrandom::fill(&mut bytes).is_err() {
        bytes = current_timestamp().to_le_bytes()[..4]
            .try_into()
            .unwrap_or([0; 4]);
    }
    let rand = u32::from_le_bytes(bytes);
    format!("op-{}-{:08x}", current_timestamp() * 1000, rand)
}

#[derive(Debug, Clone)]
struct BackupEntryDiskMetadata {
    mode: Option<u32>,
    link_target: Option<PathBuf>,
    created_dirs: Vec<PathBuf>,
}

#[derive(Debug, Clone)]
enum RestorePathState {
    Missing,
    Regular {
        content_bytes: Vec<u8>,
        mode: Option<u32>,
    },
    Symlink {
        target: PathBuf,
    },
    Directory,
}

fn backup_entry_from_path(
    path: &Path,
    backup_id: String,
    order: u128,
    description: &str,
    op_id: Option<&str>,
) -> Result<BackupEntry, AftError> {
    let metadata = std::fs::symlink_metadata(path).map_err(|error| match error.kind() {
        std::io::ErrorKind::NotFound => AftError::FileNotFound {
            path: path.display().to_string(),
        },
        _ => AftError::IoError {
            path: path.display().to_string(),
            message: error.to_string(),
        },
    })?;
    let mode = file_mode(&metadata);

    let (kind, content, content_bytes, link_target) = if metadata.file_type().is_symlink() {
        let target = std::fs::read_link(path).map_err(|error| AftError::IoError {
            path: path.display().to_string(),
            message: error.to_string(),
        })?;
        (
            BackupEntryKind::Symlink,
            target.display().to_string(),
            Vec::new(),
            Some(target),
        )
    } else if metadata.is_file() {
        let bytes = std::fs::read(path).map_err(|error| AftError::IoError {
            path: path.display().to_string(),
            message: error.to_string(),
        })?;
        (
            BackupEntryKind::Content,
            String::from_utf8_lossy(&bytes).into_owned(),
            bytes,
            None,
        )
    } else {
        return Err(AftError::InvalidRequest {
            message: format!(
                "backup: '{}' is not a regular file or symlink",
                path.display()
            ),
        });
    };

    Ok(BackupEntry {
        backup_id,
        content,
        content_bytes,
        timestamp: current_timestamp(),
        order,
        description: description.to_string(),
        op_id: op_id.map(str::to_string),
        kind,
        mode,
        link_target,
        created_dirs: Vec::new(),
    })
}

fn canonicalize_key(path: &Path) -> PathBuf {
    let absolute = if path.is_absolute() {
        path.to_path_buf()
    } else {
        std::env::current_dir()
            .unwrap_or_else(|_| PathBuf::from("."))
            .join(path)
    };

    match std::fs::symlink_metadata(&absolute) {
        Ok(metadata) if metadata.file_type().is_symlink() => {
            canonicalize_parent_join_leaf(&absolute)
        }
        Ok(_) => std::fs::canonicalize(&absolute)
            .map(|path| normalize_absolute_key(&path))
            .unwrap_or_else(|_| canonicalize_existing_ancestor(&absolute)),
        Err(_) => canonicalize_existing_ancestor(&absolute),
    }
}

fn canonicalize_parent_join_leaf(path: &Path) -> PathBuf {
    let Some(parent) = path.parent() else {
        return normalize_absolute_key(path);
    };
    let mut key = canonicalize_existing_ancestor(parent);
    if let Some(file_name) = path.file_name() {
        key.push(file_name);
    }
    key
}

fn canonicalize_existing_ancestor(path: &Path) -> PathBuf {
    let mut suffix = Vec::new();
    let mut current = path;

    loop {
        if let Ok(mut base) = std::fs::canonicalize(current) {
            for component in suffix.iter().rev() {
                base.push(Path::new(component));
            }
            return normalize_absolute_key(&base);
        }
        let Some(parent) = current.parent() else {
            return normalize_absolute_key(path);
        };
        if let Some(file_name) = current.file_name() {
            suffix.push(file_name.to_os_string());
        }
        current = parent;
    }
}

fn normalize_absolute_key(path: &Path) -> PathBuf {
    let mut normalized = PathBuf::new();

    for component in path.components() {
        match component {
            std::path::Component::CurDir => {}
            std::path::Component::ParentDir => {
                if !normalized.pop() {
                    normalized.push(component.as_os_str());
                }
            }
            other => normalized.push(other.as_os_str()),
        }
    }

    normalized
}

fn file_mode(metadata: &std::fs::Metadata) -> Option<u32> {
    #[cfg(unix)]
    {
        use std::os::unix::fs::PermissionsExt;
        Some(metadata.permissions().mode())
    }
    #[cfg(not(unix))]
    {
        let _ = metadata;
        None
    }
}

fn set_file_mode(path: &Path, mode: Option<u32>) -> std::io::Result<()> {
    #[cfg(unix)]
    {
        use std::os::unix::fs::PermissionsExt;
        if let Some(mode) = mode {
            std::fs::set_permissions(path, std::fs::Permissions::from_mode(mode))?;
        }
    }
    #[cfg(not(unix))]
    {
        let _ = (path, mode);
    }
    Ok(())
}

fn capture_path_state(path: &Path) -> Result<RestorePathState, AftError> {
    let metadata = match std::fs::symlink_metadata(path) {
        Ok(metadata) => metadata,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
            return Ok(RestorePathState::Missing);
        }
        Err(error) => {
            return Err(AftError::IoError {
                path: path.display().to_string(),
                message: error.to_string(),
            });
        }
    };

    if metadata.file_type().is_symlink() {
        let target = std::fs::read_link(path).map_err(|error| AftError::IoError {
            path: path.display().to_string(),
            message: error.to_string(),
        })?;
        Ok(RestorePathState::Symlink { target })
    } else if metadata.is_file() {
        let content_bytes = std::fs::read(path).map_err(|error| AftError::IoError {
            path: path.display().to_string(),
            message: error.to_string(),
        })?;
        Ok(RestorePathState::Regular {
            content_bytes,
            mode: file_mode(&metadata),
        })
    } else {
        Ok(RestorePathState::Directory)
    }
}

fn restore_entry_to_path(path: &Path, entry: &BackupEntry) -> std::io::Result<()> {
    match entry.kind {
        BackupEntryKind::Content => restore_regular_file(path, &entry.content_bytes, entry.mode),
        BackupEntryKind::Symlink => {
            let target = entry.link_target.as_ref().ok_or_else(|| {
                std::io::Error::new(
                    std::io::ErrorKind::InvalidData,
                    "symlink backup entry missing target",
                )
            })?;
            restore_symlink(path, target)
        }
        BackupEntryKind::Tombstone => remove_tombstone_path(path),
    }
}

fn restore_path_state(path: &Path, state: &RestorePathState) -> bool {
    match state {
        RestorePathState::Missing => remove_file_or_symlink_if_present(path).is_ok(),
        RestorePathState::Regular {
            content_bytes,
            mode,
        } => restore_regular_file(path, content_bytes, *mode).is_ok(),
        RestorePathState::Symlink { target } => restore_symlink(path, target).is_ok(),
        RestorePathState::Directory => true,
    }
}

fn restore_regular_file(
    path: &Path,
    content_bytes: &[u8],
    mode: Option<u32>,
) -> std::io::Result<()> {
    if let Some(parent) = path.parent() {
        if !parent.as_os_str().is_empty() {
            std::fs::create_dir_all(parent)?;
        }
    }
    if std::fs::symlink_metadata(path)
        .map(|metadata| metadata.file_type().is_symlink())
        .unwrap_or(false)
    {
        std::fs::remove_file(path)?;
    }
    std::fs::write(path, content_bytes)?;
    set_file_mode(path, mode)
}

fn restore_symlink(path: &Path, target: &Path) -> std::io::Result<()> {
    if let Some(parent) = path.parent() {
        if !parent.as_os_str().is_empty() {
            std::fs::create_dir_all(parent)?;
        }
    }
    remove_file_or_symlink_if_present(path)?;
    create_symlink(target, path)
}

#[cfg(unix)]
fn create_symlink(target: &Path, link: &Path) -> std::io::Result<()> {
    std::os::unix::fs::symlink(target, link)
}

#[cfg(windows)]
fn create_symlink(target: &Path, link: &Path) -> std::io::Result<()> {
    if target.is_dir() {
        std::os::windows::fs::symlink_dir(target, link)
    } else {
        std::os::windows::fs::symlink_file(target, link)
    }
}

fn remove_tombstone_path(path: &Path) -> std::io::Result<()> {
    match std::fs::symlink_metadata(path) {
        Ok(metadata) if metadata.file_type().is_symlink() || metadata.is_file() => {
            std::fs::remove_file(path)
        }
        Ok(_) => Err(std::io::Error::new(
            std::io::ErrorKind::IsADirectory,
            "tombstone target is a directory",
        )),
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => Ok(()),
        Err(error) => Err(error),
    }
}

fn remove_file_or_symlink_if_present(path: &Path) -> std::io::Result<()> {
    match std::fs::symlink_metadata(path) {
        Ok(metadata) if metadata.file_type().is_symlink() || metadata.is_file() => {
            std::fs::remove_file(path)
        }
        Ok(_) => Err(std::io::Error::new(
            std::io::ErrorKind::IsADirectory,
            "path is a directory",
        )),
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => Ok(()),
        Err(error) => Err(error),
    }
}

fn read_entry_disk_metadata(
    backup_path: &Path,
    backup_id: &str,
) -> Option<BackupEntryDiskMetadata> {
    let meta_path = if backup_path.file_name().and_then(|name| name.to_str()) == Some("meta.json") {
        backup_path.to_path_buf()
    } else {
        backup_path.parent()?.join("meta.json")
    };
    let content = std::fs::read_to_string(meta_path).ok()?;
    let meta: serde_json::Value = serde_json::from_str(&content).ok()?;
    let entries = meta.get("entries")?.as_array()?;
    let entry = entries
        .iter()
        .find(|entry| entry.get("backup_id").and_then(|value| value.as_str()) == Some(backup_id))?;
    Some(BackupEntryDiskMetadata {
        mode: entry
            .get("mode")
            .and_then(|value| value.as_u64())
            .and_then(|mode| u32::try_from(mode).ok()),
        link_target: entry
            .get("link_target")
            .and_then(|value| value.as_str())
            .map(PathBuf::from),
        created_dirs: entry
            .get("created_dirs")
            .and_then(|value| value.as_array())
            .map(|dirs| {
                dirs.iter()
                    .filter_map(|dir| dir.as_str())
                    .map(PathBuf::from)
                    .collect()
            })
            .unwrap_or_default(),
    })
}

fn rollback_transactional_restore(
    written: &[(PathBuf, RestorePathState)],
    attempted: Option<(&PathBuf, &RestorePathState)>,
) -> bool {
    let mut ok = true;

    if let Some((path, state)) = attempted {
        ok &= restore_path_state(path, state);
    }

    for (path, state) in written.iter().rev() {
        ok &= restore_path_state(path, state);
    }

    ok
}

fn rollback_deleted_tombstones(deleted: &[(PathBuf, RestorePathState)]) -> bool {
    let mut ok = true;
    for (path, state) in deleted.iter().rev() {
        ok &= restore_path_state(path, state);
    }
    ok
}

fn missing_parent_dirs(parent: &Path) -> Vec<PathBuf> {
    let mut dirs = Vec::new();
    let mut current = Some(parent);

    while let Some(dir) = current {
        if dir.as_os_str().is_empty() || dir.exists() {
            break;
        }
        dirs.push(dir.to_path_buf());
        current = dir.parent();
    }

    dirs
}

fn rollback_created_dirs(dirs: &[PathBuf]) -> bool {
    let mut dirs = dirs.to_vec();
    dirs.sort_by_key(|dir| std::cmp::Reverse(dir.components().count()));
    dirs.dedup();

    let mut ok = true;
    for dir in dirs {
        match std::fs::remove_dir(&dir) {
            Ok(()) => {}
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => {}
            Err(_) => ok = false,
        }
    }

    ok
}

fn remove_created_dirs_best_effort(dirs: &[PathBuf]) {
    let mut dirs = dirs.to_vec();
    dirs.sort_by_key(|dir| std::cmp::Reverse(dir.components().count()));
    dirs.dedup();

    for dir in dirs {
        match std::fs::remove_dir(&dir) {
            Ok(()) => {}
            Err(error) if error.kind() == std::io::ErrorKind::NotFound => {}
            Err(_) => {}
        }
    }
}

fn dir_has_entries(path: &Path) -> bool {
    std::fs::read_dir(path)
        .map(|mut entries| entries.next().is_some())
        .unwrap_or(false)
}

fn current_timestamp() -> u64 {
    std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .unwrap_or_default()
        .as_secs()
}

fn current_timestamp_nanos() -> u64 {
    let nanos = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .unwrap_or_default()
        .as_nanos();
    nanos.min(u128::from(u64::MAX)) as u64
}

fn legacy_entry_order(timestamp_secs: u64, backup_id: &str) -> u128 {
    let nanos = timestamp_secs.saturating_mul(1_000_000_000);
    ((nanos as u128) << 32) | u128::from(backup_sequence(backup_id).unwrap_or(0))
}

fn parse_order_value(value: &serde_json::Value) -> Option<u128> {
    value
        .as_str()
        .and_then(|s| s.parse::<u128>().ok())
        .or_else(|| value.as_u64().map(u128::from))
}

fn is_loadable_backup_path(key: &Path, path_dir: &Path) -> bool {
    if !key.is_absolute()
        || key
            .components()
            .any(|c| matches!(c, std::path::Component::ParentDir))
    {
        return false;
    }
    let Some(dir_name) = path_dir.file_name().and_then(|name| name.to_str()) else {
        return false;
    };
    BackupStore::path_hash(key) == dir_name
}

fn stable_hash_16(bytes: &[u8]) -> String {
    let digest = Sha256::digest(bytes);
    digest[..8]
        .iter()
        .map(|byte| format!("{:02x}", byte))
        .collect()
}

fn backup_sequence(backup_id: &str) -> Option<u64> {
    backup_id
        .strip_prefix("backup-")
        .or_else(|| backup_id.strip_prefix("disk-"))
        .and_then(|s| s.parse().ok())
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::protocol::DEFAULT_SESSION_ID;
    use std::fs;
    #[cfg(unix)]
    use std::os::unix::fs::PermissionsExt;

    fn temp_file(name: &str, content: &str) -> PathBuf {
        let dir = std::env::temp_dir().join("aft_backup_tests");
        fs::create_dir_all(&dir).unwrap();
        let path = dir.join(name);
        fs::write(&path, content).unwrap();
        path
    }

    #[test]
    fn snapshot_and_restore_round_trip() {
        let path = temp_file("round_trip.txt", "original");
        let mut store = BackupStore::new();

        let id = store
            .snapshot(DEFAULT_SESSION_ID, &path, "before edit")
            .unwrap();
        assert!(id.starts_with("backup-"));

        fs::write(&path, "modified").unwrap();
        assert_eq!(fs::read_to_string(&path).unwrap(), "modified");

        let (entry, _) = store.restore_latest(DEFAULT_SESSION_ID, &path).unwrap();
        assert_eq!(entry.content, "original");
        assert_eq!(fs::read_to_string(&path).unwrap(), "original");
    }

    #[test]
    fn multiple_snapshots_preserve_order() {
        let path = temp_file("order.txt", "v1");
        let mut store = BackupStore::new();

        store.snapshot(DEFAULT_SESSION_ID, &path, "first").unwrap();
        fs::write(&path, "v2").unwrap();
        store.snapshot(DEFAULT_SESSION_ID, &path, "second").unwrap();
        fs::write(&path, "v3").unwrap();
        store.snapshot(DEFAULT_SESSION_ID, &path, "third").unwrap();

        let history = store.history(DEFAULT_SESSION_ID, &path);
        assert_eq!(history.len(), 3);
        assert_eq!(history[0].content, "v1");
        assert_eq!(history[1].content, "v2");
        assert_eq!(history[2].content, "v3");
    }

    #[test]
    fn restore_pops_from_stack() {
        let path = temp_file("pop.txt", "v1");
        let mut store = BackupStore::new();

        store.snapshot(DEFAULT_SESSION_ID, &path, "first").unwrap();
        fs::write(&path, "v2").unwrap();
        store.snapshot(DEFAULT_SESSION_ID, &path, "second").unwrap();

        let (entry, _) = store.restore_latest(DEFAULT_SESSION_ID, &path).unwrap();
        assert_eq!(entry.description, "second");
        assert_eq!(entry.content, "v2");

        let history = store.history(DEFAULT_SESSION_ID, &path);
        assert_eq!(history.len(), 1);
    }

    #[test]
    fn empty_history_returns_empty_vec() {
        let store = BackupStore::new();
        let path = Path::new("/tmp/aft_backup_tests/nonexistent_history.txt");
        assert!(store.history(DEFAULT_SESSION_ID, path).is_empty());
    }

    #[test]
    fn snapshot_nonexistent_file_returns_error() {
        let mut store = BackupStore::new();
        let path = Path::new("/tmp/aft_backup_tests/absolutely_does_not_exist.txt");
        assert!(store.snapshot(DEFAULT_SESSION_ID, path, "test").is_err());
    }

    #[test]
    fn tracked_files_lists_snapshotted_paths() {
        let path1 = temp_file("tracked1.txt", "a");
        let path2 = temp_file("tracked2.txt", "b");
        let mut store = BackupStore::new();

        store.snapshot(DEFAULT_SESSION_ID, &path1, "snap1").unwrap();
        store.snapshot(DEFAULT_SESSION_ID, &path2, "snap2").unwrap();
        assert_eq!(store.tracked_files(DEFAULT_SESSION_ID).len(), 2);
    }

    #[test]
    fn sessions_are_isolated() {
        let path = temp_file("isolated.txt", "original");
        let mut store = BackupStore::new();

        store.snapshot("session_a", &path, "a's snapshot").unwrap();

        // Session B sees no history for this file.
        assert!(store.history("session_b", &path).is_empty());
        assert_eq!(store.tracked_files("session_b").len(), 0);

        // Session B's restore_latest fails with NoUndoHistory.
        let err = store.restore_latest("session_b", &path);
        assert!(matches!(err, Err(AftError::NoUndoHistory { .. })));

        // Session A still sees its own snapshot.
        assert_eq!(store.history("session_a", &path).len(), 1);
        assert_eq!(store.tracked_files("session_a").len(), 1);
    }

    #[test]
    fn per_session_per_file_cap_is_independent() {
        // Two sessions fill up their own stacks independently; hitting the cap
        // in session A does not evict anything from session B.
        let path = temp_file("cap_indep.txt", "v0");
        let mut store = BackupStore::new();

        for i in 0..(MAX_UNDO_DEPTH + 5) {
            fs::write(&path, format!("a{}", i)).unwrap();
            store.snapshot("session_a", &path, "a").unwrap();
        }
        fs::write(&path, "b_initial").unwrap();
        store.snapshot("session_b", &path, "b").unwrap();

        // Session A should be capped at MAX_UNDO_DEPTH.
        assert_eq!(store.history("session_a", &path).len(), MAX_UNDO_DEPTH);
        // Session B should still have its single entry.
        assert_eq!(store.history("session_b", &path).len(), 1);
    }

    #[test]
    fn sessions_with_backups_lists_all_namespaces() {
        let path_a = temp_file("sessions_list_a.txt", "a");
        let path_b = temp_file("sessions_list_b.txt", "b");
        let mut store = BackupStore::new();

        store.snapshot("alice", &path_a, "from alice").unwrap();
        store.snapshot("bob", &path_b, "from bob").unwrap();

        let sessions = store.sessions_with_backups();
        assert_eq!(sessions.len(), 2);
        assert!(sessions.iter().any(|s| s == "alice"));
        assert!(sessions.iter().any(|s| s == "bob"));
    }

    #[test]
    fn disk_persistence_survives_reload() {
        let dir = std::env::temp_dir().join("aft_backup_disk_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();

        let file_path = temp_file("disk_persist.txt", "original");

        // Create store with storage, snapshot under default session, drop.
        {
            let mut store = BackupStore::new();
            store.set_storage_dir(dir.clone(), 72);
            store
                .snapshot(DEFAULT_SESSION_ID, &file_path, "before edit")
                .unwrap();
        }

        // Modify the file externally.
        fs::write(&file_path, "externally modified").unwrap();

        // Create new store, load from disk, restore.
        let mut store2 = BackupStore::new();
        store2.set_storage_dir(dir.clone(), 72);

        let (entry, warning) = store2
            .restore_latest(DEFAULT_SESSION_ID, &file_path)
            .unwrap();
        assert_eq!(entry.content, "original");
        assert!(warning.is_some()); // modified externally
        assert_eq!(fs::read_to_string(&file_path).unwrap(), "original");

        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn snapshot_after_restart_preserves_history_and_unique_ids() {
        // Regression (bug #8): after a restart the BackupStore is fresh
        // (entries cleared, counter reset to 0). A new snapshot must EXTEND the
        // persisted undo stack — not overwrite it with a single entry — and must
        // not reuse backup-0. Two undo levels must remain available across the
        // restart boundary.
        let dir = std::env::temp_dir().join("aft_backup_restart_history_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let file_path = temp_file("restart_history.txt", "v0");

        // Run 1: edit v0 -> v1 (snapshot captures "v0"), then write v1.
        let first_id = {
            let mut store = BackupStore::new();
            store.set_storage_dir(dir.clone(), 72);
            let id = store
                .snapshot(DEFAULT_SESSION_ID, &file_path, "edit 1")
                .unwrap();
            fs::write(&file_path, "v1").unwrap();
            id
        };

        // Restart: fresh store, same storage dir. Edit v1 -> v2 (snapshot
        // captures "v1"), then write v2.
        let second_id = {
            let mut store = BackupStore::new();
            store.set_storage_dir(dir.clone(), 72);
            let id = store
                .snapshot(DEFAULT_SESSION_ID, &file_path, "edit 2")
                .unwrap();
            fs::write(&file_path, "v2").unwrap();
            id
        };

        // The post-restart snapshot must NOT reuse the first id (counter
        // advanced past persisted entries).
        assert_ne!(
            first_id, second_id,
            "post-restart snapshot reused backup id {first_id}"
        );

        // Both undo levels survive: a fresh store sees 2 entries on disk, and
        // two sequential restores walk v1 then v0.
        let mut store = BackupStore::new();
        store.set_storage_dir(dir.clone(), 72);
        assert_eq!(
            store.history(DEFAULT_SESSION_ID, &file_path).len(),
            2,
            "prior history was overwritten by the post-restart snapshot"
        );

        let (entry1, _) = store
            .restore_latest(DEFAULT_SESSION_ID, &file_path)
            .unwrap();
        assert_eq!(entry1.content, "v1", "first undo should restore v1");
        let (entry0, _) = store
            .restore_latest(DEFAULT_SESSION_ID, &file_path)
            .unwrap();
        assert_eq!(entry0.content, "v0", "second undo should restore v0");

        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn legacy_flat_layout_migrates_to_default_session() {
        // Simulate a pre-session on-disk layout (schema v1) and verify it's
        // moved under the default session namespace on set_storage_dir.
        let dir = std::env::temp_dir().join("aft_backup_migration_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let backups = dir.join("backups");
        fs::create_dir_all(&backups).unwrap();

        // Fake legacy entry for some path hash.
        let legacy_hash = "deadbeefcafebabe";
        let legacy_dir = backups.join(legacy_hash);
        fs::create_dir_all(&legacy_dir).unwrap();
        fs::write(legacy_dir.join("0.bak"), "original content").unwrap();
        let legacy_meta = serde_json::json!({
            "path": "/tmp/migrated_file.txt",
            "count": 1,
        });
        fs::write(
            legacy_dir.join("meta.json"),
            serde_json::to_string_pretty(&legacy_meta).unwrap(),
        )
        .unwrap();

        // Run migration.
        let mut store = BackupStore::new();
        store.set_storage_dir(dir.clone(), 72);

        // After migration, the legacy dir should be gone from the top level,
        // and the entry should now live under the default-session hash dir.
        let default_session_dir = backups.join(BackupStore::session_hash(DEFAULT_SESSION_ID));
        assert!(default_session_dir.exists());
        assert!(default_session_dir.join(legacy_hash).exists());
        assert!(!backups.join(legacy_hash).exists());

        // The upgraded meta.json should now include session_id + schema_version.
        let meta_content =
            fs::read_to_string(default_session_dir.join(legacy_hash).join("meta.json")).unwrap();
        let meta: serde_json::Value = serde_json::from_str(&meta_content).unwrap();
        assert_eq!(meta["session_id"], DEFAULT_SESSION_ID);
        assert_eq!(meta["schema_version"], SCHEMA_VERSION);

        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn set_storage_dir_removes_stale_backup_sessions() {
        let dir = std::env::temp_dir().join("aft_backup_gc_test");
        let _ = fs::remove_dir_all(&dir);
        let backups = dir.join("backups");
        fs::create_dir_all(&backups).unwrap();

        let stale_session_dir = backups.join("stale-session");
        fs::create_dir_all(&stale_session_dir).unwrap();
        let stale_marker = serde_json::json!({
            "schema_version": SCHEMA_VERSION,
            "session_id": "stale",
            "last_accessed": 1,
        });
        fs::write(
            stale_session_dir.join("session.json"),
            serde_json::to_string_pretty(&stale_marker).unwrap(),
        )
        .unwrap();

        let mut store = BackupStore::new();
        store.set_storage_dir(dir.clone(), 1);

        assert!(!stale_session_dir.exists());
        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn markerless_session_dir_is_skipped_not_mapped_to_default() {
        let dir = std::env::temp_dir().join("aft_backup_markerless_skip_test");
        let _ = fs::remove_dir_all(&dir);
        let file_path = temp_file("markerless.txt", "original");
        let key = canonicalize_key(&file_path);
        let path_dir = dir
            .join("backups")
            .join("corrupt-session")
            .join("path-entry");
        fs::create_dir_all(&path_dir).unwrap();
        fs::write(path_dir.join("0.bak"), "original").unwrap();
        fs::write(
            path_dir.join("meta.json"),
            serde_json::to_string_pretty(&serde_json::json!({
                "schema_version": SCHEMA_VERSION,
                "session_id": "lost-session",
                "path": key.display().to_string(),
                "count": 1,
                "entries": [{
                    "backup_id": "disk-0",
                    "timestamp": 0,
                    "description": "corrupt marker test",
                    "op_id": null,
                    "kind": "content",
                }]
            }))
            .unwrap(),
        )
        .unwrap();

        let mut store = BackupStore::new();
        store.set_storage_dir(dir.clone(), 72);

        assert_eq!(store.disk_history_count(DEFAULT_SESSION_ID, &file_path), 0);
        assert!(store.sessions_with_backups().is_empty());
        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn set_storage_dir_reconfiguration_drops_previous_disk_index() {
        let dir_a = std::env::temp_dir().join("aft_backup_storage_a_test");
        let dir_b = std::env::temp_dir().join("aft_backup_storage_b_test");
        let _ = fs::remove_dir_all(&dir_a);
        let _ = fs::remove_dir_all(&dir_b);
        fs::create_dir_all(&dir_a).unwrap();
        fs::create_dir_all(&dir_b).unwrap();
        let file_path = temp_file("storage_reconfigure.txt", "original");

        let mut store = BackupStore::new();
        store.set_storage_dir(dir_a.clone(), 72);
        store
            .snapshot(DEFAULT_SESSION_ID, &file_path, "stored in a")
            .unwrap();
        assert_eq!(store.disk_history_count(DEFAULT_SESSION_ID, &file_path), 1);

        store.set_storage_dir(dir_b.clone(), 72);

        assert_eq!(store.disk_history_count(DEFAULT_SESSION_ID, &file_path), 0);
        assert!(store.tracked_files(DEFAULT_SESSION_ID).is_empty());
        let _ = fs::remove_dir_all(&dir_a);
        let _ = fs::remove_dir_all(&dir_b);
    }

    #[test]
    fn restore_last_operation_restores_all_top_entries_for_same_op() {
        let path_a = temp_file("op_restore_a.txt", "a1");
        let path_b = temp_file("op_restore_b.txt", "b1");
        let mut store = BackupStore::new();
        let op_id = "op-test-00000001";

        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_a, "a", Some(op_id))
            .unwrap();
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_b, "b", Some(op_id))
            .unwrap();
        fs::write(&path_a, "a2").unwrap();
        fs::write(&path_b, "b2").unwrap();

        let restored = store.restore_last_operation(DEFAULT_SESSION_ID).unwrap();
        assert_eq!(restored.op_id, op_id);
        assert_eq!(restored.restored.len(), 2);
        assert_eq!(fs::read_to_string(&path_a).unwrap(), "a1");
        assert_eq!(fs::read_to_string(&path_b).unwrap(), "b1");
    }

    #[test]
    fn restore_last_operation_deletes_tombstone_destination() {
        let dir = std::env::temp_dir().join("aft_backup_tombstone_delete_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let source = dir.join("source.txt");
        let destination = dir.join("destination.txt");
        fs::write(&source, "original").unwrap();

        let mut store = BackupStore::new();
        let op_id = "op-tombstone-delete";
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &source, "move source", Some(op_id))
            .unwrap();
        fs::rename(&source, &destination).unwrap();
        store
            .snapshot_op_tombstone(DEFAULT_SESSION_ID, op_id, &destination, "created dest")
            .unwrap();

        let restored = store.restore_last_operation(DEFAULT_SESSION_ID).unwrap();
        assert_eq!(restored.op_id, op_id);
        assert_eq!(restored.restored.len(), 1);
        assert_eq!(fs::read_to_string(&source).unwrap(), "original");
        assert!(!destination.exists());
        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn restore_last_operation_rolls_back_source_when_tombstone_delete_fails() {
        let dir = std::env::temp_dir().join("aft_backup_tombstone_atomic_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let source = dir.join("source.txt");
        let destination = dir.join("destination.txt");
        fs::write(&source, "original").unwrap();

        let mut store = BackupStore::new();
        let op_id = "op-tombstone-atomic";
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &source, "move source", Some(op_id))
            .unwrap();
        fs::rename(&source, &destination).unwrap();
        store
            .snapshot_op_tombstone(DEFAULT_SESSION_ID, op_id, &destination, "created dest")
            .unwrap();

        fs::remove_file(&destination).unwrap();
        fs::create_dir(&destination).unwrap();
        let result = store.restore_last_operation(DEFAULT_SESSION_ID);

        assert!(result.is_err(), "directory tombstone target should fail");
        assert!(
            !source.exists(),
            "source restore must roll back when destination deletion fails"
        );
        assert!(
            destination.is_dir(),
            "failed tombstone target should remain"
        );
        let _ = fs::remove_dir_all(&dir);
    }

    // Uses Unix-specific PermissionsExt::set_mode to make a target file
    // read-only and force the Phase 1 write to fail. The atomicity logic
    // it exercises is platform-independent — Windows has different
    // mechanisms for forcing write failures, covered separately.
    #[cfg(unix)]
    #[test]
    fn restore_last_operation_is_atomic_when_a_write_fails() {
        let dir = std::env::temp_dir().join("aft_backup_tests_atomic_restore");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let path_a = dir.join("a.txt");
        let path_b = dir.join("b.txt");
        let path_c = dir.join("c.txt");
        fs::write(&path_a, "a-original").unwrap();
        fs::write(&path_b, "b-original").unwrap();
        fs::write(&path_c, "c-original").unwrap();

        let mut store = BackupStore::new();
        let op_id = "op-atomic-restore-01";
        let id_a = store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_a, "a", Some(op_id))
            .unwrap();
        let id_b = store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_b, "b", Some(op_id))
            .unwrap();
        let id_c = store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_c, "c", Some(op_id))
            .unwrap();
        fs::write(&path_a, "a-modified").unwrap();
        fs::write(&path_b, "b-modified").unwrap();
        fs::write(&path_c, "c-modified").unwrap();

        let original_permissions = fs::metadata(&path_b).unwrap().permissions();
        let mut readonly_permissions = original_permissions.clone();
        readonly_permissions.set_mode(0o444);
        fs::set_permissions(&path_b, readonly_permissions).unwrap();

        let result = store.restore_last_operation(DEFAULT_SESSION_ID);
        fs::set_permissions(&path_b, original_permissions).unwrap();

        assert!(result.is_err());
        assert_eq!(fs::read_to_string(&path_a).unwrap(), "a-modified");
        assert_eq!(fs::read_to_string(&path_b).unwrap(), "b-modified");
        assert_eq!(fs::read_to_string(&path_c).unwrap(), "c-modified");

        let history_a = store.history(DEFAULT_SESSION_ID, &path_a);
        let history_b = store.history(DEFAULT_SESSION_ID, &path_b);
        let history_c = store.history(DEFAULT_SESSION_ID, &path_c);
        assert_eq!(history_a.len(), 1);
        assert_eq!(history_b.len(), 1);
        assert_eq!(history_c.len(), 1);
        assert_eq!(history_a[0].backup_id, id_a);
        assert_eq!(history_b[0].backup_id, id_b);
        assert_eq!(history_c[0].backup_id, id_c);
        assert_eq!(history_a[0].op_id.as_deref(), Some(op_id));
        assert_eq!(history_b[0].op_id.as_deref(), Some(op_id));
        assert_eq!(history_c[0].op_id.as_deref(), Some(op_id));

        let restored = store.restore_last_operation(DEFAULT_SESSION_ID).unwrap();
        assert_eq!(restored.op_id, op_id);
        assert_eq!(restored.restored.len(), 3);
        assert_eq!(fs::read_to_string(&path_a).unwrap(), "a-original");
        assert_eq!(fs::read_to_string(&path_b).unwrap(), "b-original");
        assert_eq!(fs::read_to_string(&path_c).unwrap(), "c-original");

        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn restore_last_operation_restores_only_most_recent_op() {
        let path_a = temp_file("op_recent_a.txt", "a1");
        let path_b = temp_file("op_recent_b.txt", "b1");
        let mut store = BackupStore::new();

        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_a, "older", Some("op-older"))
            .unwrap();
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_b, "newer", Some("op-newer"))
            .unwrap();
        fs::write(&path_a, "a2").unwrap();
        fs::write(&path_b, "b2").unwrap();

        let restored = store.restore_last_operation(DEFAULT_SESSION_ID).unwrap();
        assert_eq!(restored.op_id, "op-newer");
        assert_eq!(restored.restored.len(), 1);
        assert_eq!(fs::read_to_string(&path_a).unwrap(), "a2");
        assert_eq!(fs::read_to_string(&path_b).unwrap(), "b1");
    }

    #[test]
    fn restore_recreates_missing_parent_directories() {
        // Simulate aft_delete files: [dir/] with recursive: true:
        // the parent directories are gone by the time we restore.
        let dir = std::env::temp_dir().join("aft_backup_tests_recreate_parents");
        let _ = fs::remove_dir_all(&dir);
        let nested = dir.join("nested");
        fs::create_dir_all(&nested).unwrap();
        let path = nested.join("inner.txt");
        fs::write(&path, "original").unwrap();

        let mut store = BackupStore::new();
        let op_id = "op-recreate-parents-01";
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path, "original", Some(op_id))
            .unwrap();

        // Real-world delete sequence: tree is wiped before undo runs.
        fs::remove_dir_all(&dir).unwrap();
        assert!(!path.exists());
        assert!(!nested.exists());
        assert!(!dir.exists());

        let restored = store.restore_last_operation(DEFAULT_SESSION_ID).unwrap();
        assert_eq!(restored.op_id, op_id);
        assert_eq!(restored.restored.len(), 1);
        assert!(
            path.exists(),
            "file should be restored even though both nested/ and dir/ were missing"
        );
        assert_eq!(fs::read_to_string(&path).unwrap(), "original");

        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn restore_last_operation_ignores_legacy_entries_without_op_id() {
        let path = temp_file("op_legacy_none.txt", "v1");
        let mut store = BackupStore::new();

        store.snapshot(DEFAULT_SESSION_ID, &path, "legacy").unwrap();
        fs::write(&path, "v2").unwrap();

        let err = store.restore_last_operation(DEFAULT_SESSION_ID);
        assert!(matches!(err, Err(AftError::NoUndoHistory { .. })));
        assert_eq!(fs::read_to_string(&path).unwrap(), "v2");
    }

    #[test]
    fn schema_v2_meta_loads_with_none_op_id_and_persists_as_v3() {
        let dir = std::env::temp_dir().join("aft_backup_v2_to_v3_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let file_path = temp_file("v2_to_v3.txt", "original");
        let key = canonicalize_key(&file_path);
        let session_dir = dir
            .join("backups")
            .join(BackupStore::session_hash(DEFAULT_SESSION_ID));
        let path_dir = session_dir.join(BackupStore::path_hash(&key));
        fs::create_dir_all(&path_dir).unwrap();
        fs::write(path_dir.join("0.bak"), "original").unwrap();
        fs::write(
            session_dir.join("session.json"),
            serde_json::to_string_pretty(&serde_json::json!({
                "schema_version": 2,
                "session_id": DEFAULT_SESSION_ID,
                "last_accessed": current_timestamp(),
            }))
            .unwrap(),
        )
        .unwrap();
        fs::write(
            path_dir.join("meta.json"),
            serde_json::to_string_pretty(&serde_json::json!({
                "schema_version": 2,
                "session_id": DEFAULT_SESSION_ID,
                "path": key.display().to_string(),
                "count": 1,
            }))
            .unwrap(),
        )
        .unwrap();

        let mut store = BackupStore::new();
        store.set_storage_dir(dir.clone(), 72);
        assert!(store.load_from_disk_if_needed(DEFAULT_SESSION_ID, &key));
        let history = store.history(DEFAULT_SESSION_ID, &file_path);
        assert_eq!(history.len(), 1);
        assert_eq!(history[0].op_id, None);

        fs::write(&file_path, "second").unwrap();
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &file_path, "second", Some("op-v3"))
            .unwrap();
        let written: serde_json::Value =
            serde_json::from_str(&fs::read_to_string(path_dir.join("meta.json")).unwrap()).unwrap();
        assert_eq!(written["schema_version"], SCHEMA_VERSION);
        assert_eq!(written["entries"][0]["op_id"], serde_json::Value::Null);
        assert_eq!(written["entries"][1]["op_id"], "op-v3");
        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn per_file_restore_latest_still_works_with_op_ids() {
        let path = temp_file("op_per_file.txt", "v1");
        let mut store = BackupStore::new();

        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path, "op", Some("op-file"))
            .unwrap();
        fs::write(&path, "v2").unwrap();

        let (entry, _) = store.restore_latest(DEFAULT_SESSION_ID, &path).unwrap();
        assert_eq!(entry.op_id.as_deref(), Some("op-file"));
        assert_eq!(fs::read_to_string(&path).unwrap(), "v1");
    }

    #[test]
    fn per_file_restore_latest_deletes_tombstone() {
        let dir = std::env::temp_dir().join("aft_backup_per_file_tombstone_test");
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).unwrap();
        let path = dir.join("created.txt");
        fs::write(&path, "created").unwrap();

        let mut store = BackupStore::new();
        let id = store
            .snapshot_op_tombstone(DEFAULT_SESSION_ID, "op-create", &path, "created")
            .unwrap();

        let (entry, _) = store.restore_latest(DEFAULT_SESSION_ID, &path).unwrap();
        assert_eq!(entry.backup_id, id);
        assert!(!path.exists(), "tombstone undo should delete the file");
        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_disk_index_skips_tampered_meta_path_hash_mismatch() {
        let dir = std::env::temp_dir().join("aft_backup_tampered_meta_skip_test");
        let _ = fs::remove_dir_all(&dir);
        let backups = dir.join("backups");
        let session_dir = backups.join(BackupStore::session_hash(DEFAULT_SESSION_ID));
        let path_dir = session_dir.join("not-the-path-hash");
        fs::create_dir_all(&path_dir).unwrap();
        fs::write(
            session_dir.join("session.json"),
            serde_json::to_string_pretty(&serde_json::json!({
                "schema_version": SCHEMA_VERSION,
                "session_id": DEFAULT_SESSION_ID,
                "last_accessed": current_timestamp(),
            }))
            .unwrap(),
        )
        .unwrap();
        fs::write(path_dir.join("0.bak"), "outside").unwrap();
        fs::write(
            path_dir.join("meta.json"),
            serde_json::to_string_pretty(&serde_json::json!({
                "schema_version": SCHEMA_VERSION,
                "session_id": DEFAULT_SESSION_ID,
                "path": "/tmp/aft-malicious-overwrite-target.txt",
                "count": 1,
                "entries": [{
                    "backup_id": "backup-0",
                    "timestamp": current_timestamp(),
                    "order": "1",
                    "description": "tampered",
                    "op_id": "op-tampered",
                    "kind": "content",
                }]
            }))
            .unwrap(),
        )
        .unwrap();

        let mut store = BackupStore::new();
        store.set_storage_dir(dir.clone(), 72);

        assert!(store.sessions_with_backups().is_empty());
        let _ = fs::remove_dir_all(&dir);
    }

    #[test]
    fn restore_last_operation_uses_only_top_entries_and_persisted_order() {
        let path_a = temp_file("op_order_a.txt", "a1");
        let path_b = temp_file("op_order_b.txt", "b1");
        let mut store = BackupStore::new();

        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_a, "buried", Some("op-buried"))
            .unwrap();
        store
            .snapshot(DEFAULT_SESSION_ID, &path_a, "top without op")
            .unwrap();
        store
            .snapshot_with_op(DEFAULT_SESSION_ID, &path_b, "top", Some("op-top"))
            .unwrap();

        let key_a = canonicalize_key(&path_a);
        let key_b = canonicalize_key(&path_b);
        let files = store.entries.get_mut(DEFAULT_SESSION_ID).unwrap();
        files.get_mut(&key_a).unwrap()[0].order = u128::MAX;
        files.get_mut(&key_a).unwrap()[1].order = 1;
        files.get_mut(&key_b).unwrap()[0].order = 2;

        fs::write(&path_a, "a2").unwrap();
        fs::write(&path_b, "b2").unwrap();

        let restored = store.restore_last_operation(DEFAULT_SESSION_ID).unwrap();
        assert_eq!(restored.op_id, "op-top");
        assert_eq!(restored.restored.len(), 1);
        assert_eq!(fs::read_to_string(&path_a).unwrap(), "a2");
        assert_eq!(fs::read_to_string(&path_b).unwrap(), "b1");
    }
}