package engine

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/cdnutter/playlink/internal/fsops"
	"github.com/cdnutter/playlink/internal/probe"
	"github.com/cdnutter/playlink/internal/store"
)

type LinkOptions struct {
	// AllowDowngrade confirms linking when the master copy scored lower.
	AllowDowngrade bool
	// KeepFolder keeps a cross-folder match in the slave's current folder
	// instead of mirroring the master's folder structure.
	KeepFolder bool
	Trigger    string
}

// LinkTarget is where the slave file ends up after linking. A match in the
// same folder keeps the slave's folder (only the file name follows the
// master). A match found in another folder, such as a single matched to its
// album version, mirrors the master's path so the slave gets the album's
// folder structure, unless keepFolder is set.
func LinkTarget(relSlave, relMaster string, keepFolder bool) string {
	if relMaster == "" {
		return relSlave
	}
	sd := filepath.Dir(relSlave)
	if keepFolder || strings.EqualFold(sd, filepath.Dir(relMaster)) {
		return filepath.Join(sd, filepath.Base(relMaster))
	}
	return relMaster
}

// snapshot holds the verified state of both files right before an operation.
type snapshot struct {
	masterAbs, slaveAbs string
	mi, si              fsops.Info
}

// preflight re-validates a match against the filesystem: both files must be
// plain regular files inside their roots, on one device, and unchanged since
// the scan recorded them.
func preflight(l *libs, m store.Match) (snapshot, error) {
	var s snapshot
	var err error
	if s.masterAbs, err = l.master.Resolve(m.RelMaster); err != nil {
		return s, err
	}
	if s.slaveAbs, err = l.slave.Resolve(m.RelSlave); err != nil {
		return s, err
	}
	if s.mi, err = fsops.RegularFile(s.masterAbs); err != nil {
		return s, fmt.Errorf("master file: %w", err)
	}
	if s.si, err = fsops.RegularFile(s.slaveAbs); err != nil {
		return s, fmt.Errorf("slave file: %w", err)
	}
	if s.mi.Ino != m.MasterIno || !s.mi.Unchanged(m.MasterSize, m.MasterMtime) {
		return s, fmt.Errorf("master %w", fsops.ErrChanged)
	}
	if s.si.Ino != m.SlaveIno || !s.si.Unchanged(m.SlaveSize, m.SlaveMtime) {
		return s, fmt.Errorf("slave %w", fsops.ErrChanged)
	}
	if s.mi.Dev != s.si.Dev || s.mi.Dev != l.dev {
		return s, fsops.ErrCrossDevice
	}
	if s.mi.SameInode(s.si) {
		return s, errors.New("already linked")
	}
	return s, nil
}

func quarantinePath(l *libs, opID int64, side, rel string) string {
	return filepath.Join(l.quarantine, strconv.FormatInt(opID, 10), side, rel)
}

// quarantine adds an extra hardlink of p (whose identity is in) inside the
// quarantine. The original stays in place; nothing is lost if a later step
// fails. The caller removes or replaces p afterwards.
func quarantine(p string, in fsops.Info, q string) error {
	if err := fsops.MkdirAllLike(filepath.Dir(q)); err != nil {
		return err
	}
	return fsops.LinkNew(p, in, q)
}

// dropQuarantine removes a quarantine link that turned out not to be needed.
func dropQuarantine(l *libs, q string, keep fsops.Info) {
	if err := fsops.RemoveLink(q, keep); err == nil {
		pruneEmpty(filepath.Dir(q), l.quarantine)
	}
}

// removeOriginal deletes an original file after it was safely superseded. The
// inode must still be the one we verified.
//
// With a quarantine copy q, identity is proven by p and q being the same
// file right now. Timestamps are deliberately not compared: FUSE filesystems
// such as Unraid's /mnt/user can report them inconsistently between calls,
// and any change to the file is preserved in quarantine anyway because both
// names point at the same data. Without a quarantine copy (a byte-identical
// file), the size and modification time must also be unchanged.
func removeOriginal(p string, expect fsops.Info, q string) error {
	now, err := fsops.RegularFile(p)
	if err != nil {
		return err
	}
	if q != "" {
		qi, err := fsops.RegularFile(q)
		if err != nil {
			return fmt.Errorf("quarantine copy: %w", err)
		}
		if !now.SameInode(qi) {
			return fmt.Errorf("%s is no longer the file that was quarantined: %w", p, fsops.ErrChanged)
		}
	} else {
		if !now.SameInode(expect) {
			return fmt.Errorf("%s was replaced by another file: %w", p, fsops.ErrChanged)
		}
		if !now.Unchanged(expect.Size, expect.MtimeNs) {
			return fmt.Errorf("%s changed after it was compared (size %d -> %d, mtime %d -> %d): %w",
				p, expect.Size, now.Size, expect.MtimeNs, now.MtimeNs, fsops.ErrChanged)
		}
	}
	if err := os.Remove(p); err != nil {
		return err
	}
	return fsops.SyncDir(filepath.Dir(p))
}

// testHookBeforeRemove lets tests change the filesystem between creating the
// new link and removing the original. Always nil outside tests.
var testHookBeforeRemove func(slaveAbs string)

// stillQuarantined reports whether the original at p is intact and still the
// quarantined file, i.e. the quarantine copy is not needed after a rollback.
func stillQuarantined(p, q string) bool {
	pi, err1 := fsops.RegularFile(p)
	qi, err2 := fsops.RegularFile(q)
	return err1 == nil && err2 == nil && pi.SameInode(qi)
}

// pruneEmpty removes empty directories from dir upward, stopping at stop.
func pruneEmpty(dir, stop string) {
	for dir != stop && fsops.Within(stop, dir) {
		if err := os.Remove(dir); err != nil {
			return
		}
		dir = filepath.Dir(dir)
	}
}

func (e *Engine) begin(ctx context.Context, o *store.Operation) error {
	o.DryRun = e.st.DryRun(ctx)
	if o.Trigger == "" {
		o.Trigger = "manual"
	}
	o.Status = store.OpRunning
	if o.DryRun {
		o.Status = store.OpPlanned
	}
	return e.st.CreateOp(ctx, o)
}

func (e *Engine) finish(ctx context.Context, o *store.Operation, err error) error {
	if err != nil {
		o.Status, o.Error = store.OpFailed, err.Error()
		e.log.Warn("operation failed", "op", o.ID, "action", o.Action, "slave", o.RelSlave, "err", err)
	} else {
		o.Status = store.OpDone
		e.log.Info("operation done", "op", o.ID, "action", o.Action, "slave", o.RelSlave, "master", o.RelMaster, "new", o.RelNew)
	}
	if ferr := e.st.FinishOp(ctx, o.ID, o.Status, o.Error); ferr != nil && err == nil {
		return ferr
	}
	return err
}

func verdictOf(m store.Match) probe.Verdict {
	var v probe.Verdict
	_ = json.Unmarshal(m.Verdict, &v)
	return v
}

// Link replaces the slave file with a hardlink of the master file.
//
// Identical files are re-compared byte for byte right before linking and no
// quarantine copy is kept (the data exists in the master). Non-identical files
// are quarantined first so the operation can be undone.
func (e *Engine) Link(ctx context.Context, matchID int64, opt LinkOptions) (store.Operation, error) {
	e.mu.Lock()
	defer e.mu.Unlock()

	m, err := e.st.Match(ctx, matchID)
	if err != nil {
		return store.Operation{}, err
	}
	l, err := e.loadPair(ctx, m.PairID)
	if err != nil {
		return store.Operation{}, err
	}
	switch m.Status {
	case store.StatusIdentical, store.StatusDifferent, store.StatusSimilar:
	default:
		return store.Operation{}, fmt.Errorf("cannot link a match with status %q", m.Status)
	}
	if m.Ignored {
		return store.Operation{}, errors.New("match is ignored; un-ignore it first")
	}
	identical := m.Status == store.StatusIdentical
	if !identical && verdictOf(m).Better == "slave" && !opt.AllowDowngrade {
		return store.Operation{}, ErrDowngrade
	}
	s, err := preflight(l, m)
	if err != nil {
		return store.Operation{}, err
	}
	if identical {
		same, err := fsops.SameContent(s.masterAbs, s.slaveAbs)
		if err != nil {
			return store.Operation{}, err
		}
		if !same {
			e.refresh(ctx, l, m.RelSlave)
			return store.Operation{}, errors.New("files are no longer identical; moved to manual review")
		}
	}

	newRel := LinkTarget(m.RelSlave, m.RelMaster, opt.KeepFolder)
	newAbs, err := l.slave.Resolve(newRel)
	if err != nil {
		return store.Operation{}, err
	}
	rename := newRel != m.RelSlave
	// preexisting: the target already is a hardlink of the master (e.g. left
	// behind by an earlier failed link). Then only the original is retired.
	preexisting := false
	if rename {
		if cur, err := fsops.Lstat(newAbs); err == nil {
			if !cur.Mode.IsRegular() || !cur.SameInode(s.mi) {
				return store.Operation{}, fmt.Errorf("slave already has a different file at %s: %w", newRel, fsops.ErrExists)
			}
			preexisting = true
		}
	}

	o := store.Operation{PairID: l.pair.ID, Action: store.ActLink, RelMaster: m.RelMaster, RelSlave: m.RelSlave,
		RelNew: newRel, Trigger: opt.Trigger, MatchKind: m.Kind, MatchReason: m.Reason, LinkPreexisted: preexisting}
	if identical {
		o.Bytes = s.si.Size // space freed
	} else {
		o.QuarantineSide = "slave"
	}
	if err := e.begin(ctx, &o); err != nil {
		return o, err
	}
	if o.DryRun {
		return o, nil
	}

	err = func() error {
		var q string
		if !identical {
			q = quarantinePath(l, o.ID, "slave", m.RelSlave)
			if err := quarantine(s.slaveAbs, s.si, q); err != nil {
				return fmt.Errorf("quarantine: %w", err)
			}
			o.QuarantinePath = q
			if _, err := e.st.DB.ExecContext(ctx, `UPDATE operations SET quarantine_path = ? WHERE id = ?`, q, o.ID); err != nil {
				dropQuarantine(l, q, s.si)
				return err
			}
		}
		if !rename {
			if err := fsops.ReplaceWithLink(s.masterAbs, s.mi, s.slaveAbs); err != nil {
				if q != "" {
					if cur, e2 := fsops.Lstat(s.slaveAbs); e2 == nil && cur.SameInode(s.si) {
						dropQuarantine(l, q, s.si) // original untouched
						e.clearQuarantine(ctx, o.ID)
					}
				}
				return err
			}
			return nil
		}
		if !preexisting {
			err := fsops.MkdirAllLike(filepath.Dir(newAbs))
			if err == nil {
				err = fsops.LinkNew(s.masterAbs, s.mi, newAbs)
			}
			if err != nil {
				pruneEmpty(filepath.Dir(newAbs), l.slave.Path)
				if q != "" {
					dropQuarantine(l, q, s.si)
					e.clearQuarantine(ctx, o.ID)
				}
				return err
			}
		}
		if testHookBeforeRemove != nil {
			testHookBeforeRemove(s.slaveAbs)
		}
		if err := removeOriginal(s.slaveAbs, s.si, q); err != nil {
			// Never leave both copies behind: undo the new link (it is only
			// an extra name for the master file) and the quarantine copy.
			if !preexisting {
				if rbErr := fsops.RemoveLink(newAbs, s.mi); rbErr == nil {
					pruneEmpty(filepath.Dir(newAbs), l.slave.Path)
				}
			}
			if q != "" && stillQuarantined(s.slaveAbs, q) {
				dropQuarantine(l, q, s.si)
				e.clearQuarantine(ctx, o.ID)
			}
			return fmt.Errorf("could not remove the original slave file, so nothing was changed: %w", err)
		}
		pruneEmpty(filepath.Dir(s.slaveAbs), l.slave.Path) // e.g. the single's now-empty folder
		return nil
	}()
	err = e.finish(ctx, &o, err)
	e.refresh(ctx, l, m.RelSlave, m)
	if rename {
		e.refresh(ctx, l, newRel, m)
	}
	return o, err
}

func (e *Engine) clearQuarantine(ctx context.Context, opID int64) {
	_, _ = e.st.DB.ExecContext(ctx, `UPDATE operations SET quarantine_path = '' WHERE id = ?`, opID)
}

// Promote is the override path: the slave copy is better, so it replaces the
// master file (whose original is quarantined) and both libraries share it.
func (e *Engine) Promote(ctx context.Context, matchID int64, confirmed bool) (store.Operation, error) {
	if !confirmed {
		return store.Operation{}, ErrConfirm
	}
	return e.promote(ctx, matchID, "manual")
}

func (e *Engine) promote(ctx context.Context, matchID int64, trigger string) (store.Operation, error) {
	e.mu.Lock()
	defer e.mu.Unlock()

	m, err := e.st.Match(ctx, matchID)
	if err != nil {
		return store.Operation{}, err
	}
	l, err := e.loadPair(ctx, m.PairID)
	if err != nil {
		return store.Operation{}, err
	}
	if m.Status != store.StatusDifferent && m.Status != store.StatusSimilar {
		return store.Operation{}, fmt.Errorf("cannot promote a match with status %q", m.Status)
	}
	s, err := preflight(l, m)
	if err != nil {
		return store.Operation{}, err
	}
	newMasterRel := filepath.Join(filepath.Dir(m.RelMaster), filepath.Base(m.RelSlave))
	newMasterAbs, err := l.master.Resolve(newMasterRel)
	if err != nil {
		return store.Operation{}, err
	}
	rename := newMasterRel != m.RelMaster
	if rename {
		if _, err := os.Lstat(newMasterAbs); err == nil {
			return store.Operation{}, fmt.Errorf("master already has %s: %w", newMasterRel, fsops.ErrExists)
		}
	}

	o := store.Operation{PairID: l.pair.ID, Action: store.ActPromote, RelMaster: m.RelMaster, RelSlave: m.RelSlave,
		RelNew: newMasterRel, QuarantineSide: "master", Trigger: trigger, MatchKind: m.Kind, MatchReason: m.Reason}
	if err := e.begin(ctx, &o); err != nil {
		return o, err
	}
	if o.DryRun {
		return o, nil
	}
	err = func() error {
		q := quarantinePath(l, o.ID, "master", m.RelMaster)
		if err := quarantine(s.masterAbs, s.mi, q); err != nil {
			return fmt.Errorf("quarantine: %w", err)
		}
		o.QuarantinePath = q
		if _, err := e.st.DB.ExecContext(ctx, `UPDATE operations SET quarantine_path = ? WHERE id = ?`, q, o.ID); err != nil {
			dropQuarantine(l, q, s.mi)
			return err
		}
		if !rename {
			if err := fsops.ReplaceWithLink(s.slaveAbs, s.si, s.masterAbs); err != nil {
				if cur, e2 := fsops.Lstat(s.masterAbs); e2 == nil && cur.SameInode(s.mi) {
					dropQuarantine(l, q, s.mi)
					e.clearQuarantine(ctx, o.ID)
				}
				return err
			}
			return nil
		}
		if err := fsops.LinkNew(s.slaveAbs, s.si, newMasterAbs); err != nil {
			dropQuarantine(l, q, s.mi)
			e.clearQuarantine(ctx, o.ID)
			return err
		}
		if err := removeOriginal(s.masterAbs, s.mi, q); err != nil {
			_ = fsops.RemoveLink(newMasterAbs, s.si)
			if stillQuarantined(s.masterAbs, q) {
				dropQuarantine(l, q, s.mi)
				e.clearQuarantine(ctx, o.ID)
			}
			return fmt.Errorf("could not remove the original master file, so nothing was changed: %w", err)
		}
		return nil
	}()
	err = e.finish(ctx, &o, err)
	promoted := m
	if err == nil {
		promoted.RelMaster = newMasterRel // the master file may have been renamed
	}
	e.refresh(ctx, l, m.RelSlave, promoted)
	return o, err
}

// ---- build (fresh slave library) and remove ----

// expand turns a list of relative files/folders into audio files under root.
func expand(ctx context.Context, l *libs, root fsops.Root, rels []string) ([]string, error) {
	set := map[string]fsops.Info{}
	for _, rel := range rels {
		abs, err := root.Resolve(rel)
		if err != nil {
			return nil, err
		}
		fi, err := os.Lstat(abs)
		if err != nil {
			return nil, err
		}
		switch {
		case fi.IsDir():
			sub, err := fsops.NewRoot(abs)
			if err != nil {
				return nil, err
			}
			files := map[string]fsops.Info{}
			if _, err := walk(ctx, l, sub, files); err != nil {
				return nil, err
			}
			for r, in := range files {
				set[filepath.Join(rel, r)] = in
			}
		case fi.Mode().IsRegular() && l.hasExt(abs):
			set[filepath.Clean(rel)] = fsops.Info{}
		}
	}
	return sortedKeys(set), nil
}

// StartBuild hardlinks the chosen master files/folders into the slave library,
// mirroring the master path. Existing slave files are never overwritten.
func (e *Engine) StartBuild(pairID int64, rels []string) (*Job, error) {
	return e.startJob(pairID, "build", func(ctx context.Context, j *Job) error {
		l, err := e.loadPair(ctx, pairID)
		if err != nil {
			return err
		}
		files, err := expand(ctx, l, l.master, rels)
		if err != nil {
			return err
		}
		j.phase("Linking into slave library", int64(len(files)))
		for _, rel := range files {
			if ctx.Err() != nil {
				return ctx.Err()
			}
			e.buildOne(ctx, j, l, rel)
			j.tick()
		}
		return nil
	})
}

func (e *Engine) buildOne(ctx context.Context, j *Job, l *libs, rel string) {
	e.mu.Lock()
	defer e.mu.Unlock()
	masterAbs, err := l.master.Resolve(rel)
	if err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	slaveAbs, err := l.slave.Resolve(rel)
	if err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	mi, err := fsops.RegularFile(masterAbs)
	if err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	if si, err := fsops.Lstat(slaveAbs); err == nil {
		if si.SameInode(mi) {
			j.count("already linked")
		} else {
			j.skip(rel + ": a different file already exists in the slave library (scan, then use Review)")
		}
		return
	}
	o := store.Operation{PairID: l.pair.ID, Action: store.ActBuild, RelMaster: rel, RelSlave: rel, RelNew: rel, Trigger: "build"}
	if err := e.begin(ctx, &o); err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	if o.DryRun {
		j.count("planned")
		return
	}
	err = fsops.MkdirAllLike(filepath.Dir(slaveAbs))
	if err == nil {
		err = fsops.LinkNew(masterAbs, mi, slaveAbs)
	}
	if err = e.finish(ctx, &o, err); err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	j.count("linked")
	e.refresh(ctx, l, rel)
}

// StartRemove deletes slave files that are hardlinks of the master file at the
// same path. Anything that is not such a link is left alone.
func (e *Engine) StartRemove(pairID int64, rels []string) (*Job, error) {
	return e.startJob(pairID, "remove", func(ctx context.Context, j *Job) error {
		l, err := e.loadPair(ctx, pairID)
		if err != nil {
			return err
		}
		files, err := expand(ctx, l, l.slave, rels)
		if err != nil {
			return err
		}
		j.phase("Removing links from slave library", int64(len(files)))
		for _, rel := range files {
			if ctx.Err() != nil {
				return ctx.Err()
			}
			e.removeOne(ctx, j, l, rel)
			j.tick()
		}
		return nil
	})
}

func (e *Engine) removeOne(ctx context.Context, j *Job, l *libs, rel string) {
	e.mu.Lock()
	defer e.mu.Unlock()
	masterAbs, err1 := l.master.Resolve(rel)
	slaveAbs, err2 := l.slave.Resolve(rel)
	if err1 != nil || err2 != nil {
		j.skip(rel + ": invalid path")
		return
	}
	mi, err := fsops.RegularFile(masterAbs)
	if err != nil {
		j.skip(rel + ": no master file at the same path, so it is not removed")
		return
	}
	si, err := fsops.RegularFile(slaveAbs)
	if err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	if !si.SameInode(mi) {
		j.skip(rel + ": not a hardlink of the master file, so it is not removed")
		return
	}
	o := store.Operation{PairID: l.pair.ID, Action: store.ActRemove, RelMaster: rel, RelSlave: rel, Trigger: "build"}
	if err := e.begin(ctx, &o); err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	if o.DryRun {
		j.count("planned")
		return
	}
	err = fsops.RemoveLink(slaveAbs, mi)
	if err = e.finish(ctx, &o, err); err != nil {
		j.skip(rel + ": " + err.Error())
		return
	}
	pruneEmpty(filepath.Dir(slaveAbs), l.slave.Path)
	j.count("removed")
	e.refresh(ctx, l, rel)
}

// ---- undo, restore and purge ----

// Undo reverts a completed operation. Every branch verifies that the current
// file is still exactly what the operation left behind before touching it.
func (e *Engine) Undo(ctx context.Context, opID int64) error {
	e.mu.Lock()
	defer e.mu.Unlock()
	o, err := e.st.Op(ctx, opID)
	if err != nil {
		return err
	}
	if o.Status != store.OpDone || o.UndoneAt != nil {
		return errors.New("only completed operations that were not undone can be reverted")
	}
	if o.PurgedAt != nil {
		return errors.New("the quarantined original was already purged")
	}
	switch o.Action {
	case store.ActLink, store.ActPromote, store.ActBuild, store.ActRemove, store.ActRename, store.ActRemoveFolder, store.ActRemoveDupe:
	default:
		// History can hold operations of features that were removed (tag
		// edits, file moves); undoing them as a link would be wrong.
		return fmt.Errorf("%q operations can no longer be undone from Playlink; their originals stay in the quarantine folder", o.Action)
	}
	l, err := e.loadPair(ctx, o.PairID)
	if err != nil {
		return err
	}
	if o.Action == store.ActRemoveFolder {
		if err := undoRemoveFolder(l, o); err != nil {
			return err
		}
		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
			return err
		}
		e.log.Info("folder restored from quarantine", "op", o.ID)
		pruneEmpty(filepath.Dir(o.QuarantinePath), l.quarantine)
		return nil
	}
	if o.Action == store.ActRemoveDupe {
		if err := e.undoRemoveDupe(ctx, l, o); err != nil {
			return err
		}
		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
			return err
		}
		e.log.Info("duplicate restored from quarantine", "op", o.ID, "slave", o.RelSlave)
		return nil
	}
	if o.Action == store.ActRename {
		if err := undoRename(l, o); err != nil {
			return err
		}
		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
			return err
		}
		e.log.Info("folder rename undone", "op", o.ID, "folder", o.RelNew)
		_, _ = e.StartScan(o.PairID, "undo") // paths changed; ErrBusy just means a job already runs
		return nil
	}
	res := func(r fsops.Root, rel string) string {
		p, e := r.Resolve(rel)
		if e != nil && err == nil {
			err = e
		}
		return p
	}
	masterAbs := res(l.master, o.RelMaster)
	slaveAbs := res(l.slave, o.RelSlave)
	if err != nil {
		return err
	}

	switch o.Action {
	case store.ActBuild:
		mi, err := fsops.RegularFile(masterAbs)
		if err != nil {
			return err
		}
		if err := fsops.RemoveLink(slaveAbs, mi); err != nil {
			return err
		}
		pruneEmpty(filepath.Dir(slaveAbs), l.slave.Path)

	case store.ActRemove:
		mi, err := fsops.RegularFile(masterAbs)
		if err != nil {
			return err
		}
		if err := fsops.MkdirAllLike(filepath.Dir(slaveAbs)); err != nil {
			return err
		}
		if err := fsops.LinkNew(masterAbs, mi, slaveAbs); err != nil {
			return err
		}

	case store.ActLink:
		newAbs := res(l.slave, o.RelNew)
		if err != nil {
			return err
		}
		mi, err := fsops.RegularFile(masterAbs)
		if err != nil {
			return err
		}
		cur, err := fsops.RegularFile(newAbs)
		if err != nil {
			return err
		}
		if !cur.SameInode(mi) {
			return errors.New("the slave file is no longer linked to the master file; refusing to undo")
		}
		renamed := o.RelNew != o.RelSlave
		if renamed {
			if _, err := os.Lstat(slaveAbs); err == nil {
				return fmt.Errorf("%s: %w", o.RelSlave, fsops.ErrExists)
			}
			// The original folder may have been pruned when the link moved
			// the file into the master's folder structure.
			if err := fsops.MkdirAllLike(filepath.Dir(slaveAbs)); err != nil {
				return err
			}
		}
		if o.QuarantinePath == "" {
			// Identical link: give the slave its own copy of the bytes again.
			if err := fsops.CopyReplace(masterAbs, slaveAbs, mi.Mode); err != nil {
				return err
			}
		} else {
			qi, err := e.quarantined(l, o.QuarantinePath)
			if err != nil {
				return err
			}
			if renamed {
				err = fsops.LinkNew(o.QuarantinePath, qi, slaveAbs)
			} else {
				err = fsops.ReplaceWithLink(o.QuarantinePath, qi, slaveAbs)
			}
			if err != nil {
				return err
			}
			dropQuarantine(l, o.QuarantinePath, qi)
		}
		if renamed && !o.LinkPreexisted {
			if err := fsops.RemoveLink(newAbs, mi); err != nil {
				return err
			}
			pruneEmpty(filepath.Dir(newAbs), l.slave.Path)
		}
		e.refresh(ctx, l, o.RelNew)

	case store.ActPromote:
		newMasterAbs := res(l.master, o.RelNew)
		if err != nil {
			return err
		}
		si, err := fsops.RegularFile(slaveAbs)
		if err != nil {
			return err
		}
		cur, err := fsops.RegularFile(newMasterAbs)
		if err != nil {
			return err
		}
		if !cur.SameInode(si) {
			return errors.New("the master file changed after promotion; refusing to undo")
		}
		qi, err := e.quarantined(l, o.QuarantinePath)
		if err != nil {
			return err
		}
		if o.RelNew == o.RelMaster {
			err = fsops.ReplaceWithLink(o.QuarantinePath, qi, masterAbs)
		} else {
			if _, e2 := os.Lstat(masterAbs); e2 == nil {
				return fmt.Errorf("%s: %w", o.RelMaster, fsops.ErrExists)
			}
			if err = fsops.LinkNew(o.QuarantinePath, qi, masterAbs); err == nil {
				err = fsops.RemoveLink(newMasterAbs, si)
			}
		}
		if err != nil {
			return err
		}
		dropQuarantine(l, o.QuarantinePath, qi)

	default:
		return fmt.Errorf("cannot undo %q", o.Action)
	}
	if err := e.st.MarkUndone(ctx, o.ID); err != nil {
		return err
	}
	e.log.Info("operation undone", "op", o.ID, "action", o.Action)
	// Restore the original pairing (the master path before any promote rename).
	e.refresh(ctx, l, o.RelSlave, store.Match{RelMaster: o.RelMaster, Kind: o.MatchKind, Reason: o.MatchReason})
	return nil
}

// quarantined validates a quarantine path recorded in the database.
func (e *Engine) quarantined(l *libs, q string) (fsops.Info, error) {
	q = filepath.Clean(q)
	if !fsops.Within(l.quarantine, q) || q == l.quarantine {
		return fsops.Info{}, fmt.Errorf("quarantine path %s: %w", q, fsops.ErrOutsideRoot)
	}
	return fsops.RegularFile(q)
}

// Purge permanently deletes a quarantined original.
func (e *Engine) Purge(ctx context.Context, opID int64) error {
	e.mu.Lock()
	defer e.mu.Unlock()
	o, err := e.st.Op(ctx, opID)
	if err != nil {
		return err
	}
	return e.purge(ctx, o)
}

func (e *Engine) purge(ctx context.Context, o store.Operation) error {
	if o.QuarantinePath == "" || o.PurgedAt != nil || o.UndoneAt != nil {
		return errors.New("nothing to purge for this operation")
	}
	l, err := e.loadPair(ctx, o.PairID)
	if err != nil {
		return err
	}
	opDir := filepath.Join(l.quarantine, strconv.FormatInt(o.ID, 10))
	if !fsops.Within(l.quarantine, filepath.Clean(o.QuarantinePath)) || !fsops.Within(opDir, filepath.Clean(o.QuarantinePath)) {
		return fsops.ErrOutsideRoot
	}
	if err := os.RemoveAll(opDir); err != nil {
		return err
	}
	e.log.Info("quarantine purged", "op", o.ID, "path", o.QuarantinePath)
	return e.st.MarkPurged(ctx, o.ID)
}

// Maintenance purges expired quarantine entries and prunes sessions.
func (e *Engine) Maintenance(ctx context.Context) {
	e.st.PruneSessions(ctx)
	pairs, err := e.st.Pairs(ctx)
	if err != nil {
		return
	}
	for _, p := range pairs {
		if p.RetentionDays <= 0 {
			continue // keep forever
		}
		cutoff := time.Now().Add(-time.Duration(p.RetentionDays) * 24 * time.Hour).Unix()
		ops, err := e.st.ExpiredQuarantine(ctx, p.ID, cutoff)
		if err != nil {
			continue
		}
		for _, o := range ops {
			e.mu.Lock()
			if err := e.purge(ctx, o); err != nil {
				e.log.Warn("retention purge failed", "op", o.ID, "err", err)
			}
			e.mu.Unlock()
		}
	}
}

// Recover runs at startup: operations left running by a crash are marked
// interrupted and their disposable staging files are removed. Originals are
// never at risk because every replacement is an atomic rename.
func (e *Engine) Recover(ctx context.Context) {
	ops, err := e.st.RunningOps(ctx)
	if err != nil {
		return
	}
	for _, o := range ops {
		if o.Action == store.ActRename {
			if p, err := e.st.Pair(ctx, o.PairID); err == nil {
				recoverRename(p, o, e.log)
			}
			_ = e.st.FinishOp(ctx, o.ID, store.OpInterrupted, "interrupted by shutdown; rescan to verify state")
			continue
		}
		if p, err := e.st.Pair(ctx, o.PairID); err == nil {
			for _, root := range []string{p.SlaveRoot, p.MasterRoot} {
				for _, rel := range []string{o.RelSlave, o.RelNew, o.RelMaster} {
					if rel == "" {
						continue
					}
					dir := filepath.Join(root, filepath.Dir(rel))
					ents, _ := os.ReadDir(dir)
					for _, en := range ents {
						if strings.HasPrefix(en.Name(), fsops.TmpPrefix) && en.Type().IsRegular() {
							_ = os.Remove(filepath.Join(dir, en.Name()))
						}
					}
				}
			}
		}
		_ = e.st.FinishOp(ctx, o.ID, store.OpInterrupted, "interrupted by shutdown; rescan to verify state")
		e.log.Warn("marked interrupted operation", "op", o.ID)
	}
}

// ---- browsing (build mode) ----

type TreeEntry struct {
	Name   string `json:"name"`
	Rel    string `json:"rel"`
	Dir    bool   `json:"dir"`
	Size   int64  `json:"size,omitempty"`
	State  string `json:"state,omitempty"` // files: linked, conflict, missing
	Linked int    `json:"linked,omitempty"`
}

// Tree lists a master directory and shows how much of it is in the slave.
func (e *Engine) Tree(ctx context.Context, pairID int64, rel string) ([]TreeEntry, error) {
	l, err := e.loadPair(ctx, pairID)
	if err != nil {
		return nil, err
	}
	dir := l.master.Path
	if rel != "" {
		if dir, err = l.master.Resolve(rel); err != nil {
			return nil, err
		}
	}
	ents, err := os.ReadDir(dir)
	if err != nil {
		return nil, err
	}
	linked, _ := e.st.LinkedMasterPaths(ctx, pairID, rel)
	perChild := map[string]int{}
	for _, p := range linked {
		sub := p
		if rel != "" {
			sub = strings.TrimPrefix(p, rel+string(filepath.Separator))
		}
		if i := strings.IndexByte(sub, filepath.Separator); i > 0 {
			perChild[sub[:i]]++
		}
	}
	out := []TreeEntry{}
	for _, en := range ents {
		name := en.Name()
		if strings.HasPrefix(name, ".") || en.Type()&fs.ModeSymlink != 0 {
			continue
		}
		child := name
		if rel != "" {
			child = filepath.Join(rel, name)
		}
		te := TreeEntry{Name: name, Rel: child, Dir: en.IsDir()}
		if en.IsDir() {
			te.Linked = perChild[name]
		} else {
			if !l.hasExt(name) {
				continue
			}
			mi, err := fsops.Lstat(filepath.Join(dir, name))
			if err != nil {
				continue
			}
			te.Size = mi.Size
			te.State = "missing"
			if sp, err := l.slave.Resolve(child); err == nil {
				if si, err := fsops.Lstat(sp); err == nil {
					te.State = "conflict"
					if si.SameInode(mi) {
						te.State = "linked"
					}
				}
			}
		}
		out = append(out, te)
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Dir != out[j].Dir {
			return out[i].Dir
		}
		return strings.ToLower(out[i].Name) < strings.ToLower(out[j].Name)
	})
	return out, nil
}