package engine

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"strings"

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

// Folder renames fix the "named differently" warning: slave folders that are
// the same as a master folder under another spelling ("City Of Evil" vs
// "City of Evil", "AC-DC" vs "AC_DC") are renamed to the master's spelling so
// their songs match by path. Only the slave library is ever renamed.

// FolderRename is one proposed rename of a slave folder.
type FolderRename struct {
	Slave  string `json:"slave"`  // current slave folder (relative)
	Master string `json:"master"` // the master's spelling = the new slave name
	Ready  bool   `json:"ready"`
	Reason string `json:"reason,omitempty"` // why it cannot be renamed
}

// maxRenamePasses bounds the loop that picks up folders revealed by renaming
// their parent (an artist rename can expose album folders that also differ).
const maxRenamePasses = 3

// planRenames lists the renames needed right now, marking conflicts.
func planRenames(l *libs) []FolderRename {
	var out []FolderRename
	for _, p := range renamedFolders(l) {
		r := FolderRename{Slave: p[0], Master: p[1], Ready: true}
		r.Reason = checkRename(l, r.Slave, r.Master)
		r.Ready = r.Reason == ""
		out = append(out, r)
	}
	return out
}

// checkRename returns why a slave folder cannot be renamed, or "".
func checkRename(l *libs, from, to string) string {
	src, err := l.slave.Resolve(from)
	if err != nil {
		return err.Error()
	}
	dst, err := l.slave.Resolve(to)
	if err != nil {
		return err.Error()
	}
	fi, err := os.Lstat(src)
	switch {
	case err != nil:
		return "folder no longer exists (rescan)"
	case fi.Mode()&os.ModeSymlink != 0:
		return "is a symlink"
	case !fi.IsDir():
		return "is not a folder"
	case src == l.quarantine || fsops.Within(src, l.quarantine):
		return "contains the quarantine folder"
	}
	if filepath.Dir(from) != filepath.Dir(to) {
		return "rename would move the folder"
	}
	// A case-only rename "exists" on case-insensitive filesystems; that is fine.
	if di, err := os.Lstat(dst); err == nil && !(strings.EqualFold(from, to) && os.SameFile(fi, di)) {
		return fmt.Sprintf("the slave already has a folder named %q; merge the two by hand", filepath.Base(to))
	}
	return ""
}

// PreviewRenames lists the folder renames that would be made.
func (e *Engine) PreviewRenames(ctx context.Context, pairID int64) ([]FolderRename, error) {
	l, err := e.loadPair(ctx, pairID)
	if err != nil {
		return nil, err
	}
	out := planRenames(l)
	if out == nil {
		out = []FolderRename{}
	}
	return out, nil
}

// StartRenames renames slave folders to the master's spelling, then rescans.
// With only set, just those renames are made (by slave path); they must be
// among the renames the server computes itself, so a request can never name
// arbitrary paths. Later passes (folders revealed inside renamed ones) only
// run when renaming everything.
func (e *Engine) StartRenames(pairID int64, only []string) (*Job, error) {
	return e.startJob(pairID, "rename-folders", func(ctx context.Context, j *Job) error {
		l, err := e.loadPair(ctx, pairID)
		if err != nil {
			return err
		}
		want := map[string]bool{}
		for _, s := range only {
			want[filepath.Clean(s)] = true
		}
		done := map[string]bool{}
		for pass := 0; pass < maxRenamePasses; pass++ {
			var todo []FolderRename
			for _, r := range planRenames(l) {
				if done[r.Slave] || (len(want) > 0 && !want[r.Slave]) {
					continue
				}
				todo = append(todo, r)
			}
			if len(todo) == 0 {
				break
			}
			j.phase(fmt.Sprintf("Renaming slave folders (pass %d)", pass+1), int64(len(todo)))
			for _, r := range todo {
				if ctx.Err() != nil {
					return ctx.Err()
				}
				done[r.Slave] = true
				if !r.Ready {
					j.skip(r.Slave + ": " + r.Reason)
				} else if err := e.renameOne(ctx, j, l, r); err != nil {
					j.skip(r.Slave + ": " + err.Error())
				}
				j.tick()
			}
			if len(want) > 0 || e.st.DryRun(ctx) {
				break // a dry run renames nothing, so there is nothing new to find
			}
		}
		if e.st.DryRun(ctx) {
			j.update(func(j *Job) { j.Message = "Dry run: renames recorded as planned in Activity" })
			return nil
		}
		// Paths changed, so the stored matches are stale.
		return e.scan(ctx, j, pairID, "rename")
	})
}

func (e *Engine) renameOne(ctx context.Context, j *Job, l *libs, r FolderRename) error {
	e.mu.Lock()
	defer e.mu.Unlock()
	if why := checkRename(l, r.Slave, r.Master); why != "" { // re-check under the lock
		return errors.New(why)
	}
	src, _ := l.slave.Resolve(r.Slave)
	dst, _ := l.slave.Resolve(r.Master)
	o := store.Operation{PairID: l.pair.ID, Action: store.ActRename, RelSlave: r.Slave, RelNew: r.Master, Trigger: "rename"}
	if err := e.begin(ctx, &o); err != nil {
		return err
	}
	if o.DryRun {
		j.count("planned")
		return nil
	}
	err := renameDir(src, dst)
	if err = e.finish(ctx, &o, err); err != nil {
		return err
	}
	j.count("renamed")
	return nil
}

// renameDir renames a folder. Case-only renames go through a temporary name
// so they also work on case-insensitive filesystems. The destination must not
// exist (checked by the caller under the engine lock).
func renameDir(src, dst string) error {
	if strings.EqualFold(src, dst) {
		var b [6]byte
		if _, err := rand.Read(b[:]); err != nil {
			return err
		}
		tmp := filepath.Join(filepath.Dir(src), fsops.TmpPrefix+"dir-"+hex.EncodeToString(b[:]))
		if err := os.Rename(src, tmp); err != nil {
			return err
		}
		if err := os.Rename(tmp, dst); err != nil {
			_ = os.Rename(tmp, src) // put it back
			return err
		}
	} else if err := os.Rename(src, dst); err != nil {
		return err
	}
	return fsops.SyncDir(filepath.Dir(dst))
}

// undoRename renames a folder back to its old name.
func undoRename(l *libs, o store.Operation) error {
	cur, err := l.slave.Resolve(o.RelNew)
	if err != nil {
		return err
	}
	old, err := l.slave.Resolve(o.RelSlave)
	if err != nil {
		return err
	}
	fi, err := os.Lstat(cur)
	if err != nil || !fi.IsDir() {
		return fmt.Errorf("the folder %q no longer exists", o.RelNew)
	}
	if di, err := os.Lstat(old); err == nil && !(strings.EqualFold(o.RelNew, o.RelSlave) && os.SameFile(fi, di)) {
		return fmt.Errorf("%q exists again: %w", o.RelSlave, fsops.ErrExists)
	}
	return renameDir(cur, old)
}

// recoverRename finishes a case-only folder rename cut short by a crash: the
// folder was moved to a temporary name and never reached its new name. The
// folder holds user data, so it is renamed, never removed.
func recoverRename(p store.Pair, o store.Operation, log *slog.Logger) {
	dir := filepath.Join(p.SlaveRoot, filepath.Dir(o.RelSlave))
	from := filepath.Join(p.SlaveRoot, o.RelSlave)
	to := filepath.Join(p.SlaveRoot, o.RelNew)
	if _, err := os.Lstat(from); err == nil {
		return // never moved
	}
	if _, err := os.Lstat(to); err == nil {
		return // completed
	}
	ents, _ := os.ReadDir(dir)
	for _, en := range ents {
		if en.IsDir() && strings.HasPrefix(en.Name(), fsops.TmpPrefix+"dir-") {
			if err := os.Rename(filepath.Join(dir, en.Name()), to); err == nil {
				log.Warn("finished interrupted folder rename", "op", o.ID, "folder", o.RelNew)
			}
			return
		}
	}
}