package library

import (
	"context"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"slices"
	"strings"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"

	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
)

// The library's folders (#6129). They are set in Settings and changed without
// a restart; MINSTREL_LIBRARY_SCAN_PATHS names them until the first save. The
// Scanner holds the live list, and the watcher and the importer read it there.
//
// Changing a folder's path is a move, not a new library: when the files that
// were under /music are now under /data/music, every row under the old path is
// rewritten to the new one, so the tracks keep their ids and their history.
// The scan never sees the change as a mass deletion followed by a mass add.

// LoadRoots is the folders the server starts with: the saved ones, or env
// when nothing has been saved. fromSettings says which.
func LoadRoots(ctx context.Context, pool *pgxpool.Pool, env []string) (roots []string, fromSettings bool, err error) {
	row, err := dbq.New(pool).GetLibrarySettings(ctx)
	if err != nil {
		return env, false, fmt.Errorf("library settings: load: %w", err)
	}
	if len(row.ScanPaths) == 0 {
		return env, false, nil
	}
	return row.ScanPaths, true, nil
}

// RootChange is one folder of a save. Was is the folder it replaces, whose
// files have moved to Path; "" is a folder added.
type RootChange struct {
	Path string
	Was  string
}

// RerootResult is what a save rewrote.
type RerootResult struct {
	Roots  []string
	Tracks int64
}

// RootsError is a save the folders refuse. Its text says why, in a sentence
// for the operator.
type RootsError struct{ msg string }

func (e *RootsError) Error() string { return e.msg }

func badRoots(format string, args ...any) error {
	return &RootsError{msg: fmt.Sprintf(format, args...)}
}

// moveSample is how many present tracks a move checks for at the new path,
// and moveSampleMinFound how many of them must be there.
const (
	moveSample         = 20
	moveSampleMinFound = 0.5
)

// SetRoots validates the folders, rewrites the paths of any that moved, saves
// them and starts reading them. Nothing is written when any check fails.
func (s *Scanner) SetRoots(ctx context.Context, changes []RootChange) (RerootResult, error) {
	s.rootsMu.Lock()
	defer s.rootsMu.Unlock()

	roots, moves, err := planRoots(s.paths, changes)
	if err != nil {
		return RerootResult{}, err
	}
	q := dbq.New(s.pool)
	// A scan walking the old folders would put their paths back, or mark the
	// moved rows missing when it reconciles.
	_, err = q.GetInFlightScanRun(ctx)
	if err == nil {
		return RerootResult{}, badRoots("A library scan is running; save again once it finishes.")
	}
	if !errors.Is(err, pgx.ErrNoRows) {
		return RerootResult{}, fmt.Errorf("in-flight scan check: %w", err)
	}
	for _, m := range moves {
		if err := checkMove(ctx, q, m); err != nil {
			return RerootResult{}, err
		}
	}

	var res RerootResult
	err = pgx.BeginFunc(ctx, s.pool, func(tx pgx.Tx) error {
		tq := dbq.New(tx)
		for _, m := range moves {
			n, err := tq.RerootTrackPaths(ctx, dbq.RerootTrackPathsParams{NewRoot: m.Path, OldRoot: m.Was})
			if err != nil {
				if isUniqueViolation(err) {
					return badRoots("Some tracks under %s are already in the library at %s, "+
						"so its tracks cannot move there.", m.Was, m.Path)
				}
				return fmt.Errorf("reroot tracks: %w", err)
			}
			res.Tracks += n
			if _, err := tq.RerootAlbumCoverPaths(ctx, dbq.RerootAlbumCoverPathsParams{NewRoot: m.Path, OldRoot: m.Was}); err != nil {
				return fmt.Errorf("reroot covers: %w", err)
			}
			if _, err := tq.RerootGrabDestinations(ctx, dbq.RerootGrabDestinationsParams{NewRoot: m.Path, OldRoot: m.Was}); err != nil {
				return fmt.Errorf("reroot downloads: %w", err)
			}
			if _, err := tq.RerootImportRoot(ctx, dbq.RerootImportRootParams{NewRoot: m.Path, OldRoot: m.Was}); err != nil {
				return fmt.Errorf("reroot import folder: %w", err)
			}
		}
		_, err := tq.UpdateLibraryScanPaths(ctx, roots)
		return err
	})
	if err != nil {
		return RerootResult{}, err
	}
	for _, m := range moves {
		s.logger.Info("library folder moved", "from", m.Was, "to", m.Path)
	}
	s.paths = roots
	select {
	case s.rootsChanged <- struct{}{}:
	default:
	}
	res.Roots = slices.Clone(roots)
	return res, nil
}

// RootsChanged is signalled after a save changes the folders. A Scanner built
// without New never signals.
func (s *Scanner) RootsChanged() <-chan struct{} { return s.rootsChanged }

// planRoots checks a save against the current folders and returns the new
// list and the folders that moved.
func planRoots(current []string, changes []RootChange) ([]string, []RootChange, error) {
	if len(changes) == 0 {
		return nil, nil, badRoots("Name at least one folder.")
	}
	roots := make([]string, 0, len(changes))
	var moves []RootChange
	replaced := map[string]bool{}
	for _, c := range changes {
		p := strings.TrimSpace(c.Path)
		if !filepath.IsAbs(p) {
			return nil, nil, badRoots("%q is not a full path; it starts with /.", c.Path)
		}
		p = filepath.Clean(p)
		if p == "/" {
			return nil, nil, badRoots("The library cannot be the whole filesystem.")
		}
		if err := checkDir(p); err != nil {
			return nil, nil, err
		}
		for _, r := range roots {
			if r == p {
				return nil, nil, badRoots("%s is listed twice.", p)
			}
			if within(p, r) || within(r, p) {
				return nil, nil, badRoots("%s and %s overlap; name one of them.", r, p)
			}
		}
		roots = append(roots, p)
		if c.Was == "" || c.Was == p {
			continue
		}
		if !slices.Contains(current, c.Was) {
			return nil, nil, badRoots("%s is not one of the library's folders.", c.Was)
		}
		if replaced[c.Was] {
			return nil, nil, badRoots("%s can only move to one place.", c.Was)
		}
		replaced[c.Was] = true
		moves = append(moves, RootChange{Path: p, Was: c.Was})
	}
	return roots, moves, nil
}

// checkDir refuses a folder the server cannot read: a typo, or a mount the
// container does not have.
func checkDir(p string) error {
	info, err := os.Stat(p)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return badRoots("%s does not exist on the server. Is it mounted into the container?", p)
		}
		return badRoots("%s cannot be read: %v.", p, err)
	}
	if !info.IsDir() {
		return badRoots("%s is a file, not a folder.", p)
	}
	if _, err := os.ReadDir(p); err != nil {
		return badRoots("%s cannot be read: %v.", p, err)
	}
	return nil
}

// checkMove looks for a sample of the folder's tracks at the new path, so a
// move to the wrong folder is refused before any row is rewritten.
func checkMove(ctx context.Context, q *dbq.Queries, m RootChange) error {
	sample, err := q.SampleTrackPathsUnder(ctx, dbq.SampleTrackPathsUnderParams{Root: m.Was, N: moveSample})
	if err != nil {
		return fmt.Errorf("sample tracks: %w", err)
	}
	if len(sample) == 0 {
		return nil
	}
	found := 0
	for _, p := range sample {
		if _, err := os.Stat(m.Path + strings.TrimPrefix(p, m.Was)); err == nil {
			found++
		}
	}
	if float64(found) < moveSampleMinFound*float64(len(sample)) {
		return badRoots("Only %d of %d tracks checked from %s are at the same place under %s. "+
			"Is that the folder they moved to?", found, len(sample), m.Was, m.Path)
	}
	return nil
}

// within reports whether p is inside dir.
func within(p, dir string) bool {
	return strings.HasPrefix(p, dir+"/")
}