// Package store implements the flat-file object store: one directory per
// object, holding the blob and a JSON metadata sidecar. There is no database;
// an in-memory index is rebuilt from disk at startup and kept in sync.
package store

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"hash"
	"io"
	"io/fs"
	"os"
	"path/filepath"
	"sync"
	"time"
)

const (
	blobName = "blob"
	partName = "blob.part"
	metaName = "meta.json"

	dirPerm  fs.FileMode = 0o775
	filePerm fs.FileMode = 0o664

	// debrisMaxAge is how long an object directory with no metadata is left
	// alone before being treated as the remains of a killed upload.
	debrisMaxAge = 24 * time.Hour
)

var (
	ErrNotFound = errors.New("object not found")
	ErrExists   = errors.New("name already taken")
	ErrTooLarge = errors.New("upload exceeds the size limit")
)

// Store owns the data directory.
type Store struct {
	dir     string
	objects string

	// root confines every object file operation to the objects directory.
	// The data directory is group-writable by design, so a symlink planted
	// there must not be able to redirect a read or a write outside it.
	root *os.Root

	mu    sync.RWMutex
	index map[string]*Meta
	total int64
}

// Open prepares the data directory and rebuilds the index from it.
func Open(dir string) (*Store, error) {
	s := &Store{
		dir:     dir,
		objects: filepath.Join(dir, "objects"),
		index:   make(map[string]*Meta),
	}
	if err := os.MkdirAll(s.objects, dirPerm); err != nil {
		return nil, err
	}
	root, err := os.OpenRoot(s.objects)
	if err != nil {
		return nil, err
	}
	s.root = root
	if err := s.load(); err != nil {
		return nil, err
	}
	return s, nil
}

// DataDir is the directory the store was opened on.
func (s *Store) DataDir() string { return s.dir }

// Close releases the handle on the objects directory.
func (s *Store) Close() error { return s.root.Close() }

// objectDir builds an object's path for display. Actual file operations go
// through s.root instead, which cannot be walked out of.
func (s *Store) objectDir(id string) string { return filepath.Join(s.objects, id) }

// within builds a root-relative path for one of an object's files.
func within(id, name string) string { return id + "/" + name }

func (s *Store) load() error {
	entries, err := os.ReadDir(s.objects)
	if err != nil {
		return err
	}
	for _, e := range entries {
		if !e.IsDir() {
			continue
		}
		id, err := CleanID(e.Name())
		if err != nil || id != e.Name() {
			// Not a name this service could have created; leave it be.
			continue
		}
		m, err := s.readMeta(id)
		if err != nil {
			continue // incomplete or unreadable; the debris sweep handles it
		}
		// A leftover .part in a committed object is always stale at startup.
		s.root.Remove(within(id, partName))
		s.index[id] = m
		s.total += m.Size
	}
	return nil
}

func (s *Store) readMeta(id string) (*Meta, error) {
	b, err := s.root.ReadFile(within(id, metaName))
	if err != nil {
		return nil, err
	}
	var m Meta
	if err := json.Unmarshal(b, &m); err != nil {
		return nil, err
	}
	if m.ID != id {
		return nil, fmt.Errorf("store: metadata for %q claims id %q", id, m.ID)
	}
	return &m, nil
}

// Exists reports whether a name is currently taken, expired objects included:
// a name stays claimed until its object is actually removed.
func (s *Store) Exists(id string) bool {
	_, err := s.root.Lstat(id)
	return err == nil
}

// Reserve claims a name by creating its directory. os.Mkdir is atomic, so this
// is the point at which a vanity collision is detected - before any of the
// caller's body has been read.
func (s *Store) Reserve(id string) (*Upload, error) {
	if err := s.root.Mkdir(id, dirPerm); err != nil {
		if errors.Is(err, fs.ErrExist) {
			return nil, ErrExists
		}
		return nil, err
	}
	f, err := s.root.OpenFile(within(id, partName), os.O_WRONLY|os.O_CREATE|os.O_EXCL, filePerm)
	if err != nil {
		s.root.RemoveAll(id)
		return nil, err
	}
	return &Upload{s: s, id: id, f: f, h: sha256.New()}, nil
}

// ReserveRandom claims a fresh UUIDv4 name.
func (s *Store) ReserveRandom() (*Upload, error) {
	for range 8 {
		id, err := NewUUID()
		if err != nil {
			return nil, err
		}
		u, err := s.Reserve(id)
		if errors.Is(err, ErrExists) {
			continue // astronomically unlikely; retry regardless
		}
		return u, err
	}
	return nil, errors.New("store: could not allocate an unused id")
}

// Upload is an in-flight object. It is an io.Writer so callers can stream a
// request body straight to disk; nothing is ever buffered in memory.
type Upload struct {
	s     *Store
	id    string
	f     *os.File
	h     hash.Hash
	n     int64
	limit int64 // 0 means unlimited
	done  bool
}

func (u *Upload) ID() string  { return u.id }
func (u *Upload) Size() int64 { return u.n }

// SetLimit caps the number of bytes the upload will accept. The cap is applied
// to bytes actually written, never to a declared Content-Length.
func (u *Upload) SetLimit(n int64) { u.limit = n }

func (u *Upload) Write(p []byte) (int, error) {
	if u.limit > 0 && u.n+int64(len(p)) > u.limit {
		return 0, ErrTooLarge
	}
	n, err := u.f.Write(p)
	u.n += int64(n)
	u.h.Write(p[:n])
	return n, err
}

// Commit makes the object visible. The ordering matters: the blob is durable
// and in place before the metadata that advertises it is written, and the
// metadata is renamed into place atomically.
func (u *Upload) Commit(m *Meta) error {
	if u.done {
		return errors.New("store: upload already finished")
	}
	m.ID = u.id
	m.Size = u.n
	m.SHA256 = hex.EncodeToString(u.h.Sum(nil))

	if err := u.f.Sync(); err != nil {
		return err
	}
	if err := u.f.Close(); err != nil {
		return err
	}
	if err := u.s.root.Rename(within(u.id, partName), within(u.id, blobName)); err != nil {
		return err
	}
	if err := u.s.writeMetaAtomic(u.id, m); err != nil {
		return err
	}
	if err := u.s.syncDir(u.id); err != nil {
		return err
	}
	u.done = true

	u.s.mu.Lock()
	u.s.index[u.id] = m
	u.s.total += m.Size
	u.s.mu.Unlock()
	return nil
}

// Abort discards an incomplete upload, releasing its name.
func (u *Upload) Abort() {
	if u.done {
		return
	}
	u.done = true
	u.f.Close()
	u.s.root.RemoveAll(u.id)
}

// writeMetaAtomic serialises m to a temporary file in the object's own
// directory, fsyncs it, and renames it into place. Only once this rename lands
// does the object become visible to a reader.
func (s *Store) writeMetaAtomic(id string, m *Meta) error {
	b, err := json.MarshalIndent(m, "", "  ")
	if err != nil {
		return err
	}
	b = append(b, '\n')

	suffix, err := NewSecret()
	if err != nil {
		return err
	}
	tmpPath := within(id, "."+metaName+"."+suffix[:16])

	tmp, err := s.root.OpenFile(tmpPath, os.O_WRONLY|os.O_CREATE|os.O_EXCL, filePerm)
	if err != nil {
		return err
	}
	defer s.root.Remove(tmpPath) // no-op once the rename succeeds

	if _, err := tmp.Write(b); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	return s.root.Rename(tmpPath, within(id, metaName))
}

// syncDir flushes a directory entry so a rename survives a power loss.
func (s *Store) syncDir(id string) error {
	d, err := s.root.Open(id)
	if err != nil {
		return err
	}
	defer d.Close()
	return d.Sync()
}

// Get returns an object's metadata, treating an expired object as absent and
// removing it on the spot. Expiry is checked here, on every read, so a stalled
// sweeper can never serve a file past its lifetime.
func (s *Store) Get(id string, now time.Time) (*Meta, error) {
	s.mu.RLock()
	m, ok := s.index[id]
	s.mu.RUnlock()
	if !ok {
		return nil, ErrNotFound
	}
	if m.Expired(now) {
		s.Delete(id)
		return nil, ErrNotFound
	}
	return m, nil
}

// OpenBlob returns the metadata and an open handle to the object's bytes.
func (s *Store) OpenBlob(id string, now time.Time) (*Meta, *os.File, error) {
	m, err := s.Get(id, now)
	if err != nil {
		return nil, nil, err
	}
	f, err := s.root.Open(within(id, blobName))
	if err != nil {
		// Metadata without a blob means the data directory was tampered with.
		s.Delete(id)
		return nil, nil, ErrNotFound
	}
	return m, f, nil
}

// Delete removes an object and frees its name.
func (s *Store) Delete(id string) error {
	s.mu.Lock()
	if m, ok := s.index[id]; ok {
		s.total -= m.Size
		delete(s.index, id)
	}
	s.mu.Unlock()
	return s.root.RemoveAll(id)
}

// Total reports the number of bytes currently stored.
func (s *Store) Total() int64 {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.total
}

// Count reports the number of live objects.
func (s *Store) Count() int {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return len(s.index)
}

// Sweep removes expired objects and long-abandoned upload directories. It
// returns the number of objects removed.
func (s *Store) Sweep(now time.Time) int {
	s.mu.RLock()
	var expired []string
	for id, m := range s.index {
		if m.Expired(now) {
			expired = append(expired, id)
		}
	}
	s.mu.RUnlock()

	for _, id := range expired {
		s.Delete(id)
	}
	s.sweepDebris(now)
	return len(expired)
}

// sweepDebris removes object directories that never gained metadata and are
// older than debrisMaxAge - the remains of an upload killed mid-flight.
func (s *Store) sweepDebris(now time.Time) {
	entries, err := os.ReadDir(s.objects)
	if err != nil {
		return
	}
	for _, e := range entries {
		if !e.IsDir() {
			continue
		}
		s.mu.RLock()
		_, live := s.index[e.Name()]
		s.mu.RUnlock()
		if live {
			continue
		}
		info, err := e.Info()
		if err != nil || now.Sub(info.ModTime()) < debrisMaxAge {
			continue
		}
		if _, err := s.root.Stat(within(e.Name(), metaName)); err == nil {
			continue // has metadata but is not indexed; leave it for a human
		}
		s.root.RemoveAll(e.Name())
	}
}

// List returns every live object's metadata, for administrative use.
func (s *Store) List() []*Meta {
	s.mu.RLock()
	defer s.mu.RUnlock()
	out := make([]*Meta, 0, len(s.index))
	for _, m := range s.index {
		out = append(out, m)
	}
	return out
}

var _ io.Writer = (*Upload)(nil)