// Package blob stores the KDBX payloads. Each database keeps a numbered
// version history, which makes a rejected push recoverable and a bad save
// reversible.
//
// The payloads live in the server's database next to the metadata, sealed
// under a key derived from the master key and named by a keyed digest of
// the database's id. KDBX is already encrypted by KeePass; sealing it again
// means the database holds no readable KeePass headers either, and that one
// backup of the database is the whole server.
package blob

import (
	"context"
	"crypto/sha256"
	"database/sql"
	"encoding/hex"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"time"

	"cc.tiredsysadmin.keeporg/internal/secret"
	"cc.tiredsysadmin.keeporg/internal/sqldb"
)

// KeepVersions is how many historical versions of each database are retained.
const KeepVersions = 20

// timeout bounds one blob operation. A payload is at most tens of megabytes.
const timeout = time.Minute

type Store struct {
	db  *sqldb.DB
	key *secret.Keyring
}

// Open returns the blob store in db, sealing with key.
func Open(db *sqldb.DB, key *secret.Keyring) *Store { return &Store{db: db, key: key} }

// SHA256 is the lowercase hex digest used everywhere as a content fingerprint.
func SHA256(data []byte) string {
	sum := sha256.Sum256(data)
	return hex.EncodeToString(sum[:])
}

func (s *Store) ref(dbID string) string { return s.key.MAC("blob\x00" + dbID) }

// aad binds a sealed payload to its database and version, so a row copied
// over another one fails to open instead of serving the wrong file.
func aad(ref string, version int64) []byte {
	return []byte(ref + "/" + strconv.FormatInt(version, 10))
}

// Put writes one version of a database. Writing a version again replaces it,
// so a retried request is harmless.
func (s *Store) Put(dbID string, version int64, data []byte) error {
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()
	return s.put(ctx, s.db, dbID, version, data)
}

// PutTx is Put inside a transaction, for a version written together with
// the metadata that names it: a version a concurrent writer committed can
// then never be overwritten by a write whose metadata turns out stale.
func (s *Store) PutTx(ctx context.Context, tx *sql.Tx, dbID string, version int64, data []byte) error {
	return s.put(ctx, tx, dbID, version, data)
}

func (s *Store) put(ctx context.Context, q sqldb.Querier, dbID string, version int64, data []byte) error {
	ref := s.ref(dbID)
	_, err := q.ExecContext(ctx, s.db.Q(`INSERT INTO keeporg_blobs (ref, version, body)
		VALUES (?, ?, ?) ON CONFLICT (ref, version) DO UPDATE SET body = excluded.body`),
		ref, version, s.key.Seal(aad(ref, version), data))
	return err
}

// Get reads one version of a database. A version that is not stored is
// fs.ErrNotExist.
func (s *Store) Get(dbID string, version int64) ([]byte, error) {
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()
	ref := s.ref(dbID)
	var body []byte
	err := s.db.QueryRowContext(ctx, s.db.Q(`SELECT body FROM keeporg_blobs
		WHERE ref = ? AND version = ?`), ref, version).Scan(&body)
	if errors.Is(err, sql.ErrNoRows) {
		return nil, fmt.Errorf("blob %s v%d: %w", dbID, version, fs.ErrNotExist)
	}
	if err != nil {
		return nil, err
	}
	return s.key.Open(aad(ref, version), body)
}

// Versions lists the stored versions of a database, oldest first.
func (s *Store) Versions(dbID string) []int64 {
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()
	rows, err := s.db.QueryContext(ctx, s.db.Q(`SELECT version FROM keeporg_blobs
		WHERE ref = ? ORDER BY version`), s.ref(dbID))
	if err != nil {
		return nil
	}
	defer rows.Close()
	out := []int64{}
	for rows.Next() {
		var v int64
		if rows.Scan(&v) == nil {
			out = append(out, v)
		}
	}
	return out
}

// Prune removes all but the newest KeepVersions versions.
func (s *Store) Prune(dbID string) error {
	vs := s.Versions(dbID)
	if len(vs) <= KeepVersions {
		return nil
	}
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()
	_, err := s.db.ExecContext(ctx, s.db.Q(`DELETE FROM keeporg_blobs WHERE ref = ? AND version < ?`),
		s.ref(dbID), vs[len(vs)-KeepVersions])
	return err
}

// Forget deletes every version of a database.
func (s *Store) Forget(dbID string) error {
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()
	_, err := s.db.ExecContext(ctx, s.db.Q(`DELETE FROM keeporg_blobs WHERE ref = ?`), s.ref(dbID))
	return err
}

// ImportDir copies the blobs/<database id>/<version>.kdbx tree that servers
// before 0.11 kept on disk into the store, and says how many it copied.
func (s *Store) ImportDir(dir string) (int, error) {
	dbs, err := os.ReadDir(dir)
	if errors.Is(err, fs.ErrNotExist) {
		return 0, nil
	}
	if err != nil {
		return 0, err
	}
	n := 0
	for _, d := range dbs {
		if !d.IsDir() {
			continue
		}
		files, err := os.ReadDir(filepath.Join(dir, d.Name()))
		if err != nil {
			return n, err
		}
		for _, f := range files {
			name := f.Name()
			if !strings.HasSuffix(name, ".kdbx") {
				continue
			}
			v, err := strconv.ParseInt(strings.TrimSuffix(name, ".kdbx"), 10, 64)
			if err != nil {
				continue
			}
			data, err := os.ReadFile(filepath.Join(dir, d.Name(), name))
			if err != nil {
				return n, err
			}
			if err := s.Put(d.Name(), v, data); err != nil {
				return n, err
			}
			n++
		}
	}
	return n, nil
}