// Package kv is meta-core's storage engine: one string→string table in an
// embedded SQLite file.
//
// # Why a key/value table and not a schema
//
// meta-core's data model is schema-free on purpose — a record is a set of flat
// `file:<rid>/<field>` keys, and a new metadata field is just a new key
// (METADATA_KEYS.md). The engine keeps exactly that model:
//
//	CREATE TABLE kv(k TEXT PRIMARY KEY, v TEXT NOT NULL) WITHOUT ROWID
//
// is the only table that holds data, and it never needs a migration. What
// Redis gave us that a bare KV would not — "list a record's fields", "find
// records by field value", free-text search — comes from the table being
// *ordered* (a record is a key range) and from two derived structures the
// storage package maintains on top: an expression index over the field name
// parsed out of the key, and an FTS5 table. Both are rebuildable from the kv
// rows and never authoritative.
//
// It replaced Redis on 2026-10: holding everything in RAM cost ~6.6 GB on a
// 169k-record corpus (Redis plus the API's in-memory copy needed to search
// it), with the OOM kills that implies. The same data is ~2.5 GB on disk here
// and the working set is a bounded page cache.
//
// # Concurrency
//
// SQLite allows one writer at a time, so every write goes through one writer
// goroutine owning one connection. Concurrent Update calls are GROUP-COMMITTED:
// the writer runs every request waiting at that moment inside one SQLite
// transaction, each under its own SAVEPOINT (a failing request rolls back
// alone), and commits once. A commit — writing the dirty pages of the table,
// the field index and the FTS shadow tables to the WAL — was ~40% of the cost
// of a record write, so sharing it is what makes concurrent ingest scale.
//
// Reads use a separate pool and see the last committed state (WAL). Inside
// Update, read through the *Tx — the reader pool cannot see uncommitted rows —
// and never call Update from inside an Update callback (the writer would wait
// for itself).
//
// # Events
//
// Redis keyspace notifications are replaced by the transaction itself: every
// Put/Delete of a key the Options.EventKey predicate selects is appended, in
// the same transaction, to Options.EventStream as {type: set|del, key, ts}.
// A change and its event therefore commit together or not at all.
package kv

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"log"
	"net/url"
	"os"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	_ "modernc.org/sqlite"
)

// Options configures a Store.
type Options struct {
	// EventStream receives a set/del entry for every changed key EventKey
	// selects. Empty disables change events.
	EventStream string
	// EventMaxLen is the approximate retention of EventStream.
	EventMaxLen int64
	// EventKey selects the keys whose changes are published.
	EventKey func(key string) bool
	// WriterCacheMB / ReaderCacheMB size SQLite's page cache per connection.
	WriterCacheMB int
	ReaderCacheMB int
	// Readers is the size of the read connection pool.
	Readers int
	// Schema is extra DDL run at open (idempotent CREATE … IF NOT EXISTS),
	// for derived indexes owned by the caller.
	Schema string
	// OnChange is called inside the transaction for every key whose value
	// actually changed (created, modified or deleted), with the old and new
	// values — the hook derived counters hang off (Tx.AddCounter). It must not
	// write kv rows itself.
	OnChange func(tx *Tx, key, old string, hadOld bool, new string, hasNew bool)
	// NoKVTable opens the file without the kv table and its prepared
	// statements: the caller owns every table through Schema and writes them
	// with Tx.Exec / Tx.Query (meta-core's truth database). The kv methods
	// (Get, Put, streams, counters) must not be used on such a store.
	NoKVTable bool
	// Synchronous is SQLite's synchronous pragma; default NORMAL. A store that
	// is rebuildable from another (meta-core's cache) runs OFF.
	Synchronous string
}

// Store is an open database.
type Store struct {
	path string
	opts Options
	w    *sql.DB // single connection: the one writer
	r    *sql.DB // read pool

	streamMu sync.Mutex
	streams  map[string]*streamState

	// Hot write statements, prepared once on the writer connection: parsing
	// SQL on every Put was the single largest cost of a record write.
	stPut, stPutIfAbsent, stDelete, stGet, stInsert *sql.Stmt

	// Write accounting: how many Puts changed a value vs found it unchanged.
	puts, noops atomic.Int64

	writes    chan *writeReq
	writeMu   sync.Mutex // held by the writer while a group runs
	closeOnce sync.Once
	stopped   chan struct{}
}

// writeReq is one Update call waiting for the writer.
type writeReq struct {
	fn   func(tx *Tx) error
	done chan error
}

// maxGroup caps how many requests share one commit, bounding how long the
// writer holds a transaction open.
const maxGroup = 128

// Open opens (creating if needed) the database file at path.
func Open(path string, opts Options) (*Store, error) {
	if opts.Readers <= 0 {
		opts.Readers = 8
	}
	if opts.WriterCacheMB <= 0 {
		opts.WriterCacheMB = 64
	}
	if opts.ReaderCacheMB <= 0 {
		opts.ReaderCacheMB = 16
	}
	if opts.EventMaxLen <= 0 {
		opts.EventMaxLen = 100_000
	}

	if opts.Synchronous == "" {
		opts.Synchronous = "NORMAL"
	}
	w, err := sql.Open("sqlite", dsn(path, opts.WriterCacheMB, false, opts.Synchronous))
	if err != nil {
		return nil, fmt.Errorf("open writer: %w", err)
	}
	w.SetMaxOpenConns(1)
	w.SetMaxIdleConns(1)
	w.SetConnMaxLifetime(0)

	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	defer cancel()
	if !opts.NoKVTable {
		if _, err := w.ExecContext(ctx, `CREATE TABLE IF NOT EXISTS kv(k TEXT PRIMARY KEY, v TEXT NOT NULL) WITHOUT ROWID`); err != nil {
			w.Close()
			return nil, fmt.Errorf("create kv: %w", err)
		}
	}
	if opts.Schema != "" {
		// No deadline: on an existing store, the first open after a new index
		// is added builds it over every row (minutes at millions of keys).
		// Cancelling that just rolls it back and restarts it on the next boot.
		started := time.Now()
		if _, err := w.ExecContext(context.Background(), opts.Schema); err != nil {
			w.Close()
			return nil, fmt.Errorf("apply schema: %w", err)
		}
		if d := time.Since(started); d > 5*time.Second {
			log.Printf("[kv] schema applied in %s (indexes built)", d.Round(time.Second))
		}
	}

	r, err := sql.Open("sqlite", dsn(path, opts.ReaderCacheMB, true, opts.Synchronous))
	if err != nil {
		w.Close()
		return nil, fmt.Errorf("open readers: %w", err)
	}
	r.SetMaxOpenConns(opts.Readers)
	r.SetMaxIdleConns(opts.Readers)

	st := &Store{path: path, opts: opts, w: w, r: r, streams: make(map[string]*streamState)}
	// Fresh deadline: the schema step above may legitimately have taken minutes.
	ctx, cancel = context.WithTimeout(context.Background(), 30*time.Second)
	defer cancel()
	stmts := []struct {
		dst   **sql.Stmt
		query string
	}{
		{&st.stPut, `INSERT INTO kv(k, v) VALUES(?, ?) ON CONFLICT(k) DO UPDATE SET v = excluded.v`},
		{&st.stPutIfAbsent, `INSERT INTO kv(k, v) VALUES(?, ?) ON CONFLICT(k) DO NOTHING`},
		{&st.stDelete, `DELETE FROM kv WHERE k = ? RETURNING v`},
		{&st.stGet, `SELECT v FROM kv WHERE k = ?`},
		{&st.stInsert, `INSERT INTO kv(k, v) VALUES(?, ?)`},
	}
	if opts.NoKVTable {
		stmts = nil
	}
	for _, p := range stmts {
		if *p.dst, err = w.PrepareContext(ctx, p.query); err != nil {
			r.Close()
			w.Close()
			return nil, fmt.Errorf("prepare: %w", err)
		}
	}
	st.writes = make(chan *writeReq)
	st.stopped = make(chan struct{})
	go st.writer()
	return st, nil
}

// dsn builds a modernc DSN. WAL + synchronous=NORMAL is the durability class
// Redis' AOF-everysec gave us: a power loss may drop the last transactions,
// never corrupt the file.
func dsn(path string, cacheMB int, readOnly bool, synchronous string) string {
	q := url.Values{}
	q.Add("_pragma", "busy_timeout(30000)")
	q.Add("_pragma", "journal_mode(WAL)")
	q.Add("_pragma", "synchronous("+synchronous+")")
	q.Add("_pragma", fmt.Sprintf("cache_size(-%d)", cacheMB*1024))
	q.Add("_pragma", "temp_store(MEMORY)")
	q.Add("_pragma", "journal_size_limit(268435456)")
	if readOnly {
		q.Add("_pragma", "query_only(1)")
	} else {
		q.Add("_txlock", "immediate")
	}
	return "file:" + path + "?" + q.Encode()
}

// Close stops the writer and closes both pools, checkpointing the WAL into the
// main file.
func (s *Store) Close() error {
	s.closeOnce.Do(func() { close(s.stopped) })
	s.writeMu.Lock() // wait for an in-flight group to finish
	defer s.writeMu.Unlock()
	_, _ = s.w.Exec(`PRAGMA wal_checkpoint(TRUNCATE)`)
	return errors.Join(s.r.Close(), s.w.Close())
}

// Ping reports whether the database answers.
func (s *Store) Ping(ctx context.Context) error {
	var one int
	return s.r.QueryRowContext(ctx, `SELECT 1`).Scan(&one)
}

// Path is the database file path.
func (s *Store) Path() string { return s.path }

// DB exposes the read pool for callers that own derived indexes (search). It
// is read-only (query_only), so it cannot bypass Update.
func (s *Store) DB() *sql.DB { return s.r }

// FileSize returns the on-disk size of the database plus its WAL.
func (s *Store) FileSize() int64 {
	var total int64
	for _, suffix := range []string{"", "-wal"} {
		if fi, err := os.Stat(s.path + suffix); err == nil {
			total += fi.Size()
		}
	}
	return total
}

// Checkpoint folds the WAL back into the main file. TRUNCATE takes the writer
// slot and waits for readers, so callers run it when idle.
func (s *Store) Checkpoint(ctx context.Context) error {
	_, err := s.w.ExecContext(ctx, `PRAGMA wal_checkpoint(TRUNCATE)`)
	return err
}

// KV is one key/value pair.
type KV struct {
	K, V string
}

// Reader is the read surface shared by Store (committed state) and Tx (the
// transaction's own view). Storage helpers take a Reader so they work both
// inside and outside a write.
type Reader interface {
	Get(ctx context.Context, k string) (string, bool, error)
	MGet(ctx context.Context, keys []string) (map[string]string, error)
	Range(ctx context.Context, prefix, after string, limit int) ([]KV, error)
	Count(ctx context.Context, prefix string) (int64, error)
}

type queryer interface {
	QueryContext(ctx context.Context, query string, args ...any) (*sql.Rows, error)
	QueryRowContext(ctx context.Context, query string, args ...any) *sql.Row
}

// PrefixEnd returns the smallest string greater than every string with the
// given prefix, so [prefix, PrefixEnd(prefix)) is exactly the prefix's range.
// An empty result means "unbounded".
func PrefixEnd(prefix string) string {
	b := []byte(prefix)
	for i := len(b) - 1; i >= 0; i-- {
		if b[i] < 0xff {
			b[i]++
			return string(b[:i+1])
		}
	}
	return ""
}

func get(ctx context.Context, q queryer, k string) (string, bool, error) {
	var v string
	err := q.QueryRowContext(ctx, `SELECT v FROM kv WHERE k = ?`, k).Scan(&v)
	if errors.Is(err, sql.ErrNoRows) {
		return "", false, nil
	}
	if err != nil {
		return "", false, err
	}
	return v, true, nil
}

// mgetBatch stays well under SQLite's default 32766 bound-parameter cap.
const mgetBatch = 500

func mget(ctx context.Context, q queryer, keys []string) (map[string]string, error) {
	out := make(map[string]string, len(keys))
	for start := 0; start < len(keys); start += mgetBatch {
		end := min(start+mgetBatch, len(keys))
		chunk := keys[start:end]
		args := make([]any, len(chunk))
		for i, k := range chunk {
			args[i] = k
		}
		rows, err := q.QueryContext(ctx, `SELECT k, v FROM kv WHERE k IN (?`+strings.Repeat(",?", len(chunk)-1)+`)`, args...)
		if err != nil {
			return nil, err
		}
		for rows.Next() {
			var k, v string
			if err := rows.Scan(&k, &v); err != nil {
				rows.Close()
				return nil, err
			}
			out[k] = v
		}
		if err := rows.Close(); err != nil {
			return nil, err
		}
	}
	return out, nil
}

// rangeQuery returns keys with prefix, strictly after `after` (when non-empty),
// in key order, at most limit (<= 0 means all).
func rangeQuery(ctx context.Context, q queryer, prefix, after string, limit int) ([]KV, error) {
	lo, loOp := prefix, ">="
	if after != "" && after >= prefix {
		lo, loOp = after, ">"
	}
	query := `SELECT k, v FROM kv WHERE k ` + loOp + ` ?`
	args := []any{lo}
	if end := PrefixEnd(prefix); end != "" {
		query += ` AND k < ?`
		args = append(args, end)
	}
	query += ` ORDER BY k`
	if limit > 0 {
		query += ` LIMIT ?`
		args = append(args, limit)
	}
	rows, err := q.QueryContext(ctx, query, args...)
	if err != nil {
		return nil, err
	}
	defer rows.Close()
	var out []KV
	for rows.Next() {
		var kv KV
		if err := rows.Scan(&kv.K, &kv.V); err != nil {
			return nil, err
		}
		out = append(out, kv)
	}
	return out, rows.Err()
}

func count(ctx context.Context, q queryer, prefix string) (int64, error) {
	query := `SELECT count(*) FROM kv WHERE k >= ?`
	args := []any{prefix}
	if end := PrefixEnd(prefix); end != "" {
		query += ` AND k < ?`
		args = append(args, end)
	}
	var n int64
	err := q.QueryRowContext(ctx, query, args...).Scan(&n)
	return n, err
}

// Get returns the committed value of k.
func (s *Store) Get(ctx context.Context, k string) (string, bool, error) { return get(ctx, s.r, k) }

// MGet returns the committed values of the keys that exist.
func (s *Store) MGet(ctx context.Context, keys []string) (map[string]string, error) {
	return mget(ctx, s.r, keys)
}

// Range returns committed keys under prefix after `after`, in key order.
func (s *Store) Range(ctx context.Context, prefix, after string, limit int) ([]KV, error) {
	return rangeQuery(ctx, s.r, prefix, after, limit)
}

// Count returns how many committed keys sit under prefix.
func (s *Store) Count(ctx context.Context, prefix string) (int64, error) {
	return count(ctx, s.r, prefix)
}

// ScanPrefix walks every committed key under prefix in pages of `page`, each
// page its own short read so a long walk never pins a WAL snapshot (which
// would stop checkpoints and let the WAL grow without bound).
func (s *Store) ScanPrefix(ctx context.Context, prefix string, page int, fn func([]KV) error) error {
	if page <= 0 {
		page = 5000
	}
	after := ""
	for {
		batch, err := rangeQuery(ctx, s.r, prefix, after, page)
		if err != nil {
			return err
		}
		if len(batch) == 0 {
			return nil
		}
		if err := fn(batch); err != nil {
			return err
		}
		if len(batch) < page {
			return nil
		}
		after = batch[len(batch)-1].K
	}
}

// CountAll counts every row. A full scan — callers cache it.
func (s *Store) CountAll(ctx context.Context) (int64, error) {
	var n int64
	err := s.r.QueryRowContext(ctx, `SELECT count(*) FROM kv`).Scan(&n)
	return n, err
}

// ErrClosed is returned by Update on a closed store.
var ErrClosed = errors.New("kv: store closed")

// Update runs fn atomically: its writes (and the change events they publish)
// commit together or not at all. fn may share a SQLite transaction with other
// concurrent Updates (group commit) but is isolated from their failures by a
// savepoint. Never call Update from inside fn.
//
// ctx bounds only the wait for the writer. Once fn starts it runs to
// completion: interrupting a statement inside a shared transaction would roll
// back every request in the group.
func (s *Store) Update(ctx context.Context, fn func(tx *Tx) error) error {
	req := &writeReq{fn: fn, done: make(chan error, 1)}
	select {
	case s.writes <- req:
	case <-ctx.Done():
		return ctx.Err()
	case <-s.stopped:
		return ErrClosed
	}
	return <-req.done
}

// writer is the single goroutine that executes writes.
func (s *Store) writer() {
	for {
		var first *writeReq
		select {
		case first = <-s.writes:
		case <-s.stopped:
			return
		}
		group := []*writeReq{first}
	drain:
		for len(group) < maxGroup {
			select {
			case r := <-s.writes:
				group = append(group, r)
			default:
				break drain
			}
		}
		s.writeMu.Lock()
		s.runGroup(group)
		s.writeMu.Unlock()
	}
}

// group is the state shared by the requests of one commit.
type group struct {
	tx       *sql.Tx
	pending  map[string]*streamPending
	stmts    map[*sql.Stmt]*sql.Stmt
	counters map[string]int64
}

func (g *group) snapshot() map[string]streamPending {
	out := make(map[string]streamPending, len(g.pending))
	for k, p := range g.pending {
		out[k] = *p
	}
	return out
}

func (g *group) restore(snap map[string]streamPending) {
	g.pending = make(map[string]*streamPending, len(snap))
	for k, p := range snap {
		p := p
		g.pending[k] = &p
	}
}

func (s *Store) runGroup(reqs []*writeReq) {
	ctx := context.Background()
	results := make([]error, len(reqs))
	fail := func(err error) {
		for i, r := range reqs {
			if results[i] == nil {
				results[i] = err
			}
			r.done <- results[i]
		}
	}

	sqlTx, err := s.w.BeginTx(ctx, nil)
	if err != nil {
		fail(fmt.Errorf("begin: %w", err))
		return
	}
	g := &group{tx: sqlTx}
	var commitHooks []func()
	for i, r := range reqs {
		if _, err := sqlTx.ExecContext(ctx, `SAVEPOINT req`); err != nil {
			_ = sqlTx.Rollback()
			fail(fmt.Errorf("savepoint: %w", err))
			return
		}
		snap := g.snapshot()
		tx := &Tx{ctx: ctx, s: s, g: g}
		err := r.fn(tx)
		if err == nil {
			err = tx.flushEvents()
		}
		if err == nil {
			g.mergeCounters(tx.counters)
			commitHooks = append(commitHooks, tx.onCommit...)
		}
		if err != nil {
			results[i] = err
			if _, rbErr := sqlTx.ExecContext(ctx, `ROLLBACK TO req`); rbErr != nil {
				_ = sqlTx.Rollback()
				fail(fmt.Errorf("rollback to savepoint: %w (after %v)", rbErr, err))
				return
			}
			g.restore(snap)
		}
		if _, err := sqlTx.ExecContext(ctx, `RELEASE req`); err != nil {
			_ = sqlTx.Rollback()
			fail(fmt.Errorf("release savepoint: %w", err))
			return
		}
	}
	if err := (&Tx{ctx: ctx, s: s, g: g}).flushCounters(); err != nil {
		_ = sqlTx.Rollback()
		fail(err)
		return
	}
	if err := (&Tx{ctx: ctx, s: s, g: g}).flushHWM(); err != nil {
		_ = sqlTx.Rollback()
		fail(err)
		return
	}
	if err := sqlTx.Commit(); err != nil {
		fail(fmt.Errorf("commit: %w", err))
		return
	}
	s.afterCommit(g.pending)
	// Hooks run in the writer goroutine, in commit order, before any request
	// returns: a caller that orders work by commit (meta-core's apply queue)
	// sees it in exactly that order.
	for _, h := range commitHooks {
		h()
	}
	for i, r := range reqs {
		r.done <- results[i]
	}
}

// Tx is one Update's view of the write transaction. Its reads see its own
// writes (and those of earlier requests in the same group).
type Tx struct {
	ctx      context.Context
	s        *Store
	g        *group
	events   []map[string]string
	counters map[string]int64
	onCommit []func()
	quiet    bool
}

// OnCommit registers fn to run after the transaction commits, only if this
// request succeeded. Hooks run in the writer goroutine in commit order and
// must not block or call Update.
func (t *Tx) OnCommit(fn func()) { t.onCommit = append(t.onCommit, fn) }

// Quiet stops this request's changes from publishing events (a bulk rebuild
// whose consumers are told to take a snapshot instead). Counters and the
// OnChange hook still run.
func (t *Tx) Quiet() { t.quiet = true }

// Query runs a raw multi-row query in the transaction.
func (t *Tx) Query(query string, args ...any) (*sql.Rows, error) {
	return t.sqlTx().QueryContext(t.ctx, query, args...)
}

func (t *Tx) sqlTx() *sql.Tx { return t.g.tx }

// stmt binds one of the store's prepared statements to the transaction.
func (t *Tx) stmt(st *sql.Stmt) *sql.Stmt {
	if t.g.stmts == nil {
		t.g.stmts = make(map[*sql.Stmt]*sql.Stmt, 5)
	}
	bound := t.g.stmts[st]
	if bound == nil {
		bound = t.g.tx.StmtContext(t.ctx, st)
		t.g.stmts[st] = bound
	}
	return bound
}

// Context is the context the transaction runs under.
func (t *Tx) Context() context.Context { return t.ctx }

func (t *Tx) Get(ctx context.Context, k string) (string, bool, error) {
	var v string
	err := t.stmt(t.s.stGet).QueryRowContext(ctx, k).Scan(&v)
	if errors.Is(err, sql.ErrNoRows) {
		return "", false, nil
	}
	if err != nil {
		return "", false, err
	}
	return v, true, nil
}
func (t *Tx) MGet(ctx context.Context, keys []string) (map[string]string, error) {
	return mget(ctx, t.sqlTx(), keys)
}
func (t *Tx) Range(ctx context.Context, prefix, after string, limit int) ([]KV, error) {
	return rangeQuery(ctx, t.sqlTx(), prefix, after, limit)
}
func (t *Tx) Count(ctx context.Context, prefix string) (int64, error) {
	return count(ctx, t.sqlTx(), prefix)
}

// Exec runs a raw statement in the transaction — for the caller's derived
// tables (FTS), never for kv rows (which would skip events).
func (t *Tx) Exec(query string, args ...any) (sql.Result, error) {
	return t.sqlTx().ExecContext(t.ctx, query, args...)
}

// QueryRow runs a raw single-row query in the transaction.
func (t *Tx) QueryRow(query string, args ...any) *sql.Row {
	return t.sqlTx().QueryRowContext(t.ctx, query, args...)
}

// Put sets k = v. A no-op — no row write, no index churn, no event — when k
// already holds v. (Redis keyspace notifications published `set` even for an
// unchanged value; with meta-core persisting the same gateway hits again on
// every repeated query, those no-op writes were most of the write load.)
func (t *Tx) Put(k, v string) error {
	_, err := t.Set(k, v)
	return err
}

// Set is Put that reports whether the value changed.
func (t *Tx) Set(k, v string) (bool, error) {
	old, had, err := t.Get(t.ctx, k)
	if err != nil {
		return false, fmt.Errorf("put %q: %w", k, err)
	}
	if had && old == v {
		t.s.noops.Add(1)
		return false, nil
	}
	if _, err := t.stmt(t.s.stPut).ExecContext(t.ctx, k, v); err != nil {
		return false, fmt.Errorf("put %q: %w", k, err)
	}
	t.s.puts.Add(1)
	t.changed("set", k, old, had, v, true)
	return true, nil
}

// PutIfAbsent sets k = v only when k does not exist. Reports whether it wrote.
func (t *Tx) PutIfAbsent(k, v string) (bool, error) {
	res, err := t.stmt(t.s.stPutIfAbsent).ExecContext(t.ctx, k, v)
	if err != nil {
		return false, fmt.Errorf("put %q: %w", k, err)
	}
	n, _ := res.RowsAffected()
	if n > 0 {
		t.changed("set", k, "", false, v, true)
	}
	return n > 0, nil
}

// Delete removes k. Reports whether it existed; publishes `del` only then,
// as Redis did.
func (t *Tx) Delete(k string) (bool, error) {
	var old string
	err := t.stmt(t.s.stDelete).QueryRowContext(t.ctx, k).Scan(&old)
	if errors.Is(err, sql.ErrNoRows) {
		return false, nil
	}
	if err != nil {
		return false, fmt.Errorf("delete %q: %w", k, err)
	}
	t.changed("del", k, old, true, "", false)
	return true, nil
}

// DeletePrefix removes up to limit keys under prefix (limit <= 0: all) and
// returns how many went. Bulk callers loop over separate Updates with a limit
// so one transaction never grows to millions of rows.
func (t *Tx) DeletePrefix(prefix string, limit int) (int64, error) {
	end := PrefixEnd(prefix)
	sel := `SELECT k FROM kv WHERE k >= ?`
	args := []any{prefix}
	if end != "" {
		sel += ` AND k < ?`
		args = append(args, end)
	}
	if limit > 0 {
		sel += ` ORDER BY k LIMIT ?`
		args = append(args, limit)
	}
	rows, err := t.sqlTx().QueryContext(t.ctx, `DELETE FROM kv WHERE k IN (`+sel+`) RETURNING k, v`, args...)
	if err != nil {
		return 0, fmt.Errorf("delete prefix %q: %w", prefix, err)
	}
	var gone []KV
	for rows.Next() {
		var kv KV
		if err := rows.Scan(&kv.K, &kv.V); err != nil {
			rows.Close()
			return 0, err
		}
		gone = append(gone, kv)
	}
	if err := rows.Close(); err != nil {
		return 0, err
	}
	for _, kv := range gone {
		t.changed("del", kv.K, kv.V, true, "", false)
	}
	return int64(len(gone)), nil
}

// changed records one real change: the event (when in scope) and the hook.
func (t *Tx) changed(op, k, old string, hadOld bool, v string, hasNew bool) {
	t.event(op, k)
	if t.s.opts.OnChange != nil {
		t.s.opts.OnChange(t, k, old, hadOld, v, hasNew)
	}
}

// AddCounter adds delta to the integer counter stored at key. Deltas are
// summed in memory and written once per commit (a hot counter touched by every
// record in a group costs one row write, not one per record); they are
// discarded with the request if it fails. A counter that reaches zero is
// deleted.
func (t *Tx) AddCounter(key string, delta int64) {
	if delta == 0 {
		return
	}
	if t.counters == nil {
		t.counters = make(map[string]int64)
	}
	t.counters[key] += delta
}

func (g *group) mergeCounters(c map[string]int64) {
	if len(c) == 0 {
		return
	}
	if g.counters == nil {
		g.counters = make(map[string]int64, len(c))
	}
	for k, d := range c {
		g.counters[k] += d
	}
}

// flushCounters applies the group's counter deltas, bypassing Put (counters
// are bookkeeping: no events, no hook).
func (t *Tx) flushCounters() error {
	for k, d := range t.g.counters {
		if d == 0 {
			continue
		}
		cur, _, err := t.Get(t.ctx, k)
		if err != nil {
			return err
		}
		n, _ := strconv.ParseInt(cur, 10, 64)
		n += d
		if n <= 0 {
			if _, err := t.sqlTx().ExecContext(t.ctx, `DELETE FROM kv WHERE k = ?`, k); err != nil {
				return fmt.Errorf("counter %s: %w", k, err)
			}
			continue
		}
		if _, err := t.stmt(t.s.stPut).ExecContext(t.ctx, k, strconv.FormatInt(n, 10)); err != nil {
			return fmt.Errorf("counter %s: %w", k, err)
		}
	}
	return nil
}

// Counter reads a committed counter (0 when absent).
func (s *Store) Counter(ctx context.Context, key string) (int64, bool, error) {
	v, ok, err := s.Get(ctx, key)
	if err != nil || !ok {
		return 0, ok, err
	}
	n, err := strconv.ParseInt(v, 10, 64)
	return n, true, err
}

// WriteStats reports how many Puts changed a value and how many found it
// already in place, since open.
func (s *Store) WriteStats() (changed, unchanged int64) {
	return s.puts.Load(), s.noops.Load()
}

func (t *Tx) event(op, k string) {
	if t.quiet || t.s.opts.EventStream == "" || t.s.opts.EventKey == nil || !t.s.opts.EventKey(k) {
		return
	}
	t.events = append(t.events, map[string]string{
		"type": op,
		"key":  k,
		"ts":   fmt.Sprintf("%d", time.Now().UnixMilli()),
	})
}

// flushEvents appends the queued change events to the event stream, inside
// the transaction, right before commit.
func (t *Tx) flushEvents() error {
	if len(t.events) == 0 {
		return nil
	}
	evs := t.events
	t.events = nil
	for _, e := range evs {
		if _, err := t.Append(t.s.opts.EventStream, t.s.opts.EventMaxLen, e); err != nil {
			return err
		}
	}
	return nil
}