// Copyright (c) 2020 Shivaram Lingamneni
// released under the MIT license

package mysql

import (
	"context"
	"database/sql"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"runtime/debug"
	"slices"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"github.com/ergochat/ergo/irc/history"
	"github.com/ergochat/ergo/irc/logger"
	"github.com/ergochat/ergo/irc/utils"
	_ "github.com/go-sql-driver/mysql"
)

var (
	ErrDisallowed = errors.New("disallowed")
)

const (
	// maximum length in bytes of any message target (nickname or channel name) in its
	// canonicalized (i.e., casefolded) state:
	MaxTargetLength  = 64
	cleanupRowLimit  = 50
	cleanupPauseTime = 10 * time.Minute
)

type e struct{}

type MySQL struct {
	db     *sql.DB
	logger *logger.Manager

	insertHistory        *sql.Stmt
	insertConversation   *sql.Stmt
	insertAccountMessage *sql.Stmt

	getReactionsQuery *sql.Stmt
	getSingleReaction *sql.Stmt
	addReaction       *sql.Stmt
	deleteReaction    *sql.Stmt

	getMessageById *sql.Stmt

	stateMutex sync.Mutex
	config     Config

	wakeForgetter chan e

	timeout              atomic.Uint64
	trackAccountMessages atomic.Uint32
}

func (mysql *MySQL) Initialize(logger *logger.Manager, config Config) {
	mysql.logger = logger
	mysql.wakeForgetter = make(chan e, 1)
	mysql.SetConfig(config)
}

func (mysql *MySQL) SetConfig(config Config) {
	mysql.timeout.Store(uint64(config.Timeout))
	var trackAccountMessages uint32
	if config.TrackAccountMessages {
		trackAccountMessages = 1
	}
	mysql.trackAccountMessages.Store(trackAccountMessages)
	mysql.stateMutex.Lock()
	mysql.config = config
	mysql.stateMutex.Unlock()
}

func (mysql *MySQL) getExpireTime() (expireTime time.Duration) {
	mysql.stateMutex.Lock()
	expireTime = mysql.config.ExpireTime
	mysql.stateMutex.Unlock()
	return
}

func (mysql *MySQL) Open() (err error) {
	var address string
	if mysql.config.SocketPath != "" {
		address = fmt.Sprintf("unix(%s)", mysql.config.SocketPath)
	} else if mysql.config.Port != 0 {
		address = fmt.Sprintf("tcp(%s:%d)", mysql.config.Host, mysql.config.Port)
	}

	mysql.db, err = sql.Open("mysql", fmt.Sprintf("%s:%s@%s/%s", mysql.config.User, mysql.config.Password, address, mysql.config.HistoryDatabase))
	if err != nil {
		return err
	}

	if mysql.config.MaxConns != 0 {
		mysql.db.SetMaxOpenConns(mysql.config.MaxConns)
		mysql.db.SetMaxIdleConns(mysql.config.MaxConns)
	}
	if mysql.config.ConnMaxLifetime != 0 {
		mysql.db.SetConnMaxLifetime(mysql.config.ConnMaxLifetime)
	}

	err = mysql.fixSchemas()
	if err != nil {
		return err
	}

	err = mysql.prepareStatements()
	if err != nil {
		return err
	}

	go mysql.cleanupLoop()
	go mysql.forgetLoop()

	return nil
}

func (mysql *MySQL) fixSchemas() (err error) {
	// 3M now handles this
	return
}

func (mysql *MySQL) createTables() (err error) {
	// 3M now handles this
	return nil
}

func (mysql *MySQL) createComplianceTables() (err error) {
	// 3M now handles this
	return nil
}

func (mysql *MySQL) cleanupLoop() {
	defer func() {
		if r := recover(); r != nil {
			mysql.logger.Error("mysql",
				fmt.Sprintf("Panic in cleanup routine: %v\n%s", r, debug.Stack()))
			time.Sleep(cleanupPauseTime)
			go mysql.cleanupLoop()
		}
	}()

	for {
		expireTime := mysql.getExpireTime()
		if expireTime != 0 {
			for {
				startTime := time.Now()
				rowsDeleted, err := mysql.doCleanup(expireTime)
				elapsed := time.Now().Sub(startTime)
				mysql.logError("error during row cleanup", err)
				// keep going as long as we're accomplishing significant work
				// (don't busy-wait on small numbers of rows expiring):
				if rowsDeleted < (cleanupRowLimit / 10) {
					break
				}
				// crude backpressure mechanism: if the database is slow,
				// give it time to process other queries
				time.Sleep(elapsed)
			}
		}
		time.Sleep(cleanupPauseTime)
	}
}

func (mysql *MySQL) doCleanup(age time.Duration) (count int, err error) {
	ctx, cancel := context.WithTimeout(context.Background(), cleanupPauseTime)
	defer cancel()

	ids, maxNanotime, err := mysql.selectCleanupIDs(ctx, age)
	if len(ids) == 0 {
		mysql.logger.Debug("mysql", "found no rows to clean up")
		return
	}

	mysql.logger.Debug("mysql", fmt.Sprintf("deleting %d history rows, max age %s", len(ids), utils.NanoToTimestamp(maxNanotime)))

	return len(ids), mysql.deleteHistoryIDs(ctx, ids)
}

func (mysql *MySQL) deleteHistoryIDs(ctx context.Context, ids []uint64) (err error) {
	// can't use ? binding for a variable number of arguments, build the IN clause manually
	var inBuf strings.Builder
	inBuf.WriteByte('(')
	for i, id := range ids {
		if i != 0 {
			inBuf.WriteRune(',')
		}
		fmt.Fprintf(&inBuf, "%d", id)
	}
	inBuf.WriteRune(')')
	inClause := inBuf.String()

	if mysql.isTrackingAccountMessages() {
		_, err = mysql.db.ExecContext(ctx, fmt.Sprintf(`DELETE FROM account_messages WHERE history_id in %s;`, inClause))
		if err != nil {
			return
		}
	}
	fmt.Printf(`DELETE FROM history WHERE msgid in %s;`, inClause)
	_, err = mysql.db.ExecContext(ctx, fmt.Sprintf(`DELETE FROM history WHERE msgid in %s;`, inClause))
	if err != nil {
		return
	}

	return
}

func (mysql *MySQL) selectCleanupIDs(ctx context.Context, age time.Duration) (ids []uint64, maxNanotime int64, err error) {
	before := timestampSnowflake(time.Now().Add(-age))
	maxNanotime = time.Now().Add(-age).Unix() * 1000000000

	rows, err := mysql.db.QueryContext(ctx, `
		SELECT history.msgid
		FROM history
		WHERE msgid < ?
		ORDER BY history.msgid LIMIT ?;`, before, cleanupRowLimit)
	if err != nil {
		return
	}
	defer rows.Close()

	ids = make([]uint64, cleanupRowLimit)

	i := 0
	for rows.Next() {
		var id uint64
		err = rows.Scan(&id)
		if err != nil {
			return
		}
		ids[i] = id
		i++
	}
	ids = ids[0:i]
	return
}

// wait for forget queue items and process them one by one
func (mysql *MySQL) forgetLoop() {
	defer func() {
		if r := recover(); r != nil {
			mysql.logger.Error("mysql",
				fmt.Sprintf("Panic in forget routine: %v\n%s", r, debug.Stack()))
			time.Sleep(cleanupPauseTime)
			go mysql.forgetLoop()
		}
	}()

	for {
		for {
			found, err := mysql.doForget()
			mysql.logError("error processing forget", err)
			if err != nil {
				time.Sleep(cleanupPauseTime)
			}
			if !found {
				break
			}
		}

		<-mysql.wakeForgetter
	}
}

// dequeue an item from the forget queue and process it
func (mysql *MySQL) doForget() (found bool, err error) {
	id, account, err := func() (id int64, account string, err error) {
		ctx, cancel := context.WithTimeout(context.Background(), cleanupPauseTime)
		defer cancel()

		row := mysql.db.QueryRowContext(ctx,
			`SELECT forget.id, forget.account FROM forget LIMIT 1;`)
		err = row.Scan(&id, &account)
		if err == sql.ErrNoRows {
			return 0, "", nil
		}
		return
	}()

	if err != nil || account == "" {
		return false, err
	}

	found = true

	var count int
	for {
		start := time.Now()
		count, err = mysql.doForgetIteration(account)
		elapsed := time.Since(start)
		if err != nil {
			return true, err
		}
		if count == 0 {
			break
		}
		time.Sleep(elapsed)
	}

	mysql.logger.Debug("mysql", "forget complete for account", account)

	ctx, cancel := context.WithTimeout(context.Background(), cleanupPauseTime)
	defer cancel()
	_, err = mysql.db.ExecContext(ctx, `DELETE FROM forget where id = ?;`, id)
	return
}

func (mysql *MySQL) doForgetIteration(account string) (count int, err error) {
	ctx, cancel := context.WithTimeout(context.Background(), cleanupPauseTime)
	defer cancel()

	rows, err := mysql.db.QueryContext(ctx, `
		SELECT account_messages.history_id
		FROM account_messages
		WHERE account_messages.account = ?
		LIMIT ?;`, account, cleanupRowLimit)
	if err != nil {
		return
	}
	defer rows.Close()

	var ids []uint64
	for rows.Next() {
		var id uint64
		err = rows.Scan(&id)
		if err != nil {
			return
		}
		ids = append(ids, id)
	}

	if len(ids) == 0 {
		return
	}

	mysql.logger.Debug("mysql", fmt.Sprintf("deleting %d history rows from account %s", len(ids), account))
	err = mysql.deleteHistoryIDs(ctx, ids)
	return len(ids), err
}

func (mysql *MySQL) prepareStatements() (err error) {
	mysql.insertHistory, err = mysql.db.Prepare(`INSERT INTO history
		(data, msgid, target, sender, nanotime) VALUES (?, ?, ?, ?, ?);`)
	if err != nil {
		return
	}
	mysql.insertAccountMessage, err = mysql.db.Prepare(`INSERT INTO account_messages
		(history_id, account) VALUES (?, ?);`)
	if err != nil {
		return
	}

	mysql.getReactionsQuery, err = mysql.db.Prepare(`select react, count(*) as total, (select JSON_ARRAYAGG(user)
                                    from reactions r
                                    where r.msgid = main.msgid
                                      and r.react = main.react
                                    limit 3) as sample
	from reactions as main
	where main.msgid = ?
	group by react, msgid;`)
	if err != nil {
		return
	}

	mysql.getSingleReaction, err = mysql.db.Prepare(`SELECT COUNT(*) FROM reactions WHERE msgid = ? AND user = ? AND react = ?`)
	if err != nil {
		return
	}

	mysql.deleteReaction, err = mysql.db.Prepare(`DELETE FROM reactions WHERE msgid = ? AND user = ? AND react = ?`)
	if err != nil {
		return
	}

	mysql.addReaction, err = mysql.db.Prepare(`INSERT INTO reactions(msgid, user, react) VALUES (?, ?, ?)`)
	if err != nil {
		return
	}

	mysql.getMessageById, err = mysql.db.Prepare(`SELECT msgid, data, target, sender, nanotime, pm FROM history WHERE msgid = ?`)
	if err != nil {
		return
	}

	return
}

func (mysql *MySQL) getTimeout() time.Duration {
	return time.Duration(mysql.timeout.Load())
}

func (mysql *MySQL) isTrackingAccountMessages() bool {
	return mysql.trackAccountMessages.Load() != 0
}

func (mysql *MySQL) logError(context string, err error) (quit bool) {
	if err != nil {
		mysql.logger.Error("mysql", context, err.Error())
		return true
	}
	return false
}

func (mysql *MySQL) Forget(account string) {
	if mysql.db == nil || account == "" {
		return
	}

	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()

	_, err := mysql.db.ExecContext(ctx, `INSERT INTO forget (account) VALUES (?);`, account)
	if mysql.logError("can't insert into forget table", err) {
		return
	}

	// wake up the forget goroutine if it's blocked:
	select {
	case mysql.wakeForgetter <- e{}:
	default:
	}
}

func (mysql *MySQL) AddChannelItem(target string, item history.Item, account string) (err error) {
	if mysql.db == nil {
		return
	}

	if target == "" {
		return utils.ErrInvalidParams
	}

	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()

	id, err := mysql.insertBase(ctx, item)
	if err != nil {
		return
	}

	err = mysql.insertAccountMessageEntry(ctx, id, account)
	if err != nil {
		return
	}

	return
}

func (mysql *MySQL) insertBase(ctx context.Context, item history.Item) (id int64, err error) {
	var value []byte
	value, err = marshalItem(&item)
	if mysql.logError("could not marshal item", err) {
		return
	}
	var account = item.Account
	if account == "" {
		account = "*"
	}
	result, err := mysql.insertHistory.ExecContext(ctx, value, item.Message.Msgid, item.Target, account, item.Message.Time.UnixNano())
	if mysql.logError("could not insert item", err) {
		return
	}

	id, err = result.LastInsertId()
	if mysql.logError("could not insert item", err) {
		return
	}

	return
}

func (mysql *MySQL) insertAccountMessageEntry(ctx context.Context, id int64, account string) (err error) {
	if account == "" || !mysql.isTrackingAccountMessages() {
		return
	}
	_, err = mysql.insertAccountMessage.ExecContext(ctx, id, account)
	mysql.logError("could not insert account-message entry", err)
	return
}

func (mysql *MySQL) AddDirectMessage(sender, senderAccount, recipient, recipientAccount string, item history.Item) (err error) {
	if mysql.db == nil {
		return
	}

	if senderAccount == "" && recipientAccount == "" {
		return
	}

	if sender == "" || recipient == "" {
		return utils.ErrInvalidParams
	}

	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()

	_, err = mysql.insertBase(ctx, item)
	if err != nil {
		return
	}

	return
}

// note that accountName is the unfolded name
func (mysql *MySQL) DeleteMsgid(msgid, account string) (err error) {
	if mysql.db == nil {
		return nil
	}

	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()

	_, id, data, err := mysql.lookupMsgid(ctx, msgid, true)
	if err != nil {
		return
	}

	if account != "*" {
		var item history.Item
		err = unmarshalItem(data, &item)
		// delete if the entry is corrupt
		if err == nil && item.Account != account {
			return ErrDisallowed
		}
	}

	err = mysql.deleteHistoryIDs(ctx, []uint64{id})
	mysql.logError("couldn't delete msgid", err)
	return
}

func (mysql *MySQL) Export(account string, writer io.Writer) {
	// no eu presence...
	// maybe fix this when i know the new schema works
	return
	/*if mysql.db == nil {
		return
	}

	var err error
	var lastSeen uint64
	for {
		rows := func() (count int) {
			ctx, cancel := context.WithTimeout(context.Background(), cleanupPauseTime)
			defer cancel()

			rows, rowsErr := mysql.db.QueryContext(ctx, `
				SELECT history.data, msgid, target FROM history
				WHERE sender = ? AND account_messages.history_id > ?
				LIMIT ?`, account, lastSeen, cleanupRowLimit)
			if rowsErr != nil {
				err = rowsErr
				return
			}
			defer rows.Close()
			for rows.Next() {
				var id uint64
				var blob, jsonBlob []byte
				var target string
				var item history.Item
				err = rows.Scan(&blob, &id, &target)
				if err != nil {
					return
				}
				err = unmarshalItem(blob, &item)
				if err != nil {
					return
				}
				item.Target = target
				jsonBlob, err = json.Marshal(item)
				if err != nil {
					return
				}
				count++
				if lastSeen < id {
					lastSeen = id
				}
				writer.Write(jsonBlob)
				writer.Write([]byte{'\n'})
			}
			return
		}()
		if rows == 0 || err != nil {
			break
		}
	}

	mysql.logError("could not export history", err)
	return*/
}

// Kinda an intermediary function due to the CEF DB structure
func (mysql *MySQL) GetMessage(msgid string) (id uint64, item history.Item, target string, sender string, nanotime uint64, pm bool, err error) {
	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()
	var data []byte

	row := mysql.getMessageById.QueryRowContext(ctx, msgid)
	err = row.Scan(&id, &data, &target, &sender, &nanotime, &pm)
	if err != nil {
		return
	}
	err = unmarshalItem(data, &item)
	return
}

func (mysql *MySQL) HasReactionFromUser(msgid string, user string, reaction string) (exists bool) {
	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()
	row := mysql.getSingleReaction.QueryRowContext(ctx, msgid, user, reaction)
	var count int
	row.Scan(&count)
	return count > 0
}

func (mysql *MySQL) AddReaction(msgid string, user string, reaction string) (err error) {
	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()
	_, err = mysql.addReaction.ExecContext(ctx, msgid, user, reaction)
	return
}

func (mysql *MySQL) DeleteReaction(msgid string, user string, reaction string) (err error) {
	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()
	_, err = mysql.deleteReaction.ExecContext(ctx, msgid, user, reaction)
	return
}

func (mysql *MySQL) lookupMsgid(ctx context.Context, msgid string, includeData bool) (result time.Time, id uint64, data []byte, err error) {
	if err != nil {
		return
	}
	cols := `history.nanotime`
	if includeData {
		cols = `history.nanotime, history.id, history.data`
	}

	// Since CEF uses snowflakes and vanilla ergo uses blobs, we cast as int to make it function.
	// May have to adjust it some day
	row := mysql.db.QueryRowContext(ctx, fmt.Sprintf(`
		SELECT %s FROM history
		WHERE history.msgid = CAST(? AS UNSIGNED) LIMIT 1;`, cols), msgid)
	var nanoSeq sql.NullInt64
	if !includeData {
		err = row.Scan(&nanoSeq)
	} else {
		err = row.Scan(&nanoSeq, &id, &data)
	}
	if err != sql.ErrNoRows {
		mysql.logError("could not resolve msgid to time", err)
	}
	if err != nil {
		return
	}
	nanotime := nanoSeq.Int64
	if nanotime == 0 {
		err = sql.ErrNoRows
		return
	}
	result = time.Unix(0, nanotime).UTC()
	return
}

func (mysql *MySQL) selectItems(ctx context.Context, query string, args ...interface{}) (results []history.Item, err error) {
	rows, err := mysql.db.QueryContext(ctx, query, args...)
	if mysql.logError("could not select history items", err) {
		return
	}

	defer rows.Close()

	for rows.Next() {
		var blob []byte
		var msgid uint64
		var item history.Item

		err = rows.Scan(&blob, &msgid)
		if mysql.logError("could not scan history item", err) {
			return
		}
		err = unmarshalItem(blob, &item)
		if mysql.logError("could not unmarshal history item", err) {
			return
		}
		reactions, rErr := mysql.getReactionsQuery.Query(msgid)
		if mysql.logError("could not get reactions", rErr) {
			return
		}
		var react string
		var total int
		var sample string
		for reactions.Next() {
			reactions.Scan(&react, &total, &sample)
			var sampleDecoded []string
			json.Unmarshal([]byte(sample), &sampleDecoded)

			item.Reactions = append(item.Reactions, history.Reaction{
				Name:        react,
				Total:       total,
				SampleUsers: sampleDecoded,
			})
		}

		results = append(results, item)
	}
	return
}

func timestampSnowflake(t time.Time) uint64 {
	var ts = t.Unix() & 0xffffffffffff
	return uint64(ts << 16)
}

func (mysql *MySQL) betweenTimestamps(ctx context.Context, target, correspondent string, after, before, cutoff time.Time, limit int) (results []history.Item, err error) {

	after, before, ascending := history.MinMaxAsc(after, before, cutoff)
	direction := "ASC"
	if !ascending {
		direction = "DESC"
	}

	var queryBuf strings.Builder
	args := make([]interface{}, 0, 7)
	if correspondent == "" {
		fmt.Fprintf(&queryBuf, "SELECT data, msgid FROM history WHERE target = ? ")
		args = append(args, target)
	} else {
		fmt.Fprintf(&queryBuf, "SELECT data, msgid FROM history WHERE (target = ? and sender = ?) OR  (target = ? and sender = ?)")
		args = append(args, target, correspondent, correspondent, target)
	}

	if !after.IsZero() {
		fmt.Fprintf(&queryBuf, " AND nanotime > ?")
		args = append(args, after.UnixNano())
	}
	if !before.IsZero() {
		fmt.Fprintf(&queryBuf, " AND nanotime <= ?")
		args = append(args, before.UnixNano())
	}

	fmt.Fprintf(&queryBuf, " ORDER BY nanotime %[1]s LIMIT ?;", direction)
	args = append(args, limit)

	results, err = mysql.selectItems(ctx, queryBuf.String(), args...)
	if err == nil && !ascending {
		slices.Reverse(results)
	}
	return
}

func (mysql *MySQL) listCorrespondentsInternal(ctx context.Context, target string, after, before, cutoff time.Time, limit int) (results []history.TargetListing, err error) {
	after, before, ascending := history.MinMaxAsc(after, before, cutoff)
	direction := "ASC"
	if !ascending {
		direction = "DESC"
	}

	var queryBuf strings.Builder
	args := make([]interface{}, 0, 5)
	queryBuf.WriteString(`SELECT target, sender, nanotime from history
		WHERE target = ? OR (sender = ? and pm = true)`)
	args = append(args, target, target)
	if !after.IsZero() {
		queryBuf.WriteString(" AND nanotime > ?")
		args = append(args, after.UnixNano())
	}
	if !before.IsZero() {
		queryBuf.WriteString(" AND nanotime < ?")
		args = append(args, before.UnixNano())
	}
	fmt.Fprintf(&queryBuf, " ORDER BY nanotime %s LIMIT ?;", direction)
	args = append(args, limit)
	query := queryBuf.String()

	rows, err := mysql.db.QueryContext(ctx, query, args...)
	if err != nil {
		return
	}
	defer rows.Close()
	var msgTarget string
	var msgSender string
	var nanotime int64
	for rows.Next() {
		err = rows.Scan(&msgTarget, &msgSender, &nanotime)
		if err != nil {
			return
		}
		if msgTarget == target {
			results = append(results, history.TargetListing{
				CfName: msgSender,
				Time:   time.Unix(0, nanotime),
			})
		} else {
			results = append(results, history.TargetListing{
				CfName: msgTarget,
				Time:   time.Unix(0, nanotime),
			})
		}

	}

	if !ascending {
		slices.Reverse(results)
	}

	return
}

func (mysql *MySQL) ListChannels(cfchannels []string) (results []history.TargetListing, err error) {
	if mysql.db == nil {
		return
	}

	if len(cfchannels) == 0 {
		return
	}

	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()

	var queryBuf strings.Builder
	args := make([]interface{}, 0, len(results))
	// https://dev.mysql.com/doc/refman/8.0/en/group-by-optimization.html
	// this should be a "loose index scan"
	queryBuf.WriteString(`SELECT sequence.target, MAX(sequence.nanotime) FROM sequence
		WHERE sequence.target IN (`)
	for i, chname := range cfchannels {
		if i != 0 {
			queryBuf.WriteString(", ")
		}
		queryBuf.WriteByte('?')
		args = append(args, chname)
	}
	queryBuf.WriteString(") GROUP BY sequence.target;")

	rows, err := mysql.db.QueryContext(ctx, queryBuf.String(), args...)
	if mysql.logError("could not query channel listings", err) {
		return
	}
	defer rows.Close()

	var target string
	var nanotime int64
	for rows.Next() {
		err = rows.Scan(&target, &nanotime)
		if mysql.logError("could not scan channel listings", err) {
			return
		}
		results = append(results, history.TargetListing{
			CfName: target,
			Time:   time.Unix(0, nanotime),
		})
	}
	return
}

func (mysql *MySQL) Close() {
	// closing the database will close our prepared statements as well
	if mysql.db != nil {
		mysql.db.Close()
	}
	mysql.db = nil
}

// implements history.Sequence, emulating a single history buffer (for a channel,
// a single user's DMs, or a DM conversation)
type mySQLHistorySequence struct {
	mysql         *MySQL
	target        string
	correspondent string
	cutoff        time.Time
}

func (s *mySQLHistorySequence) Between(start, end history.Selector, limit int) (results []history.Item, err error) {
	ctx, cancel := context.WithTimeout(context.Background(), s.mysql.getTimeout())
	defer cancel()

	startTime := start.Time

	if start.Msgid != "" {
		startTime, _, _, err = s.mysql.lookupMsgid(ctx, start.Msgid, false)
		if err != nil {
			if err == sql.ErrNoRows {
				return nil, nil
			} else {
				return nil, err
			}
		}
	}
	endTime := end.Time
	if end.Msgid != "" {
		endTime, _, _, err = s.mysql.lookupMsgid(ctx, end.Msgid, false)

		if err != nil {
			if err == sql.ErrNoRows {
				return nil, nil
			} else {
				return nil, err
			}
		}
	}

	results, err = s.mysql.betweenTimestamps(ctx, s.target, s.correspondent, startTime, endTime, s.cutoff, limit)
	return results, err
}

func (s *mySQLHistorySequence) Around(start history.Selector, limit int) (results []history.Item, err error) {
	return history.GenericAround(s, start, limit)
}

func (seq *mySQLHistorySequence) ListCorrespondents(start, end history.Selector, limit int) (results []history.TargetListing, err error) {
	ctx, cancel := context.WithTimeout(context.Background(), seq.mysql.getTimeout())
	defer cancel()

	// TODO accept msgids here?
	startTime := start.Time
	endTime := end.Time

	results, err = seq.mysql.listCorrespondentsInternal(ctx, seq.target, startTime, endTime, seq.cutoff, limit)
	seq.mysql.logError("could not read correspondents", err)
	return
}

func (seq *mySQLHistorySequence) Cutoff() time.Time {
	return seq.cutoff
}

func (seq *mySQLHistorySequence) Ephemeral() bool {
	return false
}

func (mysql *MySQL) MakeSequence(target, correspondent string, cutoff time.Time) history.Sequence {
	return &mySQLHistorySequence{
		target:        target,
		correspondent: correspondent,
		mysql:         mysql,
		cutoff:        cutoff,
	}
}

func (mysql *MySQL) GetPMs(casefoldedUser string) (results map[string]int64, err error) {
	if mysql.db == nil {
		return
	}
	results = make(map[string]int64)

	ctx, cancel := context.WithTimeout(context.Background(), mysql.getTimeout())
	defer cancel()

	var queryBuf strings.Builder
	args := make([]interface{}, 0)

	queryBuf.WriteString(`SELECT max(nanotime), target, sender FROM history WHERE target = ? OR (sender = ? and pm = true) GROUP BY target, sender;`)
	args = append(args, casefoldedUser, casefoldedUser)

	rows, err := mysql.db.QueryContext(ctx, queryBuf.String(), args...)
	if mysql.logError("could not get pms", err) {
		return
	}
	defer rows.Close()

	var last int64
	var target, sender string
	for rows.Next() {
		err = rows.Scan(&last, &target, &sender)
		if mysql.logError("could not get pms", err) {
			return
		}
		// We really don't need nanosecond precision
		if target != casefoldedUser {
			results[target] = last / 1000000
		} else {
			results[sender] = last / 1000000
		}
	}
	return
}