package history

import (
	"context"
	"fmt"
	"time"
)

// This file owns the per-session read cursor (design/16 §4.2, Android 协同批次
// D4/B3). There is a single owner account and no per-device identity at this
// layer, so the cursor is session-scoped: reading on any one end marks the session
// read for every end of the account. The cursor lives on chat_sessions.read_at in
// the same millisecond unit as chat_messages.created_at, so the unread badge is a
// plain `created_at > read_at` count (see listSessionsQuery).

// MarkSessionRead advances the session's read cursor to before and returns the
// resulting cursor as a Unix millisecond timestamp. A zero before means "now" — the
// caller's "mark everything I have seen" case, which does not have to read the
// clock itself.
//
// The cursor only ever moves forward: the UPDATE applies SQLite's two-argument
// scalar MAX, so a stale or out-of-order request (two ends racing, a retried
// request) can never un-read messages. A negative before therefore leaves the stored
// value untouched rather than moving it backwards.
//
// An unknown session id returns ErrSessionNotFound, matching RenameSession/SetPinned
// — isolated (group/stranger) sessions have no chat_sessions row and are not
// addressable here.
func (s *Store) MarkSessionRead(ctx context.Context, sessionID string, before time.Time) (int64, error) {
	ms := before.UnixMilli()
	if before.IsZero() {
		ms = time.Now().UnixMilli()
	}
	res, err := s.db.ExecContext(ctx,
		`UPDATE chat_sessions SET read_at = MAX(read_at, ?) WHERE id = ?`, ms, sessionID)
	if err != nil {
		return 0, fmt.Errorf("history: mark session read %s: %w", sessionID, err)
	}
	n, err := res.RowsAffected()
	if err != nil {
		return 0, fmt.Errorf("history: mark session read %s rows: %w", sessionID, err)
	}
	if n == 0 {
		return 0, fmt.Errorf("history: mark session read %s: %w", sessionID, ErrSessionNotFound)
	}
	var readAt int64
	if err := s.db.QueryRowContext(ctx,
		`SELECT read_at FROM chat_sessions WHERE id = ?`, sessionID).Scan(&readAt); err != nil {
		return 0, fmt.Errorf("history: read session cursor %s: %w", sessionID, err)
	}
	return readAt, nil
}