package history

import (
	"context"
	"errors"
	"fmt"
	"strings"
)

// This file owns the two session-title paths: auto-naming a session from its first
// user message and an explicit rename. A title is display-only metadata (design/16
// §2.1) and is never used to identify a session, a sender or a permission — identity
// always comes from the channel actor metadata / JWT (project red line).

const (
	// autoTitleMaxRunes bounds an auto-generated title so a session list row never
	// overflows; a longer first message is truncated with an ellipsis.
	autoTitleMaxRunes = 20
	// titleMainFallback and titleSubFallback name a session whose first user
	// message carries no text (e.g. an image-only turn), so the session list is
	// never nameless.
	titleMainFallback = "主会话"
	titleSubFallback  = "次级会话"
)

// ErrSessionNotFound is returned by RenameSession when no chat_sessions row carries
// the requested id.
var ErrSessionNotFound = errors.New("history: session not found")

// RenameSession sets the display title of sessionID. Both the fixed main session
// (id "web") and sub sessions may be renamed: renaming is not a destructive identity
// change, unlike deletion. An unknown id returns ErrSessionNotFound. The caller
// validates the title (non-empty, length-bounded); the store persists it verbatim.
func (s *Store) RenameSession(ctx context.Context, sessionID, title string) error {
	res, err := s.db.ExecContext(ctx, `UPDATE chat_sessions SET title = ? WHERE id = ?`, title, sessionID)
	if err != nil {
		return fmt.Errorf("history: rename session %s: %w", sessionID, err)
	}
	n, err := res.RowsAffected()
	if err != nil {
		return fmt.Errorf("history: rename session %s rows: %w", sessionID, err)
	}
	if n == 0 {
		return fmt.Errorf("history: rename session %s: %w", sessionID, ErrSessionNotFound)
	}
	return nil
}

// ensureTitle names a session from its first user message. It is a no-op unless the
// session row exists in chat_sessions, its title is still empty, and no user message
// has been persisted for it yet (so the caller must invoke it before inserting the
// message). A message that is all whitespace falls back to the session-type label.
// The title is display-only metadata, so a failure is logged and swallowed rather
// than failing the message write it accompanies.
func (s *Store) ensureTitle(ctx context.Context, sessionID, content string) {
	if sessionID == "" {
		return
	}
	title := deriveTitle(content)
	_, err := s.db.ExecContext(ctx, `UPDATE chat_sessions
		SET title = CASE WHEN ? = '' THEN CASE WHEN type = ? THEN ? ELSE ? END ELSE ? END
		WHERE id = ? AND title = ''
		  AND NOT EXISTS (SELECT 1 FROM chat_messages WHERE session_id = ? AND role = ?)`,
		title, SessionMain, titleMainFallback, titleSubFallback, title,
		sessionID, sessionID, RoleUser)
	if err != nil && s.log != nil {
		s.log.Warn("auto title failed", "err", err, "session_id", sessionID)
	}
}

// deriveTitle turns a user message into a session title: whitespace runs (newlines
// included) collapse to a single space, then the result is truncated to
// autoTitleMaxRunes runes with an ellipsis. A message that is all whitespace yields
// "" so ensureTitle falls back to the session-type label.
func deriveTitle(content string) string {
	fields := strings.Fields(content)
	if len(fields) == 0 {
		return ""
	}
	joined := strings.Join(fields, " ")
	runes := []rune(joined)
	if len(runes) <= autoTitleMaxRunes {
		return joined
	}
	return string(runes[:autoTitleMaxRunes]) + "…"
}