package history

import (
	"context"
	"database/sql"
	"fmt"
	"strings"
)

// This file owns the last-message preview carried in the session list
// (GET /api/v1/sessions): a display-only summary of each session's newest message so a
// client can render a conversation row without one request per session. The preview is
// truncated for display and never feeds identity or permission decisions — identity
// comes from channel actor metadata / JWT (project red line).

// lastMessagePreviewRunes bounds the preview content so a long message cannot bloat the
// session-list response. A longer message is whitespace-collapsed and truncated to this
// many runes with an ellipsis.
const lastMessagePreviewRunes = 60

// MessageSummary is a display-only preview of a session's most recent message.
type MessageSummary struct {
	Role      string // "user" | "assistant"
	Content   string // whitespace-collapsed, truncated preview
	CreatedAt int64  // Unix milliseconds
}

// listSessionsQuery reads every session plus its newest message in one statement. The
// LEFT JOIN's correlated subquery resolves each session's newest row via the
// (session_id, created_at DESC) index (rowid DESC breaks a same-millisecond tie by
// insertion order), so a session with no messages still appears with NULL preview
// columns. Ordering floats pinned sessions first, then keeps the main session before
// unpinned subs, then subs oldest-first.
const listSessionsQuery = `SELECT s.id, s.type, s.title, s.pinned, s.created_at,
		m.role, m.content, m.created_at
	FROM chat_sessions s
	LEFT JOIN chat_messages m ON m.rowid = (
		SELECT rowid FROM chat_messages
		WHERE session_id = s.id
		ORDER BY created_at DESC, rowid DESC
		LIMIT 1
	)
	ORDER BY s.pinned DESC, (s.type = ?) DESC, s.created_at ASC`

// ListSessions returns pinned sessions first, then the main session, then every sub
// session, oldest first, each carrying a LastMessage preview of its newest message (nil
// when the session has none). Sessions and previews are read in a single query, so the
// query count does not grow with the number of sessions. The returned slice is non-nil.
func (s *Store) ListSessions(ctx context.Context) ([]SessionMeta, error) {
	rows, err := s.db.QueryContext(ctx, listSessionsQuery, SessionMain)
	if err != nil {
		return nil, fmt.Errorf("history: list sessions: %w", err)
	}
	defer func() { _ = rows.Close() }()

	sessions := make([]SessionMeta, 0)
	for rows.Next() {
		meta, err := scanSessionMeta(rows)
		if err != nil {
			return nil, fmt.Errorf("history: scan session: %w", err)
		}
		sessions = append(sessions, meta)
	}
	if err := rows.Err(); err != nil {
		return nil, fmt.Errorf("history: iterate sessions: %w", err)
	}
	return sessions, nil
}

// scanSessionMeta reads one joined chat_sessions + last-message row. The three message
// columns are NULL for a session that has never received a message, so they scan into
// nullable holders and only produce a LastMessage when a row is present.
func scanSessionMeta(sc rowScanner) (SessionMeta, error) {
	var (
		meta    SessionMeta
		typeStr string
		pinned  int
		role    sql.NullString
		content sql.NullString
		lastAt  sql.NullInt64
	)
	if err := sc.Scan(&meta.ID, &typeStr, &meta.Title, &pinned, &meta.CreatedAt, &role, &content, &lastAt); err != nil {
		return SessionMeta{}, err
	}
	meta.Type = typeStr
	meta.Pinned = pinned != 0
	if role.Valid && lastAt.Valid {
		meta.LastMessage = &MessageSummary{
			Role:      role.String,
			Content:   previewContent(content.String),
			CreatedAt: lastAt.Int64,
		}
	}
	return meta, nil
}

// previewContent collapses whitespace runs (newlines included) to single spaces and
// truncates to lastMessagePreviewRunes runes with an ellipsis. It is display-only.
func previewContent(content string) string {
	joined := strings.Join(strings.Fields(content), " ")
	runes := []rune(joined)
	if len(runes) <= lastMessagePreviewRunes {
		return joined
	}
	return string(runes[:lastMessagePreviewRunes]) + "…"
}