package history

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

// This file owns the two derived per-session display fields carried in the session
// list (GET /api/v1/sessions): the last-message preview and the unread count. Both are
// read-only projections so a client can render a conversation row (with its unread
// badge) without one request per session. The preview is truncated for display, and
// neither field ever 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 and its unread count
// 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. The unread count is a correlated COUNT(*) over the same index
// (session_id = s.id AND created_at > s.read_at is an equality + range scan), so no
// per-session round trip (N+1) is needed and no extra index is required. 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,
		(SELECT COUNT(*) FROM chat_messages c
			WHERE c.session_id = s.id AND c.created_at > s.read_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) and the UnreadCount derived from the session's read cursor.
// Sessions, previews and unread counts 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 + unread-count 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. The
// unread count is a non-NULL COUNT(*), so it scans into a plain int.
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, &meta.UnreadCount); 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]) + "…"
}