import { useMemo } from 'react';
import type { Message } from '@/api/types';

export interface ChatContextStats {
  used: number;
  max: number;
  percent: number;
}

/**
 * Returns the latest context-window usage for the given chat, derived from the
 * assistant message (with both ctx_used and ctx_max populated) having the most
 * recent created_at. Returns null when no such message exists.
 *
 * Re-evaluates whenever the `messages` reference or `chatId` changes, which
 * matches the cadence of streaming updates from `useSessionStream`.
 */
export function useChatContextStats(
  chatId: string,
  messages: Message[],
): ChatContextStats | null {
  return useMemo(() => {
    let latest: Message | null = null;
    for (const m of messages) {
      if (m.chat_id !== chatId) continue;
      if (m.role !== 'assistant') continue;
      if (m.ctx_used == null || m.ctx_max == null) continue;
      if (!latest || m.created_at > latest.created_at) latest = m;
    }
    if (!latest || latest.ctx_used == null || latest.ctx_max == null) return null;
    const used = latest.ctx_used;
    const max = latest.ctx_max;
    if (max <= 0) return null;
    const percent = Math.round((used / max) * 100);
    return { used, max, percent };
  }, [chatId, messages]);
}