import type { ConversationMessage } from '../../gateway-api.js';

const CONTEXT_WINDOW = 200_000;
const CHARS_PER_TOKEN = 4;

function estimateTokens(messages: ConversationMessage[]): number {
  const totalChars = messages.reduce((sum, m) => sum + (m.content?.length ?? 0), 0);
  return Math.round(totalChars / CHARS_PER_TOKEN);
}

export interface HistoryContext {
  conversationId: string | undefined;
  conversationTitle?: string | null;
  gatewayUrl: string;
  sessionCookie: string | undefined;
  fetchMessages: (
    gatewayUrl: string,
    sessionCookie: string,
    conversationId: string,
  ) => Promise<ConversationMessage[]>;
}

export async function executeHistory(ctx: HistoryContext): Promise<string> {
  const { conversationId, conversationTitle, gatewayUrl, sessionCookie, fetchMessages } = ctx;

  if (!conversationId) {
    return 'No active conversation.';
  }

  if (!sessionCookie) {
    return 'Not authenticated — cannot fetch conversation messages.';
  }

  const messages = await fetchMessages(gatewayUrl, sessionCookie, conversationId);

  const userMessages = messages.filter((m) => m.role === 'user').length;
  const assistantMessages = messages.filter((m) => m.role === 'assistant').length;
  const totalMessages = messages.length;

  const estimatedTokens = estimateTokens(messages);
  const contextPercent = Math.round((estimatedTokens / CONTEXT_WINDOW) * 100);

  const label = conversationTitle ?? conversationId;

  const lines = [
    `Conversation: ${label}`,
    `Messages: ${totalMessages} (${userMessages} user, ${assistantMessages} assistant)`,
    `Estimated tokens: ~${estimatedTokens.toLocaleString()}`,
    `Context usage: ~${contextPercent}% of ${(CONTEXT_WINDOW / 1000).toFixed(0)}K`,
  ];

  return lines.join('\n');
}