/**
 * OpenAI-compatible provider — proxies requests to any OpenAI-compatible backend
 * (vLLM, Ollama, LiteLLM, etc.) using native fetch().
 */
export function createProvider(config) {
  const { id, baseUrl, apiKey, models } = config;

  // Normalize baseUrl: strip trailing slash, ensure /v1 is present
  let normalizedUrl = baseUrl.replace(/\/+$/, '');
  if (!normalizedUrl.endsWith('/v1')) {
    normalizedUrl += '/v1';
  }
  const completionsUrl = `${normalizedUrl}/chat/completions`;

  function buildHeaders() {
    const headers = { 'Content-Type': 'application/json' };
    if (apiKey) headers['Authorization'] = `Bearer ${apiKey}`;
    return headers;
  }

  function buildBody(messages, modelId, options, stream) {
    const providerModelId = models[modelId] || modelId;
    const body = { model: providerModelId, messages, stream };
    if (options.temperature !== undefined) body.temperature = options.temperature;
    if (options.max_tokens   !== undefined) body.max_tokens   = options.max_tokens;
    if (options.top_p        !== undefined) body.top_p        = options.top_p;
    if (options.stop         !== undefined) body.stop         = options.stop;
    return body;
  }

  async function request(messages, modelId, options, stream) {
    const body    = buildBody(messages, modelId, options, stream);
    const headers = buildHeaders();

    let response;
    try {
      response = await fetch(completionsUrl, {
        method: 'POST',
        headers,
        body: JSON.stringify(body)
      });
    } catch (error) {
      throw new Error(`OpenAI-compatible provider request failed: ${error.message}`);
    }

    if (!response.ok) {
      let errorBody = '';
      try { errorBody = await response.text(); } catch { /* ignore */ }
      throw new Error(
        `OpenAI-compatible provider returned ${response.status}${errorBody ? `: ${errorBody}` : ''}`
      );
    }

    return response;
  }

  async function chat(messages, modelId, options = {}) {
    const response = await request(messages, modelId, options, false);
    return response.json();
  }

  async function chatStream(messages, modelId, options = {}) {
    const response = await request(messages, modelId, options, true);
    return response.body;
  }

  return { id, chat, chatStream };
}