/**
 * Anthropic provider — translates OpenAI chat completion requests
 * to Anthropic Messages API calls using native fetch().
 */
export function createProvider(config) {
  const { id, apiKey, baseUrl, models } = config;

  const messagesUrl = baseUrl ? baseUrl.replace(/\/+$/, '') : 'https://api.anthropic.com/v1/messages';

  function buildHeaders() {
    return {
      'Content-Type': 'application/json',
      'x-api-key': apiKey,
      'anthropic-version': '2023-06-01'
    };
  }

  function translateMessages(openAiMessages) {
    const systemParts = [];
    const messages = [];

    for (const msg of openAiMessages) {
      if (msg.role === 'system') {
        systemParts.push(msg.content);
      } else {
        messages.push({ role: msg.role, content: msg.content });
      }
    }

    const system = systemParts.length > 0 ? systemParts.join('\n\n') : undefined;
    return { system, messages };
  }

  function buildBody(openAiMessages, modelId, options, stream) {
    const { system, messages } = translateMessages(openAiMessages);

    const body = {
      model: modelId,
      messages,
      max_tokens: options.max_tokens ?? 4096,
      stream
    };

    if (system !== undefined) {
      body.system = system;
    }
    if (options.temperature !== undefined) {
      body.temperature = options.temperature;
    }
    if (options.top_p !== undefined) {
      body.top_p = options.top_p;
    }

    return body;
  }

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

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

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

    return response;
  }

  async function chat(openAiMessages, localModelId, options = {}) {
    const providerModelId = models[localModelId];
    const response = await request(openAiMessages, providerModelId, options, false);
    const anthropicResponse = await response.json();

    return {
      id: anthropicResponse.id,
      object: 'chat.completion',
      created: Math.floor(Date.now() / 1000),
      model: localModelId,
      choices: [
        {
          index: 0,
          message: {
            role: 'assistant',
            content: anthropicResponse.content[0].text
          },
          finish_reason: 'stop'
        }
      ],
      usage: {
        prompt_tokens: anthropicResponse.usage.input_tokens,
        completion_tokens: anthropicResponse.usage.output_tokens,
        total_tokens: anthropicResponse.usage.input_tokens + anthropicResponse.usage.output_tokens
      }
    };
  }

  async function* chatStream(openAiMessages, localModelId, options = {}) {
    const providerModelId = models[localModelId];
    const response = await request(openAiMessages, providerModelId, options, true);

    const reader = response.body.getReader();
    const decoder = new TextDecoder();
    let buffer = '';

    try {
      while (true) {
        const { done, value } = await reader.read();
        if (done) break;

        buffer += decoder.decode(value, { stream: true });

        const lines = buffer.split('\n');
        buffer = lines.pop(); // keep incomplete line in buffer

        for (const line of lines) {
          const trimmed = line.trim();
          if (!trimmed || !trimmed.startsWith('data:')) continue;

          const dataStr = trimmed.slice(5).trim();
          if (dataStr === '[DONE]') continue;

          let data;
          try {
            data = JSON.parse(dataStr);
          } catch {
            continue;
          }

          if (data.type === 'content_block_delta' && data.delta?.text) {
            yield {
              id: `chunk-${Date.now()}`,
              object: 'chat.completion.chunk',
              created: Math.floor(Date.now() / 1000),
              model: localModelId,
              choices: [
                {
                  index: 0,
                  delta: { content: data.delta.text },
                  finish_reason: null
                }
              ]
            };
          } else if (data.type === 'message_stop') {
            yield {
              id: `chunk-${Date.now()}`,
              object: 'chat.completion.chunk',
              created: Math.floor(Date.now() / 1000),
              model: localModelId,
              choices: [
                {
                  index: 0,
                  delta: {},
                  finish_reason: 'stop'
                }
              ]
            };
          }
        }
      }
    } finally {
      reader.releaseLock();
    }
  }

  return { id, chat, chatStream };
}