import { HTTP_STATUS } from "../config/constants.ts";

type StreamReadinessLogger = {
  debug?: (tag: string, message: string) => void;
  warn?: (tag: string, message: string) => void;
};

export type StreamReadinessResult =
  | { ok: true; response: Response }
  | { ok: false; response: Response; reason: string; code: string; type: string };

function isRecord(value: unknown): value is Record<string, unknown> {
  return !!value && typeof value === "object" && !Array.isArray(value);
}

function hasNonEmptyString(value: unknown): boolean {
  return typeof value === "string" && value.length > 0;
}

function hasUsefulValue(value: unknown): boolean {
  if (hasNonEmptyString(value)) return true;
  if (Array.isArray(value)) return value.some(hasUsefulValue);
  if (!isRecord(value)) return false;

  for (const key of [
    "content",
    "text",
    "delta",
    "reasoning_content",
    "reasoning",
    // Mistral/Magistral thinking arrays and StepFun/OpenRouter reasoning_details are
    // valid model output — without these a reasoning-only stream was misclassified as
    // "no useful content" and turned into a spurious 502 (#2520).
    "thinking",
    "reasoning_details",
    "partial_json",
    "arguments",
    "name",
    "thought",
    "error",
    "executableCode",
    "codeExecutionResult",
  ]) {
    const candidate = value[key];
    if (hasNonEmptyString(candidate)) return true;
    if ((Array.isArray(candidate) || isRecord(candidate)) && hasUsefulValue(candidate)) return true;
  }

  for (const key of [
    "tool_calls",
    "tool_use",
    "function",
    "functionCall",
    "function_call",
    "function_call_output",
    "output",
    "content_block",
    "response",
    "choices",
    "candidates",
    "parts",
  ]) {
    if (hasUsefulValue(value[key])) return true;
  }

  return false;
}

function hasUsefulJsonPayload(payload: unknown): boolean {
  if (!isRecord(payload)) return false;
  return hasUsefulValue(payload);
}

function hasOpenAIResponseLifecyclePayload(
  payload: Record<string, unknown>,
  type: string
): boolean {
  if (type === "response.created" || type === "response.in_progress") {
    const response = payload.response;
    if (!isRecord(response)) return false;

    return (
      hasNonEmptyString(response.id) ||
      hasNonEmptyString(response.object) ||
      hasNonEmptyString(response.status) ||
      typeof response.created_at === "number"
    );
  }

  if (type === "response.output_item.added") {
    const item = payload.item;
    if (!isRecord(item)) return false;

    return (
      hasNonEmptyString(item.id) ||
      hasNonEmptyString(item.type) ||
      hasNonEmptyString(item.status) ||
      Array.isArray(item.content) ||
      isRecord(item.content)
    );
  }

  return false;
}

function hasChatCompletionToolCallStart(value: unknown): boolean {
  const hasToolCallId = (item: unknown) => isRecord(item) && hasNonEmptyString(item.id);
  if (Array.isArray(value)) return value.some(hasToolCallId);
  return hasToolCallId(value);
}

function hasChatCompletionFunctionCallStart(value: unknown): boolean {
  return isRecord(value) && hasNonEmptyString(value.name);
}

function hasChatCompletionChunkStartPayload(payload: Record<string, unknown>): boolean {
  if (payload.object !== "chat.completion.chunk" && payload.type !== "chat.completion.chunk") {
    return false;
  }

  const choices = payload.choices;
  if (!Array.isArray(choices) || choices.length === 0) return false;

  return choices.some((choice) => {
    if (!isRecord(choice)) return false;
    const delta = choice.delta;
    if (!isRecord(delta)) return false;

    return (
      hasNonEmptyString(delta.role) ||
      hasChatCompletionToolCallStart(delta.tool_calls) ||
      hasChatCompletionFunctionCallStart(delta.function_call)
    );
  });
}

function hasAcceptedStreamStartPayload(payload: unknown, eventType = ""): boolean {
  if (!isRecord(payload)) return false;

  // Anthropic/Claude streams can legitimately start with lifecycle frames and
  // OpenAI Responses streams can do the same before the first text/tool delta
  // arrives. Treating structurally valid lifecycle frames as readiness prevents
  // false 504s while ping-only/generic-empty zombie streams still fail below.
  const type = typeof payload.type === "string" ? payload.type : eventType;
  if (type === "message_start" && isRecord(payload.message)) return true;
  if (type === "content_block_start" && isRecord(payload.content_block)) return true;
  if (hasOpenAIResponseLifecyclePayload(payload, type)) return true;
  if (hasChatCompletionChunkStartPayload(payload)) return true;

  return false;
}

export function hasUsefulStreamContent(text: string): boolean {
  const lines = text.split(/\r?\n/);

  for (const line of lines) {
    const trimmed = line.trim();
    if (!trimmed || trimmed.startsWith(":")) continue;
    if (/^event:\s*(?:ping|keepalive)$/i.test(trimmed)) continue;
    if (!trimmed.startsWith("data:")) continue;

    const data = trimmed.slice(5).trim();
    if (!data || data === "[DONE]") continue;

    try {
      if (hasUsefulJsonPayload(JSON.parse(data))) return true;
    } catch {
      if (data.length > 0) return true;
    }
  }

  return false;
}

type StreamReadinessSignalState = {
  currentEvent: string;
  pendingLine: string;
};

function processStreamReadinessLine(state: StreamReadinessSignalState, line: string): boolean {
  const trimmed = line.trim();
  if (!trimmed || trimmed.startsWith(":")) {
    if (!trimmed) state.currentEvent = "";
    return false;
  }

  if (trimmed.startsWith("event:")) {
    state.currentEvent = trimmed.slice(6).trim();
    return false;
  }

  if (/^(?:ping|keepalive)$/i.test(state.currentEvent)) return false;
  if (!trimmed.startsWith("data:")) return false;

  const data = trimmed.slice(5).trim();
  if (!data || data === "[DONE]") return false;

  try {
    const parsed = JSON.parse(data);
    return (
      hasUsefulJsonPayload(parsed) || hasAcceptedStreamStartPayload(parsed, state.currentEvent)
    );
  } catch {
    return data.length > 0;
  }
}

function appendStreamReadinessSignal(state: StreamReadinessSignalState, chunk: string): boolean {
  const lines = `${state.pendingLine}${chunk}`.split(/\r?\n/);
  state.pendingLine = lines.pop() ?? "";

  for (const line of lines) {
    if (processStreamReadinessLine(state, line)) return true;
  }

  return false;
}

export function hasStreamReadinessSignal(text: string): boolean {
  const state: StreamReadinessSignalState = {
    currentEvent: "",
    pendingLine: "",
  };
  if (appendStreamReadinessSignal(state, text)) return true;
  if (state.pendingLine) return processStreamReadinessLine(state, state.pendingLine);
  return false;
}

function createErrorResponse(
  status: number,
  message: string,
  code: string,
  type: string
): Response {
  return new Response(
    JSON.stringify({
      error: {
        message,
        type,
        code,
      },
    }),
    { status, headers: { "Content-Type": "application/json" } }
  );
}

function prependBufferedChunks(
  chunks: Uint8Array[],
  reader: ReadableStreamDefaultReader<Uint8Array>
): ReadableStream<Uint8Array> {
  return new ReadableStream<Uint8Array>({
    async start(controller) {
      try {
        for (const chunk of chunks) {
          controller.enqueue(chunk);
        }

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

        controller.close();
      } catch (error) {
        controller.error(error);
      } finally {
        reader.releaseLock();
      }
    },
    async cancel(reason) {
      await reader.cancel(reason).catch(() => {});
      reader.releaseLock();
    },
  });
}

function readWithTimeout(
  reader: ReadableStreamDefaultReader<Uint8Array>,
  timeoutMs: number
): Promise<ReadableStreamReadResult<Uint8Array>> {
  return new Promise((resolve, reject) => {
    const timeout = setTimeout(() => reject(new Error("STREAM_READINESS_TIMEOUT")), timeoutMs);
    reader.read().then(
      (value) => {
        clearTimeout(timeout);
        resolve(value);
      },
      (error) => {
        clearTimeout(timeout);
        reject(error);
      }
    );
  });
}

export async function ensureStreamReadiness(
  response: Response,
  options: {
    timeoutMs: number;
    provider?: string | null;
    model?: string | null;
    log?: StreamReadinessLogger | null;
  }
): Promise<StreamReadinessResult> {
  if (!response.body || options.timeoutMs <= 0) return { ok: true, response };

  const reader = response.body.getReader();
  const chunks: Uint8Array[] = [];
  const decoder = new TextDecoder();
  const readinessState: StreamReadinessSignalState = {
    currentEvent: "",
    pendingLine: "",
  };
  const startedAt = Date.now();
  const deadline = startedAt + options.timeoutMs;
  let handedOffReader = false;

  try {
    while (true) {
      const remainingMs = deadline - Date.now();
      if (remainingMs <= 0) {
        const reason = `Stream produced no useful content within ${options.timeoutMs}ms`;
        options.log?.warn?.(
          "STREAM",
          `${reason} (${options.provider || "provider"}/${options.model || "unknown"})`
        );
        await reader.cancel(reason).catch(() => {});
        return {
          ok: false,
          reason,
          code: "STREAM_READINESS_TIMEOUT",
          type: "stream_timeout",
          response: createErrorResponse(
            HTTP_STATUS.GATEWAY_TIMEOUT,
            reason,
            "STREAM_READINESS_TIMEOUT",
            "stream_timeout"
          ),
        };
      }

      let readResult: ReadableStreamReadResult<Uint8Array>;
      try {
        readResult = await readWithTimeout(reader, remainingMs);
      } catch {
        const reason = `Stream produced no useful content within ${options.timeoutMs}ms`;
        options.log?.warn?.(
          "STREAM",
          `${reason} (${options.provider || "provider"}/${options.model || "unknown"})`
        );
        await reader.cancel(reason).catch(() => {});
        return {
          ok: false,
          reason,
          code: "STREAM_READINESS_TIMEOUT",
          type: "stream_timeout",
          response: createErrorResponse(
            HTTP_STATUS.GATEWAY_TIMEOUT,
            reason,
            "STREAM_READINESS_TIMEOUT",
            "stream_timeout"
          ),
        };
      }

      if (readResult.done) {
        const reason = "Stream ended before producing useful content";
        options.log?.warn?.(
          "STREAM",
          `${reason} (${options.provider || "provider"}/${options.model || "unknown"})`
        );
        return {
          ok: false,
          reason,
          code: "STREAM_EARLY_EOF",
          type: "stream_early_eof",
          response: createErrorResponse(
            HTTP_STATUS.BAD_GATEWAY,
            reason,
            "STREAM_EARLY_EOF",
            "stream_early_eof"
          ),
        };
      }

      if (!readResult.value) continue;
      chunks.push(readResult.value);
      const decodedChunk = decoder.decode(readResult.value, { stream: true });

      if (appendStreamReadinessSignal(readinessState, decodedChunk)) {
        options.log?.debug?.(
          "STREAM",
          `Stream readiness confirmed in ${Date.now() - startedAt}ms (${options.provider || "provider"}/${options.model || "unknown"})`
        );
        handedOffReader = true;
        return {
          ok: true,
          response: new Response(prependBufferedChunks(chunks, reader), {
            status: response.status,
            statusText: response.statusText,
            headers: response.headers,
          }),
        };
      }
    }
  } finally {
    if (!handedOffReader) {
      reader.releaseLock();
    }
  }
}