/**
 * Background task worker — survives tab close.
 *
 * Agent execution is decoupled from the HTTP request lifecycle.  `startRun`
 * spawns the agent call in a detached Promise that writes StreamEvents into
 * event-log.ts.  The POST and GET handlers in the stream route simply read
 * from the event log, so client disconnects never kill the run.
 *
 * A Map<taskId, RunHandle> tracks active runs.  Each handle carries an
 * AbortController so the run can be cancelled on demand.
 */

import { appendEvent, markDone, clearLog } from "./event-log";
import type { StreamEvent } from "@/lib/types";

export interface RunHandle {
  /** Resolves when the run finishes (success or error). */
  promise: Promise<void>;
  /** Signal abort to cancel the run. */
  abortController: AbortController;
}

const runs = new Map<string, RunHandle>();

export interface RunOptions {
  /** Phylo backend base URL (e.g. http://127.0.0.1:8601). */
  backendUrl: string;
  /** Selected knowledge base IDs (passed through to the backend). */
  kbIds?: string[];
}

/**
 * Start (or re-attach to) a background agent run for `taskId`.
 *
 * If a run is already active for this task the existing handle is returned —
 * no duplicate runs are spawned.
 *
 * The run streams from the Phylo backend SSE endpoint and pipes every event
 * into the event log.  On completion (or error) the log is marked done and
 * the handle is removed from the map.
 */
export function startRun(
  taskId: string,
  messages: Array<{ role: string; content: string }>,
  options: RunOptions,
): RunHandle {
  const existing = runs.get(taskId);
  if (existing) return existing;

  // Reset any stale event log from a previous run so follow-up turns on
  // the same task can stream new output instead of replaying + closing.
  clearLog(taskId);

  const abortController = new AbortController();

  const promise = executeRun(taskId, messages, options, abortController.signal)
    .catch((err) => {
      const msg = err instanceof Error ? err.message : "Unknown worker error";
      appendEvent(taskId, { type: "error", error: msg });
    })
    .finally(() => {
      markDone(taskId);
      runs.delete(taskId);
    });

  const handle: RunHandle = { promise, abortController };
  runs.set(taskId, handle);
  return handle;
}

/** Whether a run is currently active for `taskId`. */
export function isTaskRunning(taskId: string): boolean {
  return runs.has(taskId);
}

/** Signal abort for an active run.  No-op if the task isn't running. */
export function abortRun(taskId: string): void {
  const handle = runs.get(taskId);
  if (handle) {
    handle.abortController.abort();
  }
}

// ---------------------------------------------------------------------------
// Internal: the actual SSE fetch → event-log pipeline
// ---------------------------------------------------------------------------

async function executeRun(
  taskId: string,
  messages: Array<{ role: string; content: string }>,
  options: RunOptions,
  signal: AbortSignal,
): Promise<void> {
  const url = `${options.backendUrl}/api/tasks/${encodeURIComponent(taskId)}/messages/stream`;

  const res = await fetch(url, {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    // Backend accepts { message, knowledge_bases } — send the latest user
    // message, not the full array (which would 422).
    body: JSON.stringify({
      message: messages[messages.length - 1]?.content ?? "",
      ...(options.kbIds ? { knowledge_bases: options.kbIds } : {}),
    }),
    signal,
  });

  if (!res.ok) {
    throw new Error(`Backend returned ${res.status}: ${await res.text().catch(() => "")}`);
  }

  if (!res.body) {
    throw new Error("Backend response has no body");
  }

  const reader = res.body.getReader();
  const decoder = new TextDecoder();
  let buffer = "";

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

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

      // Parse SSE frames: each event is "data: <json>\n\n"
      const lines = buffer.split("\n");
      buffer = lines.pop() ?? "";

      for (const line of lines) {
        const trimmed = line.trim();
        if (!trimmed || trimmed.startsWith(":")) continue; // comment / keep-alive
        if (trimmed.startsWith("data: ")) {
          const json = trimmed.slice(6);
          if (json === "[DONE]") {
            appendEvent(taskId, { type: "done" });
            return;
          }
          try {
            const event = JSON.parse(json) as StreamEvent;
            appendEvent(taskId, event);
          } catch {
            // Malformed JSON — skip
          }
        }
      }
    }

    // Stream ended without a [DONE] sentinel — emit done anyway
    appendEvent(taskId, { type: "done" });
  } finally {
    reader.releaseLock();
  }
}