import { SessionManager } from "@earendil-works/pi-coding-agent";
import { stat } from "node:fs/promises";
import { resolve } from "node:path";

export interface CloneActiveSessionInput {
  parentSessionFile: string;
  sessionDir: string;
  cwd: string;
  leafId: string | null;
}

export interface ClonedSession {
  parentSessionFile: string;
  parentSessionId: string;
  childSessionFile: string;
  childSessionId: string;
  leafId: string;
}

async function requireFile(path: string, label: string): Promise<void> {
  let details;
  try {
    details = await stat(path);
  } catch (error) {
    throw new Error(`${label} does not exist: ${path}`, { cause: error });
  }
  if (!details.isFile()) throw new Error(`${label} is not a file: ${path}`);
}

/** Clone one persisted Pi branch without mutating the live SessionManager. */
export async function cloneActiveSession(input: CloneActiveSessionInput): Promise<ClonedSession> {
  const leafId = input.leafId?.trim();
  if (!leafId) throw new Error("The current Pi session has no saved leaf to clone. Continue the conversation first.");
  if (!input.parentSessionFile) throw new Error("The current Pi session is not persisted. Start Pi without --no-session and try again.");

  await requireFile(input.parentSessionFile, "Parent Pi session");
  const detached = SessionManager.open(input.parentSessionFile, input.sessionDir, input.cwd);
  if (!detached.getEntry(leafId)) {
    throw new Error(`The selected Pi session leaf is no longer present: ${leafId}`);
  }

  const branch = detached.getBranch(leafId);
  const hasAssistant = branch.some((entry) => entry.type === "message" && entry.message.role === "assistant");
  if (!hasAssistant) {
    throw new Error(
      "The selected branch has no completed assistant response. Continue on that branch, wait for an assistant response, then retry /sidetrack.",
    );
  }

  const parentSessionId = detached.getSessionId();
  const childSessionFile = detached.createBranchedSession(leafId);
  if (!childSessionFile) throw new Error("Pi did not create a persisted child session.");
  if (resolve(childSessionFile) === resolve(input.parentSessionFile)) {
    throw new Error("Pi returned the parent session path instead of a separate child session.");
  }
  await requireFile(childSessionFile, "Cloned Pi session");

  return {
    parentSessionFile: input.parentSessionFile,
    parentSessionId,
    childSessionFile,
    childSessionId: detached.getSessionId(),
    leafId,
  };
}