import * as fs from 'fs';

// Get all descendant PIDs of the given root PID by reading /proc/<pid>/stat.
// Returns an array of descendant PIDs (excludes the root itself).
export async function getChildPids(rootPid: number): Promise<number[]> {
  const parentMap = new Map<number, number[]>(); // parent → children

  let entries: string[];
  try {
    entries = fs.readdirSync('/proc');
  } catch {
    return [];
  }

  for (const entry of entries) {
    const pid = parseInt(entry, 10);
    if (isNaN(pid)) continue;

    try {
      const stat = fs.readFileSync(`/proc/${pid}/stat`, 'utf8');
      // Format: pid (comm) state ppid ...
      // comm can contain spaces and parentheses, so find the last ')' first
      const closeParenIdx = stat.lastIndexOf(')');
      if (closeParenIdx === -1) continue;
      const afterComm = stat.slice(closeParenIdx + 2); // skip ') '
      const fields = afterComm.split(' ');
      const ppid = parseInt(fields[1], 10); // field index 1 after state is ppid

      if (!parentMap.has(ppid)) {
        parentMap.set(ppid, []);
      }
      parentMap.get(ppid)!.push(pid);
    } catch {
      // Process may have exited between readdir and readFile
      continue;
    }
  }

  // BFS from rootPid to collect all descendants
  const result: number[] = [];
  const queue: number[] = [rootPid];

  while (queue.length > 0) {
    const current = queue.shift()!;
    const children = parentMap.get(current);
    if (children) {
      for (const child of children) {
        result.push(child);
        queue.push(child);
      }
    }
  }

  return result;
}