/**
 * Cross-platform shell command spawning for background terminals.
 *
 * Windows: PowerShell 7 (pwsh) with -NoProfile, -NonInteractive.
 * POSIX: /bin/bash -c (detached:true provides process group isolation).
 */

import { spawn, spawnSync, type ChildProcess } from "node:child_process";
import { platform } from "node:os";

// ── Public API ──────────────────────────────────────────────────────────────────

export interface ShellOptions {
  /** Working directory. Defaults to process.cwd(). */
  cwd?: string;
  /** Additional environment variables. */
  env?: Record<string, string>;
  /** Called for each stdout chunk. */
  onStdout?: (data: string) => void;
  /** Called for each stderr chunk. */
  onStderr?: (data: string) => void;
  /** Called when the process exits. */
  onExit?: (code: number | null, signal: NodeJS.Signals | null) => void;
  /** Called on spawn error. */
  onError?: (err: Error) => void;
}

export interface SpawnedShell {
  /** The underlying child process. */
  process: ChildProcess;
  /** Process ID (also the process group leader on POSIX via setsid). */
  pid: number;
}

/**
 * Spawn a shell command as a background process with its own process group.
 *
 * On POSIX, `detached: true` makes the shell process call setsid() after fork,
 * so it becomes session leader with its PID as the process group. This allows
 * SIGTERM→SIGKILL escalation on the entire group via `kill(-pgid)`.
 *
 * On Windows, the process must remain attached: detached PowerShell can exit
 * immediately without running the command or forwarding its output. Full tree
 * termination still uses `taskkill /T /F`.
 */
export function spawnBackground(
  command: string,
  options: ShellOptions = {},
): SpawnedShell {
  const shellPath = getShellPath();
  const shellArgs = buildShellArgs(command);

  const child = spawn(shellPath, shellArgs, {
    cwd: options.cwd ?? process.cwd(),
    env: { ...process.env, ...options.env },
    stdio: ["ignore", "pipe", "pipe"],
    detached: platform() !== "win32",
    windowsHide: true,
  });

  if (options.onStdout) {
    child.stdout?.on("data", (chunk: Buffer) =>
      options.onStdout!(chunk.toString()),
    );
  }
  if (options.onStderr) {
    child.stderr?.on("data", (chunk: Buffer) =>
      options.onStderr!(chunk.toString()),
    );
  }
  if (options.onExit) {
    child.on("exit", options.onExit);
  }
  if (options.onError) {
    child.on("error", options.onError);
  }

  return {
    process: child,
    pid: child.pid ?? 0,
  };
}

/**
 * Return the shell path for the current platform.
 */
export function getShellPath(): string {
  return platform() === "win32" ? getPwshPath() : "/bin/bash";
}

/**
 * Build platform-appropriate shell arguments for `spawn()`.
 *
 * Windows: pwsh -NoLogo -NoProfile -NonInteractive -Command <command>
 * POSIX:  bash -c <command> (detached:true isolates process group)
 */
export function buildShellArgs(command: string, isWin?: boolean): string[] {
  const win = isWin ?? platform() === "win32";
  if (win) {
    return [
      "-NoLogo",
      "-NoProfile",
      "-NonInteractive",
      "-ExecutionPolicy",
      "Bypass",
      "-Command",
      command,
    ];
  }
  // POSIX: spawn with detached:true already calls setsid() after fork,
  // so bash becomes session leader and its process group equals its PID.
  // SIGTERM/SIGKILL on -<pid> will target the entire group.
  return ["-c", command];
}

// ── Internal ────────────────────────────────────────────────────────────────────

let cachedWindowsShell: string | undefined;

function getPwshPath(): string {
  if (process.env.PI_USER_BASH_PWSH) return process.env.PI_USER_BASH_PWSH;
  if (process.env.PI_USER_BASH_SHELL) return process.env.PI_USER_BASH_SHELL;
  if (cachedWindowsShell) return cachedWindowsShell;

  const probe = spawnSync("where.exe", ["pwsh.exe"], {
    stdio: "ignore",
    windowsHide: true,
    timeout: 2_000,
  });
  cachedWindowsShell = probe.status === 0 ? "pwsh.exe" : "powershell.exe";
  return cachedWindowsShell;
}