import { tmpdir } from "node:os";

export type ProcessCaptureCapability =
  | { readonly available: true; readonly backend: "node-pty" }
  | {
      readonly available: false;
      readonly backend: "node-pty";
      readonly reason: string;
    };

/** Explain why process capture cannot claim owned-process cleanup on a host. */
export const processCaptureOwnershipUnavailableReason = (
  platform: NodeJS.Platform,
): string | undefined =>
  platform === "win32"
    ? "Windows process-tree ownership and cleanup are unavailable; REA cannot verify that descendants have stopped."
    : undefined;

/** Dependencies that make the native PTY probe's host boundary testable. */
export interface ProcessCaptureProbeOptions {
  readonly platform?: NodeJS.Platform;
  readonly loadPty?: () => Promise<typeof import("@lydell/node-pty")>;
}

/** Probe the native PTY seam and explain loader failures at their source. */
export const processCaptureProbeFailureReason = (cause: unknown): string => {
  const error = cause instanceof Error ? cause : new Error(String(cause));
  const code = "code" in error ? String(error.code) : "";
  const diagnostic = error.message;
  if (
    code === "ERR_MODULE_NOT_FOUND" ||
    code === "MODULE_NOT_FOUND" ||
    /cannot find module|cannot find package/iu.test(diagnostic)
  )
    return `the native PTY module is missing (${diagnostic}); reinstall REA for this host with optional dependencies enabled (for a source checkout, run npm ci)`;
  if (
    /NODE_MODULE_VERSION|compiled against a different Node\.js version|module version mismatch/iu.test(
      diagnostic,
    )
  )
    return `the native PTY module is incompatible with this Node.js runtime (${diagnostic}); reinstall REA for this host and active Node.js version with optional dependencies enabled (for a source checkout, run npm ci after selecting that Node.js version)`;
  if (code === "ERR_DLOPEN_FAILED")
    return `the native PTY library failed to load (${diagnostic}); check the loader error, then reinstall REA for this host with optional dependencies enabled (for a source checkout, run npm ci)`;
  return `the native PTY probe failed (${diagnostic}); check the reported operating-system or process-startup error and reinstall REA for this host with optional dependencies enabled if the native binary is missing`;
};

/** Probe the actual native PTY seam instead of inferring support from the OS name. */
export const probeProcessCaptureCapability = async (
  options: ProcessCaptureProbeOptions = {},
): Promise<ProcessCaptureCapability> => {
  const platform = options.platform ?? process.platform;
  const ownershipReason = processCaptureOwnershipUnavailableReason(platform);
  if (ownershipReason !== undefined)
    return {
      available: false,
      backend: "node-pty",
      reason: ownershipReason,
    };
  try {
    const { spawn } = await (
      options.loadPty ?? (() => import("@lydell/node-pty"))
    )();
    const terminal = spawn(
      platform === "win32" ? "cmd.exe" : "/bin/sh",
      platform === "win32" ? ["/c", "exit", "0"] : ["-c", "exit 0"],
      {
        cwd: tmpdir(),
        env: { HOME: tmpdir(), TERM: "xterm-256color" },
        cols: 80,
        rows: 24,
        name: "xterm-256color",
      },
    );
    await new Promise<void>((resolveExit) =>
      terminal.onExit(() => resolveExit()),
    );
    return { available: true, backend: "node-pty" };
  } catch (cause: unknown) {
    return {
      available: false,
      backend: "node-pty",
      reason: processCaptureProbeFailureReason(cause),
    };
  }
};