/**
 * Loads template file contents for {@link Engine.importTemplate}.
 *
 * - `.json` files are read directly.
 * - `.ts`, `.js`, `.mts`, `.cts`, `.mjs`, `.cjs` files are evaluated in a
 *   short-lived child process and serialized to Extended JSON on stdout.
 *   TypeScript templates (and the loader in dev) run via tsx; plain JS uses node.
 */

import { spawn } from "node:child_process";
import { createRequire } from "node:module";
import fs from "node:fs";
import path from "node:path";
import { fileURLToPath } from "node:url";

/** Extensions loaded via subprocess module evaluation. */
const MODULE_TEMPLATE_EXTENSIONS = new Set([
  ".ts",
  ".tsx",
  ".mts",
  ".cts",
  ".js",
  ".jsx",
  ".mjs",
  ".cjs",
]);

/** Maximum time allowed for a template module child process. */
const MODULE_LOAD_TIMEOUT_MS = 30_000;

/** Maximum stdout size from the loader child (50 MiB). */
const MODULE_LOAD_MAX_BUFFER_BYTES = 50 * 1024 * 1024;

/**
 * Thrown when a template file cannot be read or loaded.
 */
export class TemplateLoadError extends Error {
  constructor(message: string) {
    super(message);
    this.name = "TemplateLoadError";
  }
}

/**
 * Resolves the tsx CLI binary shipped with this package.
 */
function resolveTsxCliPath(): string {
  const require = createRequire(import.meta.url);
  const tsxPackageJsonPath = require.resolve("tsx/package.json");
  return path.join(path.dirname(tsxPackageJsonPath), "dist/cli.mjs");
}

/**
 * Resolves the loader script path for dev (.ts) and production (.js) layouts.
 */
function resolveTemplateModuleLoaderPath(): string {
  const directory = path.dirname(fileURLToPath(import.meta.url));
  const compiledLoaderPath = path.join(directory, "template-module-loader.js");
  if (fs.existsSync(compiledLoaderPath)) {
    return compiledLoaderPath;
  }

  const sourceLoaderPath = path.join(directory, "template-module-loader.ts");
  if (fs.existsSync(sourceLoaderPath)) {
    return sourceLoaderPath;
  }

  throw new TemplateLoadError(
    "Template module loader script was not found in the xo-cli package.",
  );
}

/** TypeScript extensions that require tsx to evaluate the template module. */
const TYPESCRIPT_TEMPLATE_EXTENSIONS = new Set([
  ".ts",
  ".tsx",
  ".mts",
  ".cts",
]);

/**
 * Loads a TS/JS template module in an isolated child process.
 * Returns Extended JSON suitable for {@link parseTemplate}.
 */
async function loadTemplateModuleViaChildProcess(
  absolutePath: string,
): Promise<string> {
  const loaderPath = resolveTemplateModuleLoaderPath();
  const extension = path.extname(absolutePath).toLowerCase();
  const loaderIsTypeScript = loaderPath.endsWith(".ts");
  const useTsx =
    TYPESCRIPT_TEMPLATE_EXTENSIONS.has(extension) || loaderIsTypeScript;
  const executable = useTsx ? resolveTsxCliPath() : process.execPath;
  const args = [loaderPath, absolutePath];

  return new Promise<string>((resolve, reject) => {
    const child = spawn(executable, args, {
      cwd: path.dirname(absolutePath),
      stdio: ["ignore", "pipe", "pipe"],
      env: process.env,
    });

    let stdout = "";
    let stderr = "";
    let stdoutBytes = 0;

    const timeout = setTimeout(() => {
      child.kill("SIGTERM");
      reject(
        new TemplateLoadError(
          `Template module load timed out after ${MODULE_LOAD_TIMEOUT_MS / 1000}s`,
        ),
      );
    }, MODULE_LOAD_TIMEOUT_MS);

    child.stdout.on("data", (chunk: Buffer | string) => {
      const text = typeof chunk === "string" ? chunk : chunk.toString("utf8");
      stdoutBytes += Buffer.byteLength(text, "utf8");
      if (stdoutBytes > MODULE_LOAD_MAX_BUFFER_BYTES) {
        child.kill("SIGTERM");
        reject(
          new TemplateLoadError(
            "Template module output exceeded the maximum allowed size.",
          ),
        );
        return;
      }
      stdout += text;
    });

    child.stderr.on("data", (chunk: Buffer | string) => {
      stderr += typeof chunk === "string" ? chunk : chunk.toString("utf8");
    });

    child.on("error", (error) => {
      clearTimeout(timeout);
      reject(
        new TemplateLoadError(
          `Failed to start template module loader: ${error.message}`,
        ),
      );
    });

    child.on("close", (code) => {
      clearTimeout(timeout);

      if (code !== 0) {
        reject(
          new TemplateLoadError(
            stderr.trim() ||
              `Template module loader exited with code ${code ?? "unknown"}`,
          ),
        );
        return;
      }

      if (stdout.trim().length === 0) {
        reject(new TemplateLoadError("Template module loader returned no output."));
        return;
      }

      resolve(stdout);
    });
  });
}

/**
 * Loads template contents from disk.
 *
 * @param filePath - Absolute or relative path to a JSON or module template file.
 * @returns Extended JSON string for {@link Engine.importTemplate}.
 */
export async function loadTemplateFromFile(filePath: string): Promise<string> {
  const absolutePath = path.resolve(filePath);

  if (!fs.existsSync(absolutePath)) {
    throw new TemplateLoadError(`Template file does not exist: ${absolutePath}`);
  }

  const extension = path.extname(absolutePath).toLowerCase();

  if (extension === ".json") {
    return fs.promises.readFile(absolutePath, "utf8");
  }

  if (MODULE_TEMPLATE_EXTENSIONS.has(extension)) {
    return loadTemplateModuleViaChildProcess(absolutePath);
  }

  throw new TemplateLoadError(
    `Unsupported template file extension "${extension}". ` +
      "Use .json or a JavaScript/TypeScript module that exports an XOTemplate.",
  );
}