/**
 * Decide whether a given file is "generated" (regenerated by a build step,
 * unsafe to write variants into) or "source" (safe to edit, changes persist).
 *
 * Why this matters: when the user picks an element on a page whose underlying
 * file is regenerated by a build step (e.g. `scripts/build-sub-pages.js`
 * rewriting `public/docs/*.html`), writing variants or accepted changes into
 * that file is silent data loss — the next build wipes them.
 *
 * Signals, in order of reliability:
 *   1. Git check-ignore: gitignored files are assumed generated.
 *   2. File-header markers ("GENERATED", "DO NOT EDIT", "AUTO-GENERATED")
 *      within the first ~300 characters — catches non-git projects.
 */

import { execSync } from 'node:child_process';
import fs from 'node:fs';
import path from 'node:path';

const HEADER_SCAN_BYTES = 300;
const HEADER_MARKERS = [
  /@generated\b/i,
  /\bGENERATED\s+FILE\b/,
  /\bAUTO-?GENERATED\b/i,
  /\bDO\s+NOT\s+EDIT\b/i,
];

/**
 * @param {string} filePath - absolute or cwd-relative path
 * @param {object} [options]
 * @param {string} [options.cwd] - project root (defaults to process.cwd())
 */
export function isGeneratedFile(filePath, options = {}) {
  const cwd = options.cwd || process.cwd();
  const absPath = path.isAbsolute(filePath) ? filePath : path.resolve(cwd, filePath);

  if (isGitIgnored(absPath, cwd)) return true;
  if (hasGeneratedHeader(absPath)) return true;
  return false;
}

function isGitIgnored(absPath, cwd) {
  try {
    execSync(`git check-ignore --quiet ${JSON.stringify(absPath)}`, {
      cwd,
      stdio: 'ignore',
    });
    return true; // exit 0 = ignored
  } catch (err) {
    // Exit code 1 = not ignored. Exit code 128 = not a git repo or other error.
    // In both cases, treat as "not known to be ignored."
    return false;
  }
}

function hasGeneratedHeader(absPath) {
  let fd;
  try {
    fd = fs.openSync(absPath, 'r');
    const buf = Buffer.alloc(HEADER_SCAN_BYTES);
    const bytesRead = fs.readSync(fd, buf, 0, HEADER_SCAN_BYTES, 0);
    const head = buf.slice(0, bytesRead).toString('utf-8');
    return HEADER_MARKERS.some((re) => re.test(head));
  } catch {
    return false;
  } finally {
    if (fd !== undefined) { try { fs.closeSync(fd); } catch {} }
  }
}
