const { spawnSync } = require('node:child_process');
const fs = require('node:fs');
const os = require('node:os');
const path = require('node:path');

function runGit(args, { cwd, env, accept = [0] }) {
  const result = spawnSync('git', args, { cwd, env, encoding: 'utf8' });
  if (result.error) throw new Error(`Failed to run git: ${result.error.message}`);
  if (!accept.includes(result.status)) {
    throw new Error((result.stderr || result.stdout || `git exited ${result.status}`).trim());
  }
  return result;
}

function syncRepository({ forgeUrl, upstreamUrl, branch, env }) {
  const directory = fs.mkdtempSync(path.join(os.tmpdir(), 'stoke-repo-sync-'));
  const forgeRef = 'refs/stoke/forge-branch';
  const upstreamRef = 'refs/stoke/upstream-branch';

  try {
    runGit(['init', '--bare', directory], { cwd: directory, env });
    runGit(['fetch', '--no-tags', forgeUrl, `refs/heads/${branch}:${forgeRef}`], {
      cwd: directory,
      env,
    });
    runGit(['fetch', '--no-tags', upstreamUrl, `refs/heads/${branch}:${upstreamRef}`], {
      cwd: directory,
      env,
    });

    const oldSha = runGit(['rev-parse', forgeRef], { cwd: directory, env }).stdout.trim();
    const newSha = runGit(['rev-parse', upstreamRef], { cwd: directory, env }).stdout.trim();
    const ancestry = runGit(['merge-base', '--is-ancestor', oldSha, newSha], {
      cwd: directory,
      env,
      accept: [0, 1],
    });
    if (ancestry.status !== 0) {
      throw new Error(`Refusing diverged branch ${branch}: forge ${oldSha}, upstream ${newSha}. Diverged trees are out of scope; follow ceremony docs/UPSTREAM-SYNC.md.`);
    }

    if (oldSha !== newSha) {
      runGit(['push', forgeUrl, `${upstreamRef}:refs/heads/${branch}`], {
        cwd: directory,
        env,
      });
    }
    return { branch, oldSha, newSha };
  } finally {
    fs.rmSync(directory, { recursive: true, force: true });
  }
}

module.exports = { syncRepository };