TypeScriptでGitを自作した話

const GIT_INDEX_SIGNATURE = "DIRC";
const GIT_INDEX_VERSION = 2;

export const add = (): void => {
  // リポジトリ内の各ファイルのスナップショットデータを作成
  const entries = createEntries();
  const entryBuffers: Buffer[] = [];

　　　　// .git/indexにはバイナリデータとして書き込む必要があるのでそれの準備
  entries.forEach((entry) => {
    const buffers: Buffer[] = [];

    entry.forEach((value, key) => {
      if (key === "filePath") {
        buffers.push(Buffer.from(value, "ascii"));
      } else {
        buffers.push(Buffer.from(value, "hex"));
      }
    });

    const buffer = Buffer.concat([...buffers]);
    const padding = calculatePadding(buffer.length);
    const bufferAfterPadding = Buffer.concat([
      buffer,
      Buffer.from("0".repeat(padding), "hex"),
    ]);
    entryBuffers.push(bufferAfterPadding);
  });

  // 各スナップショットデータをバイナリ化したものを結合 & ヘッダを作成し、.git/indexに書き込む
  const indexBody = Buffer.concat([...entryBuffers]);
  const indexHeader = createHeader(entries.length);
  const indexData = Buffer.concat([indexHeader, indexBody]);

  const indexFilePath = join(getGitRootPath(), "index");
  writeFileSync(indexFilePath, indexData);
};

// 再帰的にリポジトリ内を探索し、各ファイルのスナップショットデータを作成する
const createEntries = (
  path = getCheckoutRepoRootPath(),
  entries: GitIndexEntryDict[] = []
) => {
  const items = readdirSync(path);
  for (const item of items) {
    if (IGNORE_FILES_AND_DIRS.includes(item)) {
      continue;
    }

    const itemPath = join(path, item);
    if (lstatSync(itemPath).isDirectory()) {
      createEntries(itemPath, entries);
    } else {
      entries.push(createEntry(itemPath));
    }
  }

  return entries;
};

// スナップショットデータを作成する処理
// フォーマットはhttps://github.com/git/git/blob/v2.12.0/Documentation/technical/index-format.txtに従う（一部省略している）
const createEntry = (filePath: string) => {
  const fileMeta = lstatSync(filePath, { bigint: true });
  const dict: GitIndexEntryDict = new Map();

  // ctime
  const ctimeNanosecond = fileMeta.ctimeNs;
  dict.set(
    "ctimeSecond",
    alignLength(
      parseInt(ctimeNanosecond.toString().slice(0, 10)).toString(16),
      8
    )
  );

  // ctime nanosecond
  dict.set(
    "ctimeNanosecond",
    alignLength(parseInt(ctimeNanosecond.toString().slice(10)).toString(16), 8)
  );

  // mtime
  const mtimeNanosecond = fileMeta.mtimeNs;
  dict.set(
    "mtimeSecond",
    alignLength(
      parseInt(mtimeNanosecond.toString().slice(0, 10)).toString(16),
      8
    )
  );

  // mtime nanosecond
  dict.set(
    "mtimeNanosecond",
    alignLength(parseInt(mtimeNanosecond.toString().slice(10)).toString(16), 8)
  );

  // dev
  const dev = fileMeta.dev;
  dict.set("dev", alignLength(dev.toString(16), 8));

  // ino
  const ino = fileMeta.ino;
  dict.set("ino", alignLength(ino.toString(16), 8));

  // mode
  const mode = fileMeta.mode;
  dict.set("mode", alignLength(mode.toString(16), 8));

  // uid
  const uid = fileMeta.uid;
  dict.set("uid", alignLength(uid.toString(16), 8));

  // gid
  const gid = fileMeta.gid;
  dict.set("gid", alignLength(gid.toString(16), 8));

  // size
  const size = fileMeta.size;
  dict.set("size", alignLength(size.toString(16), 8));

  // create git object and get sha
  const sha = createBlobObject(filePath);
  dict.set("sha", sha);

  const filePathInRepo = filePath.replace(`${getCheckoutRepoRootPath()}/`, "");

  // filePathLength
  dict.set(
    "filePathLength",
    alignLength(filePathInRepo.length.toString(16), 4)
  );

  // filePath
  dict.set("filePath", filePath.replace(`${getCheckoutRepoRootPath()}/`, ""));

  return dict;
};

type Blob = {
  mode: typeof BLOB_MODE;
  path: string;
  sha: string;
};

type Tree = {
  mode: typeof TREE_MODE;
  path: string;
  leafs: Map<string, Blob | Tree>;
};

export const commit = async (commitMessage: string): Promise<void> => {
  // .git/indexをread
  const gitIndexRaw = readFileSync(join(getGitRootPath(), "index"));
  // 内容をパース
  const gitIndex = indexParser(gitIndexRaw);
  // 中間状態としてリポジトリ構造を表した木構造を作成
  const tree = createTreeStruct(gitIndex.entries);
  // 上記のデータ構造を用いてプロジェクトルートのtreeディレクトリを作成
  const rootTreeSha = await createGitTreeObject(tree.leafs);

  // commit情報を作成
  const parentCommit = refResolve(join(getGitRootPath(), "HEAD"));
  // when first commit, parent commit is none.
  const parentCommitMessage = parentCommit ? `parent ${parentCommit}` : "";
  const author = "";

  const commitContent = `tree ${rootTreeSha}
${parentCommitMessage}
author ${author}

${commitMessage}
`;

  // commitオブジェクトのヘッダを作成
  const commitData =
    "commit" + " " + commitContent.length + "\0" + commitContent;
  // commitオブジェクトの保存先を計算
  const sha = createHash("sha1").update(commitData).digest("hex");
  const gitObjectPath = getGitObjectPathFromHash(sha);

  if (existsSync(gitObjectPath)) {
    return;
  }

  if (!existsSync(getGitObjectDirNameFromHash(sha))) {
    mkdirSync(getGitObjectDirNameFromHash(sha));
  }

  gzip(commitData, (_, buf) => {
    // 計算した保存先にcommit情報を書き込む
    writeFileSync(gitObjectPath, buf);
    // 現在のブランチの参照先を更新
    updateRef(sha);
  });