引用体系：HEAD 与 branch

$ git log --oneline
a3f2b8c feat: add user authentication
7d1e4f2 fix: resolve null pointer in parser
b9c0a3d chore: update dependencies

# 查看一个引用指向的实际 commit
$ cat .git/refs/heads/master
a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9

# 引用可以参与所有期望 commit 哈希的命令
$ git diff master~3 master    # 等价于 git diff <hash1> <hash2>
$ git log master --oneline    # 等价于 git log <hash> --oneline

.git/
├── HEAD                    # 当前位置引用
├── refs/
│   ├── heads/              # 本地分支
│   │   ├── master
│   │   ├── feature/login
│   │   └── hotfix/issue-42
│   ├── tags/               # 标签
│   │   ├── v1.0
│   │   └── v2.0
│   └── remotes/            # 远程追踪分支
│       └── origin/
│           ├── master
│           └── develop

ref: refs/heads/master

a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9

# 查看当前 HEAD 的内容
$ cat .git/HEAD
ref: refs/heads/master

# 查看 HEAD 指向的实际 commit（两种方式等价）
$ git rev-parse HEAD
a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9

$ git rev-parse --short HEAD
a3f2b8c

$ git checkout v1.0
Note: switching to 'v1.0'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

HEAD is now at a3f2b8c feat: add user authentication

# 当前处于 detached HEAD，已创建若干 commit
$ git switch -c save-my-work
# 现在 HEAD → save-my-work → 最新 commit，提交安全了

# 记住当前 commit 哈希
$ git log --oneline -1
abc1234 my valuable work

# 切回目标分支
$ git checkout main

# 合并进来
$ git merge abc1234

$ git reflog
a3f2b8c HEAD@{0}: checkout: moving from abc1234 to main
abc1234 HEAD@{1}: commit: my valuable work    # 这就是我们要找回的
7d1e4f2 HEAD@{2}: checkout: moving from main to v1.0

# 基于该 commit 创建分支
$ git branch recovered-work abc1234

# 创建分支的底层操作（等价于 git branch feature）
$ echo "a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9" > .git/refs/heads/feature

# 查看分支指向的 commit
$ git rev-parse feature
a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9

# 当前在 master 分支，HEAD → master → commit A
$ git branch feature
# feature 也指向 commit A，HEAD 不变

# 在 detached HEAD 状态下创建分支
$ git checkout a3f2b8c   # 进入 detached HEAD
$ git branch from-detached
# from-detached 指向 a3f2b8c，HEAD 不变（仍是直接引用）

$ git switch -c feature
# 等价于：
# 1. git branch feature    （创建分支，指向当前 HEAD 对应的 commit）
# 2. git checkout feature  （将 HEAD 指向 feature）

# HEAD → master → commit A
$ git commit -m "new work"
# HEAD → master → commit B → commit A
#                ^ master 前进了

$ git merge feature  # 如果 master 是 feature 的祖先
# master 前进到 feature 指向的 commit

$ git reset --hard HEAD~3
# 当前分支回退 3 个 commit

$ git init
# 初始化空仓库，此时没有任何分支

$ git commit --allow-empty -m "initial commit"
# 创建了第一个 commit，同时创建了 master 分支指向该 commit
# .git/refs/heads/master 被自动创建

$ cat .git/HEAD
ref: refs/heads/master    # HEAD 默认指向 master

# 配置默认分支名（Git 2.28+）
$ git config --global init.defaultBranch main

# 查看当前默认分支名
$ git config --global init.defaultBranch
main

$ git clone https://github.com/user/repo.git
# 等价于以下操作的组合：
# 1. git init
# 2. git remote add origin <url>
# 3. git fetch origin
# 4. git checkout main  （检出远程默认分支，创建本地追踪分支）

.git/refs/
├── heads/                     # 本地分支引用
│   ├── master                 # 内容：a3f2b8c...
│   ├── main                   # 内容：7d1e4f2...
│   └── feature/
│       ├── login              # 内容：b9c0a3d...
│       └── dashboard          # 内容：e5f6a7b...
├── tags/                      # 标签引用
│   ├── v1.0                   # 内容：c9d0e1f...（轻量标签）
│   └── v2.0                   # 内容：d0e1f2a...（附注标签指向标签对象）
└── remotes/                   # 远程追踪引用
    └── origin/
        ├── master             # 内容：a3f2b8c...
        └── develop            # 内容：f2a3b4c...

$ cat .git/refs/heads/master
a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9

$ wc -c .git/refs/heads/master
      41 .git/refs/heads/master    # 40 字符 + 1 换行符

# 以下操作会失败
$ git branch feature
$ git branch feature/login
fatal: a branch named 'feature' already exists

# 反过来也一样
$ git branch feature/login
$ git branch feature
fatal: a branch named 'feature' already exists

# 引用存在但文件不在 refs/ 目录下
$ git branch -a
* main
  feature/login
  remotes/origin/master

$ ls .git/refs/heads/
main
# feature/login 不在这里，可能在 packed-refs 中

$ git rev-parse feature/login
b9c0a3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9
# 但引用依然可以正常解析

# 危险操作！不要这样做！
$ echo "7d1e4f2c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a" > .git/refs/heads/master
# 这会让 master 指向一个可能不存在的 commit，导致仓库损坏

# 安全的方式
$ git update-ref refs/heads/master 7d1e4f2c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a
# git update-ref 会进行合法性校验

# pack-refs with: peeled fully-peeled sorted
a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9 refs/heads/master
7d1e4f2c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a refs/heads/feature/login
b9c0a3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9 refs/remotes/origin/master
c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8 refs/tags/v1.0
^d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9 refs/tags/v2.0
^e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0

# 当 master 分支前进时
$ git commit -m "update"

# Git 的内部操作：
# 1. 计算新 commit 的 SHA-1
# 2. 写入 .git/refs/heads/master（创建或覆盖松散文件）
# 3. packed-refs 中的旧条目被松散文件覆盖（不修改 packed-refs）

# 手动触发
$ git pack-refs --all

# git gc 自动触发
$ git gc
# 内部会调用 git pack-refs --all

# git fetch 大量远程引用后可能触发
$ git fetch --all

# HEAD 的解析过程
$ git rev-parse HEAD
# 1. 读取 .git/HEAD
# 2. 发现内容为 "ref: refs/heads/master"
# 3. 递归解析 refs/heads/master
#    a. 检查 .git/refs/heads/master 松散文件
#    b. 若不存在，检查 .git/packed-refs
# 4. 返回最终的 SHA-1 哈希值

# 任意引用的解析
$ git rev-parse feature/login
# 1. 补全为 refs/heads/feature/login
# 2. 检查 .git/refs/heads/feature/login
# 3. 若不存在，检查 .git/packed-refs
# 4. 返回 SHA-1

# 查看符号引用的目标
$ git symbolic-ref HEAD
refs/heads/master
# 只对符号引用有效，直接引用会报错