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package checkpoint
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// LogEntry WAL 日志条目
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type LogEntry struct {
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    ID        int
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    Operation string
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    Key       string
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    Value     any
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}
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// stateSnapshot 检查点快照
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type stateSnapshot struct {
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    state       map[string]any
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    walPosition int // 快照对应的 WAL 位置
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}
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// CheckpointableStore 带检查点的可恢复存储
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type CheckpointableStore struct {
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    state      map[string]any
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    wal        []LogEntry
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    nextID     int
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    checkpoint *stateSnapshot
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}
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func NewStore() *CheckpointableStore {
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    return &CheckpointableStore{
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        state:  make(map[string]any),
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        nextID: 1,
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    }
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}
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// Apply 执行操作并记录到 WAL
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func (s *CheckpointableStore) Apply(operation, key string, value any) {
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    entry := LogEntry{ID: s.nextID, Operation: operation, Key: key, Value: value}
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    s.nextID++
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    s.wal = append(s.wal, entry)
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    s.executeOp(entry)
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}
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// Get 读取当前状态
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func (s *CheckpointableStore) Get(key string) (any, bool) {
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    v, ok := s.state[key]
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    return v, ok
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}
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// TakeCheckpoint 创建检查点：快照当前状态和 WAL 位置
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func (s *CheckpointableStore) TakeCheckpoint() {
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    snap := make(map[string]any, len(s.state))
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    for k, v := range s.state {
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        snap[k] = v
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    }
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    s.checkpoint = &stateSnapshot{state: snap, walPosition: len(s.wal)}
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}
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// SimulateCrash 模拟崩溃：清空内存状态
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func (s *CheckpointableStore) SimulateCrash() {
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    s.state = make(map[string]any)
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}
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// Recover 从检查点 + WAL 恢复，返回重放的条目数
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func (s *CheckpointableStore) Recover() int {
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    if s.checkpoint != nil {
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        s.state = make(map[string]any, len(s.checkpoint.state))
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        for k, v := range s.checkpoint.state {
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            s.state[k] = v
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        }
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        replayed := 0
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        for i := s.checkpoint.walPosition; i < len(s.wal); i++ {
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            s.executeOp(s.wal[i])
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            replayed++
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        }
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        return replayed
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    }
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    // 无检查点：重放整个 WAL
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    s.state = make(map[string]any)
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    for _, entry := range s.wal {
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        s.executeOp(entry)
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    }
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    return len(s.wal)
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}
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func (s *CheckpointableStore) executeOp(entry LogEntry) {
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    switch entry.Operation {
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    case "SET":
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        s.state[entry.Key] = entry.Value
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    case "DELETE":
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        delete(s.state, entry.Key)
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    }
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}