Initial commit: SRDB - High-performance LSM-Tree database

- Core engine with MemTable, SST, WAL
- B+Tree indexing for SST files  
- Leveled compaction strategy
- Multi-table database management
- Schema validation and secondary indexes
- Query builder with complex conditions
- Web UI with HTMX for data visualization
- Command-line tools for diagnostics

sst/reader.go

@@ -0,0 +1,152 @@
+package sst
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"code.tczkiot.com/srdb/btree"
+	"github.com/edsrzf/mmap-go"
+	"github.com/golang/snappy"
+)
+
+// Reader SST 文件读取器
+type Reader struct {
+	path     string
+	file     *os.File
+	mmap     mmap.MMap
+	header   *Header
+	btReader *btree.Reader
+}
+
+// NewReader 创建 SST 读取器
+func NewReader(path string) (*Reader, error) {
+	// 1. 打开文件
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	// 2. mmap 映射
+	mmapData, err := mmap.Map(file, mmap.RDONLY, 0)
+	if err != nil {
+		file.Close()
+		return nil, err
+	}
+
+	// 3. 读取 Header
+	if len(mmapData) < HeaderSize {
+		mmapData.Unmap()
+		file.Close()
+		return nil, fmt.Errorf("file too small")
+	}
+
+	header := UnmarshalHeader(mmapData[:HeaderSize])
+	if header == nil || !header.Validate() {
+		mmapData.Unmap()
+		file.Close()
+		return nil, fmt.Errorf("invalid header")
+	}
+
+	// 4. 创建 B+Tree Reader
+	btReader := btree.NewReader(mmapData, header.RootOffset)
+
+	return &Reader{
+		path:     path,
+		file:     file,
+		mmap:     mmapData,
+		header:   header,
+		btReader: btReader,
+	}, nil
+}
+
+// Get 查询一行数据
+func (r *Reader) Get(key int64) (*Row, error) {
+	// 1. 检查范围
+	if key < r.header.MinKey || key > r.header.MaxKey {
+		return nil, fmt.Errorf("key out of range")
+	}
+
+	// 2. 在 B+Tree 中查找
+	dataOffset, dataSize, found := r.btReader.Get(key)
+	if !found {
+		return nil, fmt.Errorf("key not found")
+	}
+
+	// 3. 读取数据
+	if dataOffset+int64(dataSize) > int64(len(r.mmap)) {
+		return nil, fmt.Errorf("invalid data offset")
+	}
+
+	compressed := r.mmap[dataOffset : dataOffset+int64(dataSize)]
+
+	// 4. 解压缩
+	var data []byte
+	var err error
+	if r.header.Compression == CompressionSnappy {
+		data, err = snappy.Decode(nil, compressed)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		data = compressed
+	}
+
+	// 5. 反序列化
+	row, err := decodeRow(data)
+	if err != nil {
+		return nil, err
+	}
+
+	return row, nil
+}
+
+// GetHeader 获取文件头信息
+func (r *Reader) GetHeader() *Header {
+	return r.header
+}
+
+// GetPath 获取文件路径
+func (r *Reader) GetPath() string {
+	return r.path
+}
+
+// GetAllKeys 获取文件中所有的 key（按顺序）
+func (r *Reader) GetAllKeys() []int64 {
+	return r.btReader.GetAllKeys()
+}
+
+// Close 关闭读取器
+func (r *Reader) Close() error {
+	if r.mmap != nil {
+		r.mmap.Unmap()
+	}
+	if r.file != nil {
+		return r.file.Close()
+	}
+	return nil
+}
+
+// decodeRow 解码行数据
+func decodeRow(data []byte) (*Row, error) {
+	// 尝试使用二进制格式解码
+	row, err := decodeRowBinary(data)
+	if err == nil {
+		return row, nil
+	}
+
+	// 降级到 JSON (兼容旧数据)
+	var decoded map[string]interface{}
+	err = json.Unmarshal(data, &decoded)
+	if err != nil {
+		return nil, err
+	}
+
+	row = &Row{
+		Seq:  int64(decoded["_seq"].(float64)),
+		Time: int64(decoded["_time"].(float64)),
+		Data: decoded["data"].(map[string]interface{}),
+	}
+
+	return row, nil
+}