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/writer.go

@@ -0,0 +1,155 @@
+package sst
+
+import (
+	"encoding/json"
+	"os"
+
+	"code.tczkiot.com/srdb/btree"
+	"github.com/golang/snappy"
+)
+
+// Writer SST 文件写入器
+type Writer struct {
+	file        *os.File
+	builder     *btree.Builder
+	dataOffset  int64
+	dataStart   int64 // 数据起始位置
+	rowCount    int64
+	minKey      int64
+	maxKey      int64
+	minTime     int64
+	maxTime     int64
+	compression uint8
+}
+
+// NewWriter 创建 SST 写入器
+func NewWriter(file *os.File) *Writer {
+	return &Writer{
+		file:        file,
+		builder:     btree.NewBuilder(file, HeaderSize),
+		dataOffset:  0, // 先写数据，后面会更新
+		compression: CompressionSnappy,
+		minKey:      -1,
+		maxKey:      -1,
+		minTime:     -1,
+		maxTime:     -1,
+	}
+}
+
+// Row 表示一行数据
+type Row struct {
+	Seq  int64          // _seq
+	Time int64          // _time
+	Data map[string]any // 用户数据
+}
+
+// Add 添加一行数据
+func (w *Writer) Add(row *Row) error {
+	// 更新统计信息
+	if w.minKey == -1 || row.Seq < w.minKey {
+		w.minKey = row.Seq
+	}
+	if w.maxKey == -1 || row.Seq > w.maxKey {
+		w.maxKey = row.Seq
+	}
+	if w.minTime == -1 || row.Time < w.minTime {
+		w.minTime = row.Time
+	}
+	if w.maxTime == -1 || row.Time > w.maxTime {
+		w.maxTime = row.Time
+	}
+	w.rowCount++
+
+	// 序列化数据 (简单的 JSON 序列化，后续可以优化)
+	data := encodeRow(row)
+
+	// 压缩数据
+	var compressed []byte
+	if w.compression == CompressionSnappy {
+		compressed = snappy.Encode(nil, data)
+	} else {
+		compressed = data
+	}
+
+	// 写入数据块
+	// 第一次写入时，确定数据起始位置
+	if w.dataStart == 0 {
+		// 预留足够空间给 B+Tree 索引
+		// 假设索引最多占用 10% 的空间，最少 1 MB
+		estimatedIndexSize := int64(10 * 1024 * 1024) // 10 MB
+		w.dataStart = HeaderSize + estimatedIndexSize
+		w.dataOffset = w.dataStart
+	}
+
+	offset := w.dataOffset
+	_, err := w.file.WriteAt(compressed, offset)
+	if err != nil {
+		return err
+	}
+
+	// 添加到 B+Tree
+	err = w.builder.Add(row.Seq, offset, int32(len(compressed)))
+	if err != nil {
+		return err
+	}
+
+	// 更新数据偏移
+	w.dataOffset += int64(len(compressed))
+
+	return nil
+}
+
+// Finish 完成写入
+func (w *Writer) Finish() error {
+	// 1. 构建 B+Tree 索引
+	rootOffset, err := w.builder.Build()
+	if err != nil {
+		return err
+	}
+
+	// 2. 计算索引大小
+	indexSize := w.dataStart - HeaderSize
+
+	// 3. 创建 Header
+	header := &Header{
+		Magic:       MagicNumber,
+		Version:     Version,
+		Compression: w.compression,
+		IndexOffset: HeaderSize,
+		IndexSize:   indexSize,
+		RootOffset:  rootOffset,
+		DataOffset:  w.dataStart,
+		DataSize:    w.dataOffset - w.dataStart,
+		RowCount:    w.rowCount,
+		MinKey:      w.minKey,
+		MaxKey:      w.maxKey,
+		MinTime:     w.minTime,
+		MaxTime:     w.maxTime,
+	}
+
+	// 4. 写入 Header
+	headerData := header.Marshal()
+	_, err = w.file.WriteAt(headerData, 0)
+	if err != nil {
+		return err
+	}
+
+	// 5. Sync 到磁盘
+	return w.file.Sync()
+}
+
+// encodeRow 编码行数据 (使用二进制格式)
+func encodeRow(row *Row) []byte {
+	// 使用二进制格式编码
+	encoded, err := encodeRowBinary(row)
+	if err != nil {
+		// 降级到 JSON (不应该发生)
+		data := map[string]interface{}{
+			"_seq":  row.Seq,
+			"_time": row.Time,
+			"data":  row.Data,
+		}
+		encoded, _ = json.Marshal(data)
+	}
+	return encoded
+}