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

btree/builder.go

@@ -0,0 +1,122 @@
+package btree
+
+import (
+	"os"
+)
+
+// Builder 从下往上构建 B+Tree
+type Builder struct {
+	order     int          // B+Tree 阶数
+	file      *os.File     // 输出文件
+	offset    int64        // 当前写入位置
+	leafNodes []*BTreeNode // 叶子节点列表
+}
+
+// NewBuilder 创建构建器
+func NewBuilder(file *os.File, startOffset int64) *Builder {
+	return &Builder{
+		order:     Order,
+		file:      file,
+		offset:    startOffset,
+		leafNodes: make([]*BTreeNode, 0),
+	}
+}
+
+// Add 添加一个 key-value 对 (数据必须已排序)
+func (b *Builder) Add(key int64, dataOffset int64, dataSize int32) error {
+	// 获取或创建当前叶子节点
+	var leaf *BTreeNode
+	if len(b.leafNodes) == 0 || b.leafNodes[len(b.leafNodes)-1].IsFull() {
+		// 创建新的叶子节点
+		leaf = NewLeafNode()
+		b.leafNodes = append(b.leafNodes, leaf)
+	} else {
+		leaf = b.leafNodes[len(b.leafNodes)-1]
+	}
+
+	// 添加到叶子节点
+	leaf.AddData(key, dataOffset, dataSize)
+
+	return nil
+}
+
+// Build 构建完整的 B+Tree，返回根节点的 offset
+func (b *Builder) Build() (rootOffset int64, err error) {
+	if len(b.leafNodes) == 0 {
+		return 0, nil
+	}
+
+	// 1. 写入所有叶子节点，记录它们的 offset
+	leafOffsets := make([]int64, len(b.leafNodes))
+	for i, leaf := range b.leafNodes {
+		leafOffsets[i] = b.offset
+		data := leaf.Marshal()
+		_, err := b.file.WriteAt(data, b.offset)
+		if err != nil {
+			return 0, err
+		}
+		b.offset += NodeSize
+	}
+
+	// 2. 如果只有一个叶子节点，它就是根
+	if len(b.leafNodes) == 1 {
+		return leafOffsets[0], nil
+	}
+
+	// 3. 从下往上构建内部节点
+	currentLevel := b.leafNodes
+	currentOffsets := leafOffsets
+	level := 1
+
+	for len(currentLevel) > 1 {
+		nextLevel, nextOffsets, err := b.buildLevel(currentLevel, currentOffsets, level)
+		if err != nil {
+			return 0, err
+		}
+		currentLevel = nextLevel
+		currentOffsets = nextOffsets
+		level++
+	}
+
+	// 4. 返回根节点的 offset
+	return currentOffsets[0], nil
+}
+
+// buildLevel 构建一层内部节点
+func (b *Builder) buildLevel(children []*BTreeNode, childOffsets []int64, level int) ([]*BTreeNode, []int64, error) {
+	var parents []*BTreeNode
+	var parentOffsets []int64
+
+	// 每 order 个子节点创建一个父节点
+	for i := 0; i < len(children); i += b.order {
+		end := min(i+b.order, len(children))
+
+		// 创建父节点
+		parent := NewInternalNode(byte(level))
+
+		// 添加第一个子节点 (没有对应的 key)
+		parent.AddChild(childOffsets[i])
+
+		// 添加剩余的子节点和分隔 key
+		for j := i + 1; j < end; j++ {
+			// 分隔 key 是子节点的第一个 key
+			separatorKey := children[j].Keys[0]
+			parent.AddKey(separatorKey)
+			parent.AddChild(childOffsets[j])
+		}
+
+		// 写入父节点
+		parentOffset := b.offset
+		data := parent.Marshal()
+		_, err := b.file.WriteAt(data, b.offset)
+		if err != nil {
+			return nil, nil, err
+		}
+		b.offset += NodeSize
+
+		parents = append(parents, parent)
+		parentOffsets = append(parentOffsets, parentOffset)
+	}
+
+	return parents, parentOffsets, nil
+}