Bolt源码分析（二）--数据结构解读

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// node represents an in-memory, deserialized page.
type node struct {
	bucket     *Bucket
	isLeaf     bool  // 是否是叶子分支节点
	unbalanced bool
	spilled    bool
	key        []byte // 存储该node中最小key
	pgid       pgid  // 对应page页ID
	parent     *node
	children   nodes
	inodes     inodes
}

type nodes []*node

type inode struct {
	flags uint32  // leafPage时有意义
	pgid  pgid    // branchPage时有意义，存储对应key对应的数据pageID
	key   []byte  // 对应的key
	value []byte  // leafPage时，key对应value值
}

type inodes []inode

// read initializes the node from a page.
func (n *node) read(p *page) {
	n.pgid = p.id // 将page id写入node节点中的pgid
	n.isLeaf = (p.flags & leafPageFlag) != 0 // 记录是否为叶子节点
	n.inodes = make(inodes, int(p.count)) // 生成该node节点中的数据列表

	for i := 0; i < int(p.count); i++ {
		inode := &n.inodes[i]
		if n.isLeaf {
			elem := p.leafPageElement(uint16(i))
			inode.flags = elem.flags 
			inode.key = elem.key()
			inode.value = elem.value()
		} else {
			elem := p.branchPageElement(uint16(i))
			inode.pgid = elem.pgid
			inode.key = elem.key()
		}
		_assert(len(inode.key) > 0, "read: zero-length inode key")
	}

	// Save first key so we can find the node in the parent when we spill.
	if len(n.inodes) > 0 {
		n.key = n.inodes[0].key
		_assert(len(n.key) > 0, "read: zero-length node key")
	} else {
		n.key = nil
	}
}

// write writes the items onto one or more pages.
func (n *node) write(p *page) {
	// Initialize page.
	if n.isLeaf {
		p.flags |= leafPageFlag
	} else {
		p.flags |= branchPageFlag
	}

	if len(n.inodes) >= 0xFFFF {
		panic(fmt.Sprintf("inode overflow: %d (pgid=%d)", len(n.inodes), p.id))
	}
	p.count = uint16(len(n.inodes))

	// Stop here if there are no items to write.
	if p.count == 0 {
		return
	}

	// Loop over each item and write it to the page.
	b := (*[maxAllocSize]byte)(unsafe.Pointer(&p.ptr))[n.pageElementSize()*len(n.inodes):]
	for i, item := range n.inodes {
		_assert(len(item.key) > 0, "write: zero-length inode key")

		// Write the page element.
		if n.isLeaf {
			elem := p.leafPageElement(uint16(i))
			elem.pos = uint32(uintptr(unsafe.Pointer(&b[0])) - uintptr(unsafe.Pointer(elem)))
			elem.flags = item.flags
			elem.ksize = uint32(len(item.key))
			elem.vsize = uint32(len(item.value))
		} else {
			elem := p.branchPageElement(uint16(i))
			elem.pos = uint32(uintptr(unsafe.Pointer(&b[0])) - uintptr(unsafe.Pointer(elem)))
			elem.ksize = uint32(len(item.key))
			elem.pgid = item.pgid
			_assert(elem.pgid != p.id, "write: circular dependency occurred")
		}

		// If the length of key+value is larger than the max allocation size
		// then we need to reallocate the byte array pointer.
		//
		// See: https://github.com/boltdb/bolt/pull/335
		klen, vlen := len(item.key), len(item.value)
		if len(b) < klen+vlen {
			b = (*[maxAllocSize]byte)(unsafe.Pointer(&b[0]))[:]
		}

		// Write data for the element to the end of the page.
		copy(b[0:], item.key)
		b = b[klen:]
		copy(b[0:], item.value)
		b = b[vlen:]
	}

	// DEBUG ONLY: n.dump()
}

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b := (*[maxAllocSize]byte)(unsafe.Pointer(&p.ptr))[n.pageElementSize()*len(n.inodes):]