bolt/cursor.go

package bolt

import (
	"bytes"
	"sort"
)

// Cursor represents an iterator that can traverse over all key/value pairs in a bucket in sorted order.
// Cursors can be obtained from a Transaction and are valid as long as the Transaction is open.
type Cursor struct {
	transaction *Transaction
	root        pgid
	stack       []elemRef
}

// First moves the cursor to the first item in the bucket and returns its key and value.
// If the bucket is empty then a nil key and value are returned.
func (c *Cursor) First() (key []byte, value []byte) {
	c.stack = c.stack[:0]
	p, n := c.transaction.pageNode(c.root)
	c.stack = append(c.stack, elemRef{page: p, node: n, index: 0})
	c.first()
	return c.keyValue()
}

// Last moves the cursor to the last item in the bucket and returns its key and value.
// If the bucket is empty then a nil key and value are returned.
func (c *Cursor) Last() (key []byte, value []byte) {
	c.stack = c.stack[:0]
	p, n := c.transaction.pageNode(c.root)
	ref := elemRef{page: p, node: n}
	ref.index = ref.count() - 1
	c.stack = append(c.stack, ref)
	c.last()
	return c.keyValue()
}

// Next moves the cursor to the next item in the bucket and returns its key and value.
// If the cursor is at the end of the bucket then a nil key and value are returned.
func (c *Cursor) Next() (key []byte, value []byte) {
	// Attempt to move over one element until we're successful.
	// Move up the stack as we hit the end of each page in our stack.
	for i := len(c.stack) - 1; i >= 0; i-- {
		elem := &c.stack[i]
		if elem.index < elem.count()-1 {
			elem.index++
			break
		}
		c.stack = c.stack[:i]
	}

	// If we've hit the end then return nil.
	if len(c.stack) == 0 {
		return nil, nil
	}

	// Move down the stack to find the first element of the first leaf under this branch.
	c.first()
	return c.keyValue()
}

// Prev moves the cursor to the previous item in the bucket and returns its key and value.
// If the cursor is at the beginning of the bucket then a nil key and value are returned.
func (c *Cursor) Prev() (key []byte, value []byte) {
	// Attempt to move back one element until we're successful.
	// Move up the stack as we hit the beginning of each page in our stack.
	for i := len(c.stack) - 1; i >= 0; i-- {
		elem := &c.stack[i]
		if elem.index > 0 {
			elem.index--
			break
		}
		c.stack = c.stack[:i]
	}

	// If we've hit the end then return nil.
	if len(c.stack) == 0 {
		return nil, nil
	}

	// Move down the stack to find the last element of the last leaf under this branch.
	c.last()
	return c.keyValue()
}

// Seek moves the cursor to a given key and returns it.
// If the key does not exist then the next key is used. If no keys
// follow, a nil value is returned.
func (c *Cursor) Seek(seek []byte) (key []byte, value []byte) {
	// Start from root page/node and traverse to correct page.
	c.stack = c.stack[:0]
	c.search(seek, c.root)
	ref := &c.stack[len(c.stack)-1]

	// If the cursor is pointing to the end of page/node then return nil.
	if ref.index >= ref.count() {
		return nil, nil
	}

	return c.keyValue()
}

// first moves the cursor to the first leaf element under the last page in the stack.
func (c *Cursor) first() {
	for {
		// Exit when we hit a leaf page.
		ref := &c.stack[len(c.stack)-1]
		if ref.isLeaf() {
			break
		}

		// Keep adding pages pointing to the first element to the stack.
		var pgid pgid
		if ref.node != nil {
			pgid = ref.node.inodes[ref.index].pgid
		} else {
			pgid = ref.page.branchPageElement(uint16(ref.index)).pgid
		}
		p, n := c.transaction.pageNode(pgid)
		c.stack = append(c.stack, elemRef{page: p, node: n, index: 0})
	}
}

// last moves the cursor to the last leaf element under the last page in the stack.
func (c *Cursor) last() {
	for {
		// Exit when we hit a leaf page.
		ref := &c.stack[len(c.stack)-1]
		if ref.isLeaf() {
			break
		}

		// Keep adding pages pointing to the last element in the stack.
		var pgid pgid
		if ref.node != nil {
			pgid = ref.node.inodes[ref.index].pgid
		} else {
			pgid = ref.page.branchPageElement(uint16(ref.index)).pgid
		}
		p, n := c.transaction.pageNode(pgid)

		var nextRef = elemRef{page: p, node: n}
		nextRef.index = nextRef.count() - 1
		c.stack = append(c.stack, nextRef)
	}
}

// search recursively performs a binary search against a given page/node until it finds a given key.
func (c *Cursor) search(key []byte, pgid pgid) {
	p, n := c.transaction.pageNode(pgid)
	if p != nil {
		_assert((p.flags&(branchPageFlag|leafPageFlag)) != 0, "invalid page type: "+p.typ())
	}
	e := elemRef{page: p, node: n}
	c.stack = append(c.stack, e)

	// If we're on a leaf page/node then find the specific node.
	if e.isLeaf() {
		c.nsearch(key)
		return
	}

	if n != nil {
		c.searchNode(key, n)
		return
	}
	c.searchPage(key, p)
}

func (c *Cursor) searchNode(key []byte, n *node) {
	var exact bool
	index := sort.Search(len(n.inodes), func(i int) bool {
		// TODO(benbjohnson): Optimize this range search. It's a bit hacky right now.
		// sort.Search() finds the lowest index where f() != -1 but we need the highest index.
		ret := bytes.Compare(n.inodes[i].key, key)
		if ret == 0 {
			exact = true
		}
		return ret != -1
	})
	if !exact && index > 0 {
		index--
	}
	c.stack[len(c.stack)-1].index = index

	// Recursively search to the next page.
	c.search(key, n.inodes[index].pgid)
}

func (c *Cursor) searchPage(key []byte, p *page) {
	// Binary search for the correct range.
	inodes := p.branchPageElements()

	var exact bool
	index := sort.Search(int(p.count), func(i int) bool {
		// TODO(benbjohnson): Optimize this range search. It's a bit hacky right now.
		// sort.Search() finds the lowest index where f() != -1 but we need the highest index.
		ret := bytes.Compare(inodes[i].key(), key)
		if ret == 0 {
			exact = true
		}
		return ret != -1
	})
	if !exact && index > 0 {
		index--
	}
	c.stack[len(c.stack)-1].index = index

	// Recursively search to the next page.
	c.search(key, inodes[index].pgid)
}

// nsearch searches the leaf node on the top of the stack for a key.
func (c *Cursor) nsearch(key []byte) {
	e := &c.stack[len(c.stack)-1]
	p, n := e.page, e.node

	// If we have a node then search its inodes.
	if n != nil {
		index := sort.Search(len(n.inodes), func(i int) bool {
			return bytes.Compare(n.inodes[i].key, key) != -1
		})
		e.index = index
		return
	}

	// If we have a page then search its leaf elements.
	inodes := p.leafPageElements()
	index := sort.Search(int(p.count), func(i int) bool {
		return bytes.Compare(inodes[i].key(), key) != -1
	})
	e.index = index
}

// keyValue returns the key and value of the current leaf element.
func (c *Cursor) keyValue() ([]byte, []byte) {
	ref := &c.stack[len(c.stack)-1]
	if ref.index >= ref.count() {
		return nil, nil
	}

	// Retrieve value from node.
	if ref.node != nil {
		inode := &ref.node.inodes[ref.index]
		return inode.key, inode.value
	}

	// Or retrieve value from page.
	elem := ref.page.leafPageElement(uint16(ref.index))
	return elem.key(), elem.value()
}

// node returns the node that the cursor is currently positioned on.
func (c *Cursor) node(t *RWTransaction) *node {
	_assert(len(c.stack) > 0, "accessing a node with a zero-length cursor stack")

	// If the top of the stack is a leaf node then just return it.
	if ref := &c.stack[len(c.stack)-1]; ref.node != nil && ref.isLeaf() {
		return ref.node
	}

	// Start from root and traverse down the hierarchy.
	var n = c.stack[0].node
	if n == nil {
		n = t.node(c.stack[0].page.id, nil)
	}
	for _, ref := range c.stack[:len(c.stack)-1] {
		_assert(!n.isLeaf, "expected branch node")
		n = n.childAt(int(ref.index))
	}
	_assert(n.isLeaf, "expected leaf node")
	return n
}

// elemRef represents a reference to an element on a given page/node.
type elemRef struct {
	page  *page
	node  *node
	index int
}

// isLeaf returns whether the ref is pointing at a leaf page/node.
func (r *elemRef) isLeaf() bool {
	if r.node != nil {
		return r.node.isLeaf
	}
	return (r.page.flags & leafPageFlag) != 0
}

// count returns the number of inodes or page elements.
func (r *elemRef) count() int {
	if r.node != nil {
		return len(r.node.inodes)
	}
	return int(r.page.count)
}